Develop Reference

Android sending messages between fragment and service

I have a fragment with a button. When clicked it tells a service to start polling sensors and then insert the sensor data into a database on a background thread. When the button is pushed again, the service will stop. When the Stop button is pushed, there may still be tasks in the executor queue that is inserting into the DB, so during this time I want to display a progress dialog, and dismiss it once the entire queue is clear. The fragment with the button looks like this:
public class StartFragment extends Fragment implements View.OnClickListener {
Button startButton;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_start, container, false);
startButton = (Button) view.findViewById(R.id.startButton);
startButton.setOnClickListener(this);
return view;
}
#Override
public void onClick(View v) {
if (recording has not yet started){
mainActivity.startService(new Intent(mainActivity, SensorService.class));
} else {
//I want to display a progress dialog here when the service is told to stop
//Once all executor task queue is clear, I want to dismiss this dialog
mainActivity.stopService(new Intent(mainActivity, SensorService.class));
}
}
}
When the button is clicked the first time, the following service will start:
public class SensorService extends Service implements SensorEventListener {
public static final int SCREEN_OFF_RECEIVER_DELAY = 100;
private SensorManager sensorManager = null;
private WakeLock wakeLock = null;
ExecutorService executor;
Runnable insertHandler;
private void registerListener() {
//register 4 sensor listeners (acceleration, gyro, magnetic, gravity)
}
private void unregisterListener() {
sensorManager.unregisterListener(this);
}
public BroadcastReceiver receiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
Log.i(TAG, "onReceive("+intent+")");
if (!intent.getAction().equals(Intent.ACTION_SCREEN_OFF)) {
return;
}
Runnable runnable = new Runnable() {
public void run() {
Log.i(TAG, "Runnable executing...");
unregisterListener();
registerListener();
}
};
new Handler().postDelayed(runnable, SCREEN_OFF_RECEIVER_DELAY);
}
};
public void onSensorChanged(SensorEvent event) {
//get sensor values and store into 4 different arrays here
//insert into database in background thread
executor.execute(insertHandler);
}
#Override
public void onCreate() {
super.onCreate();
//get sensor manager and sensors here
PowerManager manager = (PowerManager) getSystemService(Context.POWER_SERVICE);
wakeLock = manager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);
registerReceiver(receiver, new IntentFilter(Intent.ACTION_SCREEN_OFF));
//Executor service and runnable for DB inserts
executor = Executors.newSingleThreadExecutor();
insertHandler = new InsertHandler();
}
#Override
public IBinder onBind(Intent intent) {
return null;
}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
super.onStartCommand(intent, flags, startId);
startForeground(Process.myPid(), new Notification());
registerListener();
wakeLock.acquire();
return START_STICKY;
}
#Override
public void onDestroy() {
//Prevent new tasks from being added to thread
executor.shutdown();
try {
//Wait for all tasks to finish before we proceed
while (!executor.awaitTermination(1, TimeUnit.SECONDS)) {
Log.i(TAG, "Waiting for current tasks to finish");
}
} catch (InterruptedException e) {
executor.shutdownNow();
Thread.currentThread().interrupt();
}
if (executor.isTerminated()){
//Stop everything else once the task queue is clear
unregisterReceiver(receiver);
unregisterListener();
wakeLock.release();
dbHelper.close();
stopForeground(true);
//Once the queue is clear, I want to send a message back to the fragment to dismiss the progress dialog here
}
}
class InsertHandler implements Runnable {
public void run() {
//get sensor values from 4 arrays, and insert into db here
}
}
So I want to display the dialog on the 2nd button press. Then once it is pressed again, service will stop, and I want to wait until the queue is clear and then send a dismiss event back to the fragment to dismiss the progress dialog.
Showing the dialog is easy. I can just add progress dialog code in the onClick method of the fragment, before stopService is called
I'm having difficulty with figuring out how to send a message back in onDestroy of the SensorService to dismiss that dialog
Whats the best way of doing this without resorting to external libraries?
Is there some way that the BroadcastReceiver I'm using in SensorService can be used? Or maybe it's better to create a new Handler in the fragment and somehow pass it through to the service so it can send a message back to the fragment?
EDIT:
I have tried the following based on one of the answers below:
Added a MessageHandler class to my fragment class:
public static class MessageHandler extends Handler {
#Override
public void handleMessage(Message message) {
int state = message.arg1;
switch (state) {
case 0:
stopDialog.dismiss();
break;
case 1:
stopDialog = new ProgressDialog(mainActivity);
stopDialog.setMessage("Stopping...");
stopDialog.setTitle("Saving data");
stopDialog.setProgressNumberFormat(null);
stopDialog.setCancelable(false);
stopDialog.setMax(100);
stopDialog.show();
break;
}
}
}
Created a new instance of MessageHandler in my fragment (tried placing this in a variety of places...same results):
public static Handler messageHandler = new MessageHandler();
The service is then started from my fragment using:
Intent startService = new Intent(mainActivity, SensorService.class);
startService.putExtra("MESSENGER", new Messenger(messageHandler));
getContext().startService(startService);
In my SensorService BroadcastReceiver I create the messageHandler:
Bundle extras = intent.getExtras();
messageHandler = (Messenger) extras.get("MESSENGER");
Then I show the dialog at the very beginning of SensorService onDestroy:
sendMessage("SHOW");
and dismiss it at the very end of that same method:
sendMessage("HIDE");
My sendMessage method looks like this:
public void sendMessage(String state) {
Message message = Message.obtain();
switch (state) {
case "SHOW":
message.arg1 = 1;
break;
case "HIDE" :
message.arg1 = 0;
break;
}
try {
messageHandler.send(message);
} catch (RemoteException e) {
e.printStackTrace();
}
}
So I can start the Service OK, but when I press it again to stop, I get this:
java.lang.RuntimeException: Unable to stop service com.example.app.SensorService#21124f0: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.os.Messenger.send(android.os.Message)' on a null object reference
and its referring to Line 105 of SensorService where I have messageHandler.send(message)
Thoughts on what might be wrong?

In activity:
protected BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, final Intent intent) {
runOnUiThread(new Runnable() {
#Override
public void run() {
if(intent.hasExtra("someExtraMessage")){
doSomething(intent.getStringExtra("someExtraMessage"));
}
}
});
}
};
#Override
public void onCreate(Bundle savedInstanceState, PersistableBundle persistentState) {
super.onCreate(savedInstanceState, persistentState);
LocalBroadcastManager.getInstance(this).registerReceiver(mMessageReceiver,
new IntentFilter("message-id"));
}
protected void onDestroy() {
super.onDestroy();
LocalBroadcastManager.getInstance(this).unregisterReceiver(mMessageReceiver);
}
public void doSomething(){
//...
}
Then somewhere from service:
Context context = BamBamApplication.getApplicationContext(); // Can be application or activity context.
// BamBamApplicaiton extends Application ;)
Intent intent = new Intent("message-id");
intent.putExtra("someExtraMessage", "Some Message :)");
LocalBroadcastManager.getInstance(context).sendBroadcast(intent);
Actually you are doing wrong from the very beginning :) all the services are running on main thread, so here you must better start all hard processing to async task to move this in background otherwise you will stuck your app, or you will get sudden unexpected crashes.
Here are you sample of async task that parses json api response in background with Typed result by parameter.
class ParseJsonInBackground<T> extends AsyncTask<String, Void, ApiResponseModel<T>> {
private ProcessResponse<T> func;
private Type inClass;
public ParseJsonInBackground(ProcessResponse<T> f, Type inClass){
this.func = f;
this.inClass = inClass;
}
#Override
protected void onPreExecute() {
super.onPreExecute();
}
#Override
protected ApiResponseModel<T> doInBackground(String... json) {
Gson gson = new Gson();
try {
ApiResponseModel<T> result = (ApiResponseModel<T>) gson.fromJson(json[0], inClass);
return result;
}catch(Exception e){
ApiResponseModel<T> result = new ApiResponseModel<T>();
result.data = null;
result.success = false;
result.error = new ArrayList<>();
result.error.add(new ErrorModel(0, "Parsing error", "Parsing error"));
return result;
}
}
#Override
protected void onPostExecute(ApiResponseModel<T> result) {
Utils.hideLoadingProgress(mContext);
if(result != null && func != null){
if(result.success){
func.onSuccess(result);
}else{
func.onError(result);
}
}
}
}
and sample how to call:
new ParseJsonInBackground<T>(responseFunc, inClass).execute(json.toString());
make attention! - don't use any views in processing coz this will stuck main thread, make database processing in similar async task, don't write to often to database make recording with transactions.

I would propose doing this via Handler messages: you send a message from the Service to your Activity which has to register as a callback handler (implement http://developer.android.com/reference/android/os/Handler.Callback.html). Use a custom message code (message.what) and listen for it. Keep in mind to send this to the main looper of your application (from the service).
You may also check this comment which illustrates this kind of interaction with some more code: https://stackoverflow.com/a/20595215/4310905

It turns out that the code in the Edit of my original question works, but I have to shuffle around some of my code:
Bundle extras = intent.getExtras();
messageHandler = (Messenger) extras.get("MESSENGER");
The above needs to be moved to onStartCommand of SensorService instead of being in the BroadcastReceiver

Communication between Activity and Service

I am trying to make my own MusicPlayer for android. Where i came to a problem is running some things in background. Main activity manages GUI and up to now all the songs are playing. I wanted to separate GUI and music playing classes. I want to put music managing part in Service and leave other things as they are now.
My problem is that i can't organize communication between Activity and Service as lot of communication is happening between them including moving objects in both directions. I tried many techniques that I searched here on Stack Overflow but every time I had problems. I need Service to be able to send objects to Activity and vice versa. When I add widget i also want it to be able to communicate with Service.
Any tips are appreciated, if you need source code place comment bellow but now in this transition it became chaotic.
Is there any more advanced tutorial on this than calling one method that returns random number from service? :P
EDIT: Possible solution is to use RoboGuice library and move objects with injection

I have implemented communication between Activity and Service using Bind and Callbacks interface.
For sending data to the service I used Binder which retruns the Service instace to the Activity, and then the Activity can access public methods in the Service.
To send data back to the Activity from the Service, I used Callbacks interface like you are using when you want to communicate between Fragment and Activity.
Here is some code samples for each:
The following example shows Activity and Service bidirectional relationship:
The Activity has 2 buttons:
The first button will start and stop the service.
The second button will start a timer which runs in the service.
The service will update the Activity through callback with the timer progress.
My Activity:
//Activity implements the Callbacks interface which defined in the Service
public class MainActivity extends ActionBarActivity implements MyService.Callbacks{
ToggleButton toggleButton;
ToggleButton tbStartTask;
TextView tvServiceState;
TextView tvServiceOutput;
Intent serviceIntent;
MyService myService;
int seconds;
int minutes;
int hours;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
serviceIntent = new Intent(MainActivity.this, MyService.class);
setViewsWidgets();
}
private void setViewsWidgets() {
toggleButton = (ToggleButton)findViewById(R.id.toggleButton);
toggleButton.setOnClickListener(btListener);
tbStartTask = (ToggleButton)findViewById(R.id.tbStartServiceTask);
tbStartTask.setOnClickListener(btListener);
tvServiceState = (TextView)findViewById(R.id.tvServiceState);
tvServiceOutput = (TextView)findViewById(R.id.tvServiceOutput);
}
private ServiceConnection mConnection = new ServiceConnection() {
#Override
public void onServiceConnected(ComponentName className,
IBinder service) {
Toast.makeText(MainActivity.this, "onServiceConnected called", Toast.LENGTH_SHORT).show();
// We've binded to LocalService, cast the IBinder and get LocalService instance
MyService.LocalBinder binder = (MyService.LocalBinder) service;
myService = binder.getServiceInstance(); //Get instance of your service!
myService.registerClient(MainActivity.this); //Activity register in the service as client for callabcks!
tvServiceState.setText("Connected to service...");
tbStartTask.setEnabled(true);
}
#Override
public void onServiceDisconnected(ComponentName arg0) {
Toast.makeText(MainActivity.this, "onServiceDisconnected called", Toast.LENGTH_SHORT).show();
tvServiceState.setText("Service disconnected");
tbStartTask.setEnabled(false);
}
};
View.OnClickListener btListener = new View.OnClickListener() {
#Override
public void onClick(View v) {
if(v == toggleButton){
if(toggleButton.isChecked()){
startService(serviceIntent); //Starting the service
bindService(serviceIntent, mConnection, Context.BIND_AUTO_CREATE); //Binding to the service!
Toast.makeText(MainActivity.this, "Button checked", Toast.LENGTH_SHORT).show();
}else{
unbindService(mConnection);
stopService(serviceIntent);
Toast.makeText(MainActivity.this, "Button unchecked", Toast.LENGTH_SHORT).show();
tvServiceState.setText("Service disconnected");
tbStartTask.setEnabled(false);
}
}
if(v == tbStartTask){
if(tbStartTask.isChecked()){
myService.startCounter();
}else{
myService.stopCounter();
}
}
}
};
#Override
public void updateClient(long millis) {
seconds = (int) (millis / 1000) % 60 ;
minutes = (int) ((millis / (1000*60)) % 60);
hours = (int) ((millis / (1000*60*60)) % 24);
tvServiceOutput.setText((hours>0 ? String.format("%d:", hours) : "") + ((this.minutes<10 && this.hours > 0)? "0" + String.format("%d:", minutes) : String.format("%d:", minutes)) + (this.seconds<10 ? "0" + this.seconds: this.seconds));
}
}
And here is the service:
public class MyService extends Service {
NotificationManager notificationManager;
NotificationCompat.Builder mBuilder;
Callbacks activity;
private long startTime = 0;
private long millis = 0;
private final IBinder mBinder = new LocalBinder();
Handler handler = new Handler();
Runnable serviceRunnable = new Runnable() {
#Override
public void run() {
millis = System.currentTimeMillis() - startTime;
activity.updateClient(millis); //Update Activity (client) by the implementd callback
handler.postDelayed(this, 1000);
}
};
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
//Do what you need in onStartCommand when service has been started
return START_NOT_STICKY;
}
#Override
public IBinder onBind(Intent intent) {
return mBinder;
}
//returns the instance of the service
public class LocalBinder extends Binder{
public MyService getServiceInstance(){
return MyService.this;
}
}
//Here Activity register to the service as Callbacks client
public void registerClient(Activity activity){
this.activity = (Callbacks)activity;
}
public void startCounter(){
startTime = System.currentTimeMillis();
handler.postDelayed(serviceRunnable, 0);
Toast.makeText(getApplicationContext(), "Counter started", Toast.LENGTH_SHORT).show();
}
public void stopCounter(){
handler.removeCallbacks(serviceRunnable);
}
//callbacks interface for communication with service clients!
public interface Callbacks{
public void updateClient(long data);
}
}

Update: July 10 2016
IMO I think using BroadcastReceiver for custom events is better way
as the Messengers mentioned don't handle activity recreation on device
rotation as well as possible memory leaks.
You may create custom BroadCast Receiver for events in the activity, Then you may also use Messengers.
In your Activity
create a MessageHandler class as
public static class MessageHandler extends Handler {
#Override
public void handleMessage(Message message) {
int state = message.arg1;
switch (state) {
case HIDE:
progressBar.setVisibility(View.GONE);
break;
case SHOW:
progressBar.setVisibility(View.VISIBLE);
break;
}
}
}
Now you can have it's instance as
public static Handler messageHandler = new MessageHandler();
Start your Service with this Handler object as an extra data as
Intent startService = new Intent(context, SERVICE.class)
startService.putExtra("MESSENGER", new Messenger(messageHandler));
context.startService(startService);
In your Service you receive this object from the intent and initialize the Messenger variable in Service as
private Messenger messageHandler;
Bundle extras = intent.getExtras();
messageHandler = (Messenger) extras.get("MESSENGER");
sendMessage(ProgressBarState.SHOW);
And then write a method sendMessage to send messages to activity.
public void sendMessage(ProgressBarState state) {
Message message = Message.obtain();
switch (state) {
case SHOW :
message.arg1 = Home.SHOW;
break;
case HIDE :
message.arg1 = Home.HIDE;
break;
}
try {
messageHandler.send(message);
} catch (RemoteException e) {
e.printStackTrace();
}
}
The sample code above shows and hides a ProgressBar in Activity as messages are received from Service.

Intents are good solution for communication between Activitiy and Service.
A fast solution for receive intents in your service is subclassing IntentService class. It handles asynchronous requests expressed as Intents using a queue and worker thread.
For communication from service to Activity you can broadcast the intent but instead of using normal sendBroadcast() from Context, a more efficent way is to use LocalBroadcastManager from support library.
Example service.
public class MyIntentService extends IntentService {
private static final String ACTION_FOO = "com.myapp.action.FOO";
private static final String EXTRA_PARAM_A = "com.myapp.extra.PARAM_A";
public static final String BROADCAST_ACTION_BAZ = "com.myapp.broadcast_action.FOO";
public static final String EXTRA_PARAM_B = "com.myapp.extra.PARAM_B";
// called by activity to communicate to service
public static void startActionFoo(Context context, String param1) {
Intent intent = new Intent(context, MyIntentService.class);
intent.setAction(ACTION_FOO);
intent.putExtra(EXTRA_PARAM1, param1);
context.startService(intent);
}
public MyIntentService() {
super("MyIntentService");
}
#Override
protected void onHandleIntent(Intent intent) {
if (intent != null) {
final String action = intent.getAction();
if (ACTION_FOO.equals(action)) {
final String param1 = intent.getStringExtra(EXTRA_PARAM_A);
// do something
}
}
}
// called to send data to Activity
public static void broadcastActionBaz(String param) {
Intent intent = new Intent(BROADCAST_ACTION_BAZ);
intent.putExtra(EXTRA_PARAM_B, param);
LocalBroadcastManager bm = LocalBroadcastManager.getInstance(this);
bm.sendBroadcast(intent);
}
}
Example Activity
public class MainActivity extends ActionBarActivity {
// handler for received data from service
private final BroadcastReceiver mBroadcastReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
if (intent.getAction().equals(MyIntentService.BROADCAST_ACTION_BAZ)) {
final String param = intent.getStringExtra(EXTRA_PARAM_B);
// do something
}
}
};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
IntentFilter filter = new IntentFilter();
filter.addAction(MyIntentService.BROADCAST_ACTION_BAZ);
LocalBroadcastManager bm = LocalBroadcastManager.getInstance(this);
bm.registerReceiver(mBroadcastReceiver, filter);
}
#Override
protected void onDestroy() {
LocalBroadcastManager bm = LocalBroadcastManager.getInstance(this);
bm.unregisterReceiver(mBroadcastReceiver);
super.onDestroy();
}
// send data to MyService
protected void communicateToService(String parameter) {
MyIntentService.startActionFoo(this, parameter);
}
}

I think there is a problem with the correct answer. I have not enough reputation to comment on it.
Right in the answer:
Activity call bindService() to get pointer to Service is ok. Because service context is maintained when connection is maintained.
wrong in the answer:
service pointer to Activity class to call back is bad way. Activity instance maybe not null during Activity context is being Release => exception here.
solution for the wrong in the answer:
service send intent to Activity. and Activity receiver intent via BroadcastReceiver.
Note:
in this case, Service and Activity in the same Process, you should use LocalBroadcastManager to send intent. It make performance and security better

This is a simple example of communication between activity and service
Activity
MyReceiver myReceiver; //my global var receiver
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.layourAwesomexD);
registerReceiver();
}
//When the activity resume, the receiver is going to register...
#Override
protected void onResume() {
super.onResume();
checkStatusService(); // verficarStatusServicio(); <- name change
registerReceiver();
}
//when the activity stop, the receiver is going to unregister...
#Override
protected void onStop() {
unregisterReceiver(myReceiver); //unregister my receiver...
super.onStop();
}
//function to register receiver :3
private void registerReceiver(){
//Register BroadcastReceiver
//to receive event from our service
myReceiver = new MyReceiver();
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(MyService.SENDMESAGGE);
registerReceiver(myReceiver, intentFilter);
}
// class of receiver, the magic is here...
private class MyReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context arg0, Intent arg1) {
//verify if the extra var exist
System.out.println(arg1.hasExtra("message")); // true or false
//another example...
System.out.println(arg1.getExtras().containsKey("message")); // true or false
//if var exist only print or do some stuff
if (arg1.hasExtra("message")) {
//do what you want to
System.out.println(arg1.getStringExtra("message"));
}
}
}
public void checkStatusService(){
if(MyService.serviceStatus!=null){
if(MyService.serviceStatus == true){
//do something
//textview.text("Service is running");
}else{
//do something
//textview.text("Service is not running");
}
}
}
Service
public class MyService extends Service {
final static String SENDMESAGGE = "passMessage";
public static Boolean serviceStatus = false;
#Override
public void onCreate() {
super.onCreate();
serviceStatus=true;
}
#Nullable
#Override
public IBinder onBind(Intent intent) {return null;}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
//you service etc...
passMessageToActivity("hello my friend this an example of send a string...");
return START_STICKY;
}
#Override
public void onDestroy() {
super.onDestroy();
passMessageToActivity("The service is finished, This is going to be more cooler than the heart of your ex...");
System.out.println("onDestroy");
serviceStatus=false;
}
private void passMessageToActivity(String message){
Intent intent = new Intent();
intent.setAction(SENDMESAGGE);
intent.putExtra("message",message);
sendBroadcast(intent);
}
}
if we don't unregister BroadcastReceiver we will have an error, you need to unregister when the activity go onPause, onStop, onDestroy...
if you don't register BroadcastReceiver when you back to activity, it will not listen anything from the service... the service will send information to BroadcastReceiver but it will not receive anything because it isn't registered.
When you create more than one service, the following services are going to begin in onStartCommand.
You can pass information to service with intent and you get it in onStartCommand
Difference about return in onStartCommand: Difference between START_STICKY and START_REDELIVER_INTENT? and check the official website of google: Services

The best way in this case is to communicate by doing broadcasting from your service for different actions and receiving it in your activity. You can create a custom broadcast and send some codes defining specific events like complete, change, prepare etc...

Most easy and efficient way will be using EventBus from GreenRobot.
Use simple 3 steps:
1 Define events
public static class MessageEvent { /* Additional fields if needed */ }
2 Prepare subscribers: Declare and annotate your subscribing method, optionally specify a thread mode:
#Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent event) {/* Do something */};
Register and unregister your subscriber. For example on Android, activities and fragments should usually register according to their life cycle:
#Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
#Override
public void onStop() {
super.onStop();
EventBus.getDefault().unregister(this);
}
3 Post events:
EventBus.getDefault().post(new MessageEvent());

Very easy yet powerful way is to use EventBus you can add it to your gradle build and enjoy the easy publisher/subscriber pattern .

Android update activity UI from service

I have a service which is checking for new task all the time. If there is new task, I want to refresh the activity UI to show that info.
I did find https://github.com/commonsguy/cw-andtutorials/tree/master/18-LocalService/ this example. Is that a good approch ? Any other examples?
Thanks.

See below for my original answer - that pattern has worked well, but recently I've started using a different approach to Service/Activity communication:
Use a bound service which enables the Activity to get a direct
reference to the Service, thus allowing direct calls on it, rather
than using Intents.
Use RxJava to execute asynchronous operations.
If the Service needs to continue background operations even when no
Activity is running, also start the service from the Application
class so that it does not get stopped when unbound.
The advantages I have found in this approach compared to the startService()/LocalBroadcast technique are
No need for data objects to implement Parcelable - this is particularly important to me as I am now sharing code between Android and iOS (using RoboVM)
RxJava provides canned (and cross-platform) scheduling, and easy composition of sequential asynchronous operations.
This should be more efficient than using a LocalBroadcast, though the overhead of using RxJava may outweigh that.
Some example code. First the service:
public class AndroidBmService extends Service implements BmService {
private static final int PRESSURE_RATE = 500000; // microseconds between pressure updates
private SensorManager sensorManager;
private SensorEventListener pressureListener;
private ObservableEmitter<Float> pressureObserver;
private Observable<Float> pressureObservable;
public class LocalBinder extends Binder {
public AndroidBmService getService() {
return AndroidBmService.this;
}
}
private IBinder binder = new LocalBinder();
#Nullable
#Override
public IBinder onBind(Intent intent) {
logMsg("Service bound");
return binder;
}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
return START_NOT_STICKY;
}
#Override
public void onCreate() {
super.onCreate();
sensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
Sensor pressureSensor = sensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE);
if(pressureSensor != null)
sensorManager.registerListener(pressureListener = new SensorEventListener() {
#Override
public void onSensorChanged(SensorEvent event) {
if(pressureObserver != null) {
float lastPressure = event.values[0];
float lastPressureAltitude = (float)((1 - Math.pow(lastPressure / 1013.25, 0.190284)) * 145366.45);
pressureObserver.onNext(lastPressureAltitude);
}
}
#Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {
}
}, pressureSensor, PRESSURE_RATE);
}
#Override
public Observable<Float> observePressure() {
if(pressureObservable == null) {
pressureObservable = Observable.create(emitter -> pressureObserver = emitter);
pressureObservable = pressureObservable.share();
}
return pressureObservable;
}
#Override
public void onDestroy() {
if(pressureListener != null)
sensorManager.unregisterListener(pressureListener);
}
}
And an Activity that binds to the service and receives pressure altitude updates:
public class TestActivity extends AppCompatActivity {
private ContentTestBinding binding;
private ServiceConnection serviceConnection;
private AndroidBmService service;
private Disposable disposable;
#Override
protected void onDestroy() {
if(disposable != null)
disposable.dispose();
unbindService(serviceConnection);
super.onDestroy();
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = DataBindingUtil.setContentView(this, R.layout.content_test);
serviceConnection = new ServiceConnection() {
#Override
public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
logMsg("BlueMAX service bound");
service = ((AndroidBmService.LocalBinder)iBinder).getService();
disposable = service.observePressure()
.observeOn(AndroidSchedulers.mainThread())
.subscribe(altitude ->
binding.altitude.setText(
String.format(Locale.US,
"Pressure Altitude %d feet",
altitude.intValue())));
}
#Override
public void onServiceDisconnected(ComponentName componentName) {
logMsg("Service disconnected");
}
};
bindService(new Intent(
this, AndroidBmService.class),
serviceConnection, BIND_AUTO_CREATE);
}
}
The layout for this Activity is:
<?xml version="1.0" encoding="utf-8"?>
<layout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.controlj.mfgtest.TestActivity">
<TextView
tools:text="Pressure"
android:id="#+id/altitude"
android:gravity="center_horizontal"
android:layout_gravity="center_vertical"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
</LinearLayout>
</layout>
If the service needs to run in the background without a bound Activity it can be started from the Application class as well in OnCreate() using Context#startService().
My Original Answer (from 2013):
In your service: (using COPA as service in example below).
Use a LocalBroadCastManager. In your service's onCreate, set up the broadcaster:
broadcaster = LocalBroadcastManager.getInstance(this);
When you want to notify the UI of something:
static final public String COPA_RESULT = "com.controlj.copame.backend.COPAService.REQUEST_PROCESSED";
static final public String COPA_MESSAGE = "com.controlj.copame.backend.COPAService.COPA_MSG";
public void sendResult(String message) {
Intent intent = new Intent(COPA_RESULT);
if(message != null)
intent.putExtra(COPA_MESSAGE, message);
broadcaster.sendBroadcast(intent);
}
In your Activity:
Create a listener on onCreate:
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
super.setContentView(R.layout.copa);
receiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
String s = intent.getStringExtra(COPAService.COPA_MESSAGE);
// do something here.
}
};
}
and register it in onStart:
#Override
protected void onStart() {
super.onStart();
LocalBroadcastManager.getInstance(this).registerReceiver((receiver),
new IntentFilter(COPAService.COPA_RESULT)
);
}
#Override
protected void onStop() {
LocalBroadcastManager.getInstance(this).unregisterReceiver(receiver);
super.onStop();
}

for me the simplest solution was to send a broadcast, in the activity oncreate i registered and defined the broadcast like this (updateUIReciver is defined as a class instance) :
IntentFilter filter = new IntentFilter();
filter.addAction("com.hello.action");
updateUIReciver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
//UI update here
}
};
registerReceiver(updateUIReciver,filter);
And from the service you send the intent like this:
Intent local = new Intent();
local.setAction("com.hello.action");
this.sendBroadcast(local);
don't forget to unregister the recover in the activity on destroy :
unregisterReceiver(updateUIReciver);

I would use a bound service to do that and communicate with it by implementing a listener in my activity. So if your app implements myServiceListener, you can register it as a listener in your service after you have bound with it, call listener.onUpdateUI from your bound service and update your UI in there!

I would recommend checking out Otto, an EventBus tailored specifically to Android. Your Activity/UI can listen to events posted on the Bus from the Service, and decouple itself from the backend.

Clyde's solution works, but it is a broadcast, which I am pretty sure will be less efficient than calling a method directly. I could be mistaken, but I think the broadcasts are meant more for inter-application communication.
I'm assuming you already know how to bind a service with an Activity.
I do something sort of like the code below to handle this kind of problem:
class MyService extends Service {
MyFragment mMyFragment = null;
MyFragment mMyOtherFragment = null;
private void networkLoop() {
...
//received new data for list.
if(myFragment != null)
myFragment.updateList();
}
...
//received new data for textView
if(myFragment !=null)
myFragment.updateText();
...
//received new data for textView
if(myOtherFragment !=null)
myOtherFragment.updateSomething();
...
}
}
class MyFragment extends Fragment {
public void onResume() {
super.onResume()
//Assuming your activity bound to your service
getActivity().mMyService.mMyFragment=this;
}
public void onPause() {
super.onPause()
//Assuming your activity bound to your service
getActivity().mMyService.mMyFragment=null;
}
public void updateList() {
runOnUiThread(new Runnable() {
public void run() {
//Update the list.
}
});
}
public void updateText() {
//as above
}
}
class MyOtherFragment extends Fragment {
public void onResume() {
super.onResume()
//Assuming your activity bound to your service
getActivity().mMyService.mMyOtherFragment=this;
}
public void onPause() {
super.onPause()
//Assuming your activity bound to your service
getActivity().mMyService.mMyOtherFragment=null;
}
public void updateSomething() {//etc... }
}
I left out bits for thread safety, which is essential. Make sure to use locks or something like that when checking and using or changing the fragment references on the service.

Callback from service to activity to update UI.
ResultReceiver receiver = new ResultReceiver(new Handler()) {
protected void onReceiveResult(int resultCode, Bundle resultData) {
//process results or update UI
}
}
Intent instructionServiceIntent = new Intent(context, InstructionService.class);
instructionServiceIntent.putExtra("receiver", receiver);
context.startService(instructionServiceIntent);

My solution might not be the cleanest but it should work with no problems.
The logic is simply to create a static variable to store your data on the Service and update your view each second on your Activity.
Let's say that you have a String on your Service that you want to send it to a TextView on your Activity. It should look like this
Your Service:
public class TestService extends Service {
public static String myString = "";
// Do some stuff with myString
Your Activty:
public class TestActivity extends Activity {
TextView tv;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
tv = new TextView(this);
setContentView(tv);
update();
Thread t = new Thread() {
#Override
public void run() {
try {
while (!isInterrupted()) {
Thread.sleep(1000);
runOnUiThread(new Runnable() {
#Override
public void run() {
update();
}
});
}
} catch (InterruptedException ignored) {}
}
};
t.start();
startService(new Intent(this, TestService.class));
}
private void update() {
// update your interface here
tv.setText(TestService.myString);
}
}

You could use android Jetpack's LiveData
As per the documentation:
You can extend a LiveData object using the singleton pattern to wrap
system services so that they can be shared in your app. The LiveData
object connects to the system service once, and then any observer that
needs the resource can just watch the LiveData object. For more
information, see Extend
LiveData.
Below is what I did for communication between Service and Activity and also Service and Fragment.
In this example, I have:
a class SyncLogLiveData extending LiveData that contains a SpannableStringBuilder
a service SyncService
a fragment SyncFragment
The Fragment "observes" the LiveData (ie SyncLogLiveData) and performs an action when the LiveData changes.
The LiveData is updated by the Service.
I could also update the LiveData from the Fragment in the same way but don't show it here.
class SyncLogLiveData
public class SyncLogLiveData extends LiveData<SpannableStringBuilder> {
private static SyncLogLiveData sInstance;
private final static SpannableStringBuilder log = new SpannableStringBuilder("");
#MainThread
public static SyncLogLiveData get() {
if (sInstance == null) {
sInstance = new SyncLogLiveData();
}
return sInstance;
}
private SyncLogLiveData() {
}
public void appendLog(String text) {
log.append(text);
postValue(log);
}
public void appendLog(Spanned text) {
log.append(text);
postValue(log);
}
}
in class SyncService
This line of code will update the content of the LiveData
SyncLogLiveData.get().appendLog(message);
You could also make direct use of setValue(...) or postValue(...) methods of LiveData
SyncLogLiveData.get().setValue(message);
class SyncFragment
public void onViewCreated(#NonNull View view, Bundle savedInstanceState) {
//...
// Create the observer which updates the UI.
final Observer<SpannableStringBuilder> ETAObserver = new Observer<SpannableStringBuilder>() {
#Override
public void onChanged(#Nullable final SpannableStringBuilder spannableLog) {
// Update the UI, in this case, a TextView.
getActivity().runOnUiThread(new Runnable() {
#Override
public void run() {
textViewLog.setText(spannableLog);
}
});
}
};
// Observe the LiveData, passing in this activity/fragment as the LifecycleOwner and the observer.
SyncLogLiveData.get().observe(getViewLifecycleOwner(), ETAObserver);
//...
}
From within an activity it works the same way, but for .observe(...), you may use this instead
SyncLogLiveData.get().observe(this, ETAObserver);
You could also fetch the current value of the LiveData this way at anytime in your code.
SyncLogLiveData.get().getValue();
Hopefully this will help someone. There wasn't any mention of LiveData in this answer yet.

Set a Listener in a Service-based Class

Hy i have a problem to set the ServiceUpdateUIListener in the service to update the UI. It's wrong to make a new Service object and set there the listener and put it in an intent.
Code source is at http://developerlife.com/tutorials/?p=356 there i can't find how the set the listener and start the service right.
Calling:
TimerService service = new TimerService();
TimerService.setUpdateListener(new ServiceUpdateUIListener() {
#Override
public void updateUI(String time) {
clock.setText(time);
}
});
Intent i = new Intent(Timer.this,service.class); //service cannot be resolved to a type
i.putExtra("ms", ms);
startService(i);
Service:
public class TimerService extends Service{
CountDownTimer timer;
Chronometer clock;
public static ServiceUpdateUIListener UI_UPDATE_LISTENER;
#Override
public IBinder onBind(Intent intent) {
return null;
}
#Override
public void onStart(Intent intent, int startId) {
// TODO Auto-generated method stub
int ms = intent.getIntExtra("ms", 0);
timer = new CountDownTimer(ms,1000){
#Override
public void onTick(long millisUntilFinished) {
int seconds = (int) (millisUntilFinished / 1000) % 60 ;
int minutes = (int) ((millisUntilFinished / (1000*60)) % 60);
int hours = (int) ((millisUntilFinished / (1000*60*60)) % 24);
clock.setText( String.format("%02d:%02d:%02d", hours,minutes,seconds));
Log.e("Timer", String.valueOf(millisUntilFinished));
}
#Override
public void onFinish() {
// TODO Auto-generated method stub
}
}.start();
super.onStart(intent, startId);
}
public static void setUpdateListener(ServiceUpdateUIListener l) {
UI_UPDATE_LISTENER = l;
}

The Service documentation has fairly complete sample code for implementing a service in your app that another part of your app can bind to and make calls on:
http://developer.android.com/reference/android/app/Service.html#LocalServiceSample
Just put your setUpdateListener() method on the Service, and call it once you get onServiceConnected() with the service.
So your code would be something like this:
public interface UpdateListener {
public void onUpdate(long value);
}
class LocalService {
// Like in the Service sample code, plus:
public static String ACTION_START = "com.mypackage.START";
private final ArrayList<UpdateListener> mListeners
= new ArrayList<UpdateListener>();
private final Handler mHandler = new Handler();
private long mTick = 0;
private final Runnable mTickRunnable = new Runnable() {
public void run() {
mTick++;
sendUpdate(mTick);
mHandler.postDelayed(mTickRunnable, 1000);
}
}
public void registerListener(UpdateListener listener) {
mListeners.add(listener);
}
public void unregisterListener(UpdateListener listener) {
mListeners.remove(listener);
}
private void sendUpdate(long value) {
for (int i=mListeners.size()-1; i>=0; i--) {
mListeners.get(i).onUpdate(value);
}
}
public int onStartCommand(Intent intent, int flags, int startId) {
if (ACTION_START.equals(intent.getAction()) {
mTick = 0;
mHandler.removeCallbacks(mTickRunnable);
mHandler.post(mTickRunnable);
}
return START_STICKY;
}
public void onDestroy() {
mHandler.removeCallbacks(mTickRunnable);
}
Now you can start the service to get it to start counting, and anyone can bind to it to register a listener to receive callbacks as it counts.
It is really hard though to answer your question very well because you aren't really saying what you actually want to accomplish. There are a lot of ways to use services, either starting or binding or mixing the two together, depending on exactly what you want to accomplish.
Now you can implement your client code again based on the sample:
public class SomeActivity extends Activity implements UpdateListener {
private LocalService mBoundService;
private ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
mBoundService = ((LocalService.LocalBinder)service).getService();
mBoundService.registerListener(this);
}
public void onServiceDisconnected(ComponentName className) {
mBoundService = null;
}
};
void doBindService() {
bindService(new Intent(Binding.this,
LocalService.class), mConnection, Context.BIND_AUTO_CREATE);
mIsBound = true;
}
void doUnbindService() {
if (mIsBound) {
if (mBoundService != null) {
mBoundService.unregisterListener(this);
}
unbindService(mConnection);
mIsBound = false;
}
}
protected void onDestroy() {
super.onDestroy();
doUnbindService();
}

I don't know exactly what you want, but this is not the way to do it. It seems you're mixing up a lot of things.
The tutorial itself is a bad example to my opinion, keeping a static reference to an activity in a service seems to me bad practice; you would use binding to bind your service to an activity, or if you don't want to you can pass Intents around.
As far as I know instantiating a service like you do and setting a listener on it like that doesn't work. You get an error in the startService() call because the service instance isn't a class obviously; you should use TimerService.class instead. In your service you have an onStart(); onStart() is a deprecated function, you should use onStartCommand() instead.
Now, if you have an activity in which you want to show a clock you don't need nor want the service to update its UI directly of course, but if you'd want the service to calculate a new clock tick for you, just call startService(); As long as your service is alive, sending a new start service intent will just call the onStartCommand() with the intent you're sending along.
If your clock is in an activity, setup a broadcast receiver inside your activity that and let your service broadcast an intent that can be received by the broadcast receiver you setup, with your new clock value passed along.

MrJre is correct that onStart is depreciated and that you should be using onStartCommand().
If you want to get this to work, there is a better way.
I am doing something similar, as in wanting to update a UI from results happening in a service. This was not particularly easy. (In my opinion)
Here's how to do it: (First off scrap your existing code)
In UI class add:
public Intent service;
service = new Intent(thisContext, TimerService.class);
service.putExtra("ms", ms);
startService(service);
//bind service to the UI **Important**
bindService();
IntentFilter timerFilter = new IntentFilter("TimerIntent"); // Filter that gets stuff from the service
registerReceiver(myReceiver, timerFilter);
void bindService() {
Intent newIntent = new Intent(this, TimerService.class);
bindService(newIntent, mConnection, Context.BIND_AUTO_CREATE);
mIsBound = true;
}
private ServiceConnection mConnection = new ServiceConnection() {
#Override
public void onServiceConnected(ComponentName className, IBinder binder) {
s = ((TimerService.MyBinder) binder).getService();
}
#Override
public void onServiceDisconnected(ComponentName className) {
s = null;
}
};
public void releaseBind() {
if (mIsBound) {
unbindService(mConnection);
mIsBound = false;
}
}
// Now in this class we need to add in the listener that will update the UI (the receiver registered above)
private BroadcastReceiver myReceiver = new BroadcastReceiver() {
#Override
public void onReceive(Context context, Intent intent) {
// TODO Auto-generated method stub
//Get Bundles
Bundle extras = intent.getExtras();
/* DO ANY UI UPDATING YOU WANT HERE (set text boxes, etc.) TAKING INFO FROM THE "extras" Bundle ie: setting the clock*/
//ie: int timerTest = extras.getInt("0");
// Now update screen with value from timerTest
}
};
Service File:
public class TimerService extends Service {
public TimerService () {
super();
}
private final IBinder mBinder = new MyBinder();
public Timer clockTimer = new Timer();
public int timer = 0;
// We return the binder class upon a call of bindService
#Override
public IBinder onBind(Intent arg0) {
return mBinder;
}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
// After service starts this executes
Bundle extras;
extras = intent.getExtras();
/* Call a function to do stuff here. Like if you are a clock call a timer function updates every second */
// Here's an example, modify to fit your needs.
clock();
return START_STICKY;
}
public class MyBinder extends Binder {
TimerService getService() {
return TimerService.this;
}
}
public void clock() {
clockTimer.scheduleAtFixedRate(new TimerTask() {
#Override
public void run() {
try {
// Some function ie: Time = Time + 1 //
/* MAKE SURE YOU BROADCAST THE RECEIVER HERE. This is what you send back to the UI. IE:*/
timer = timer+ 1; // increment counter
Intent intent = new
//Bundle the timervalue with Intent
intent.putExtra("0", timer);
intent.setAction("TimerIntent");
sendBroadcast(intent); // finally broadcast to the UI
} catch(Exception ie) {
}
}
},
0, // Delay to start timer
1000); // how often this loop iterates in ms (so look runs every second)
}
There might be some syntax errors in this code as I've just modified my existing and working code to try and fit your needs. There will obviously need to also be some modifications depending on what you want to do. But follow this framework and you will be able to do what you are trying to do.
This works for me, so hopefully you can modify this to work for you. (Only thing I've left out are the imports, but you should be able to easily figure that out)
Key points:
Bind service to UI
Register listener in UI file to respond to the broadcast from inside the service.
Cheers.

How to have Android Service communicate with Activity

I'm writing my first Android application and trying to get my head around communication between services and activities. I have a Service that will run in the background and do some gps and time based logging. I will have an Activity that will be used to start and stop the Service.
So first, I need to be able to figure out if the Service is running when the Activity is started. There are some other questions here about that, so I think I can figure that out (but feel free to offer advice).
My real problem: if the Activity is running and the Service is started, I need a way for the Service to send messages to the Activity. Simple Strings and integers at this point - status messages mostly. The messages will not happen regularly, so I don't think polling the service is a good way to go if there is another way. I only want this communication when the Activity has been started by the user - I don't want to start the Activity from the Service. In other words, if you start the Activity and the Service is running, you will see some status messages in the Activity UI when something interesting happens. If you don't start the Activity, you will not see these messages (they're not that interesting).
It seems like I should be able to determine if the Service is running, and if so, add the Activity as a listener. Then remove the Activity as a listener when the Activity pauses or stops. Is that actually possible? The only way I can figure out to do it is to have the Activity implement Parcelable and build an AIDL file so I can pass it through the Service's remote interface. That seems like overkill though, and I have no idea how the Activity should implement writeToParcel() / readFromParcel().
Is there an easier or better way? Thanks for any help.
EDIT:
For anyone who's interested in this later on, there is sample code from Google for handling this via AIDL in the samples directory: /apis/app/RemoteService.java

The asker has probably long since moved past this, but in case someone else searches for this...
There's another way to handle this, which I think might be the simplest.
Add a BroadcastReceiver to your activity. Register it to receive some custom intent in onResume and unregister it in onPause. Then send out that intent from your service when you want to send out your status updates or what have you.
Make sure you wouldn't be unhappy if some other app listened for your Intent (could anyone do anything malicious?), but beyond that, you should be alright.
Code sample was requested:
In my service, I have this:
// Do stuff that alters the content of my local SQLite Database
sendBroadcast(new Intent(RefreshTask.REFRESH_DATA_INTENT));
(RefreshTask.REFRESH_DATA_INTENT is just a constant string.)
In my listening activity, I define my BroadcastReceiver:
private class DataUpdateReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
if (intent.getAction().equals(RefreshTask.REFRESH_DATA_INTENT)) {
// Do stuff - maybe update my view based on the changed DB contents
}
}
}
I declare my receiver at the top of the class:
private DataUpdateReceiver dataUpdateReceiver;
I override onResume to add this:
if (dataUpdateReceiver == null) dataUpdateReceiver = new DataUpdateReceiver();
IntentFilter intentFilter = new IntentFilter(RefreshTask.REFRESH_DATA_INTENT);
registerReceiver(dataUpdateReceiver, intentFilter);
And I override onPause to add:
if (dataUpdateReceiver != null) unregisterReceiver(dataUpdateReceiver);
Now my activity is listening for my service to say "Hey, go update yourself." I could pass data in the Intent instead of updating database tables and then going back to find the changes within my activity, but since I want the changes to persist anyway, it makes sense to pass the data via DB.

There are three obvious ways to communicate with services:
Using Intents
Using AIDL
Using the service object itself (as singleton)
In your case, I'd go with option 3. Make a static reference to the service it self and populate it in onCreate():
void onCreate(Intent i) {
sInstance = this;
}
Make a static function MyService getInstance(), which returns the static sInstance.
Then in Activity.onCreate() you start the service, asynchronously wait until the service is actually started (you could have your service notify your app it's ready by sending an intent to the activity.) and get its instance. When you have the instance, register your service listener object to you service and you are set. NOTE: when editing Views inside the Activity you should modify them in the UI thread, the service will probably run its own Thread, so you need to call Activity.runOnUiThread().
The last thing you need to do is to remove the reference to you listener object in Activity.onPause(), otherwise an instance of your activity context will leak, not good.
NOTE: This method is only useful when your application/Activity/task is the only process that will access your service. If this is not the case you have to use option 1. or 2.

Use LocalBroadcastManager to register a receiver to listen for a broadcast sent from local service inside your app, reference goes here:
http://developer.android.com/reference/android/support/v4/content/LocalBroadcastManager.html

You may also use LiveData that works like an EventBus.
class MyService : LifecycleService() {
companion object {
val BUS = MutableLiveData<Any>()
}
override fun onStartCommand(intent: Intent, flags: Int, startId: Int): Int {
super.onStartCommand(intent, flags, startId)
val testItem : Object
// expose your data
if (BUS.hasActiveObservers()) {
BUS.postValue(testItem)
}
return START_NOT_STICKY
}
}
Then add an observer from your Activity.
MyService.BUS.observe(this, Observer {
it?.let {
// Do what you need to do here
}
})
You can read more from this blog.

I am surprised that no one has given reference to Otto event Bus library
http://square.github.io/otto/
I have been using this in my android apps and it works seamlessly.

Using a Messenger is another simple way to communicate between a Service and an Activity.
In the Activity, create a Handler with a corresponding Messenger. This will handle messages from your Service.
class ResponseHandler extends Handler {
#Override public void handleMessage(Message message) {
Toast.makeText(this, "message from service",
Toast.LENGTH_SHORT).show();
}
}
Messenger messenger = new Messenger(new ResponseHandler());
The Messenger can be passed to the service by attaching it to a Message:
Message message = Message.obtain(null, MyService.ADD_RESPONSE_HANDLER);
message.replyTo = messenger;
try {
myService.send(message);
catch (RemoteException e) {
e.printStackTrace();
}
A full example can be found in the API demos: MessengerService and MessengerServiceActivity. Refer to the full example for how MyService works.

The other method that's not mentioned in the other comments is to bind to the service from the activity using bindService() and get an instance of the service in the ServiceConnection callback. As described here http://developer.android.com/guide/components/bound-services.html

Binding is another way to communicate
Create a callback
public interface MyCallBack{
public void getResult(String result);
}
Activity side:
Implement the interface in the Activity
Provide the implementation for the method
Bind the Activity to Service
Register and Unregister Callback when the Service gets bound and unbound with
Activity.
public class YourActivity extends AppCompatActivity implements MyCallBack{
private Intent notifyMeIntent;
private GPSService gpsService;
private boolean bound = false;
#Override
public void onCreate(Bundle sis){
// activity code ...
startGPSService();
}
#Override
public void getResult(String result){
// show in textView textView.setText(result);
}
#Override
protected void onStart()
{
super.onStart();
bindService();
}
#Override
protected void onStop() {
super.onStop();
unbindService();
}
private ServiceConnection serviceConnection = new ServiceConnection() {
#Override
public void onServiceConnected(ComponentName className, IBinder service) {
GPSService.GPSBinder binder = (GPSService.GPSBinder) service;
gpsService= binder.getService();
bound = true;
gpsService.registerCallBack(YourActivity.this); // register
}
#Override
public void onServiceDisconnected(ComponentName arg0) {
bound = false;
}
};
private void bindService() {
bindService(notifyMeIntent, serviceConnection, Context.BIND_AUTO_CREATE);
}
private void unbindService(){
if (bound) {
gpsService.registerCallBack(null); // unregister
unbindService(serviceConnection);
bound = false;
}
}
// Call this method somewhere to start Your GPSService
private void startGPSService(){
notifyMeIntent = new Intent(this, GPSService.class);
startService(myIntent );
}
}
Service Side:
Initialize callback
Invoke the callback method whenever needed
public class GPSService extends Service{
private MyCallBack myCallback;
private IBinder serviceBinder = new GPSBinder();
public void registerCallBack(MyCallBack myCallback){
this.myCallback= myCallback;
}
public class GPSBinder extends Binder{
public GPSService getService(){
return GPSService.this;
}
}
#Nullable
#Override
public IBinder onBind(Intent intent){
return serviceBinder;
}
}

Another way could be using observers with a fake model class through the activity and the service itself, implementing an MVC pattern variation. I don't know if it's the best way to accomplish this, but it's the way that worked for me. If you need some example ask for it and i'll post something.

Besides LocalBroadcastManager , Event Bus and Messenger already answered in this question,we can use Pending Intent to communicate from service.
As mentioned here in my blog post
Communication between service and Activity can be done using
PendingIntent.For that we can use
createPendingResult().createPendingResult() creates a new
PendingIntent object which you can hand to service to use and to send
result data back to your activity inside onActivityResult(int, int,
Intent) callback.Since a PendingIntent is Parcelable , and can
therefore be put into an Intent extra,your activity can pass this
PendingIntent to the service.The service, in turn, can call send()
method on the PendingIntent to notify the activity via
onActivityResult of an event.
Activity
public class PendingIntentActivity extends AppCompatActivity
{
#Override
protected void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
PendingIntent pendingResult = createPendingResult(
100, new Intent(), 0);
Intent intent = new Intent(getApplicationContext(), PendingIntentService.class);
intent.putExtra("pendingIntent", pendingResult);
startService(intent);
}
#Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
if (requestCode == 100 && resultCode==200) {
Toast.makeText(this,data.getStringExtra("name"),Toast.LENGTH_LONG).show();
}
super.onActivityResult(requestCode, resultCode, data);
}
}
Service
public class PendingIntentService extends Service {
private static final String[] items= { "lorem", "ipsum", "dolor",
"sit", "amet", "consectetuer", "adipiscing", "elit", "morbi",
"vel", "ligula", "vitae", "arcu", "aliquet", "mollis", "etiam",
"vel", "erat", "placerat", "ante", "porttitor", "sodales",
"pellentesque", "augue", "purus" };
private PendingIntent data;
#Override
public void onCreate() {
super.onCreate();
}
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
data = intent.getParcelableExtra("pendingIntent");
new LoadWordsThread().start();
return START_NOT_STICKY;
}
#Override
public IBinder onBind(Intent intent) {
return null;
}
#Override
public void onDestroy() {
super.onDestroy();
}
class LoadWordsThread extends Thread {
#Override
public void run() {
for (String item : items) {
if (!isInterrupted()) {
Intent result = new Intent();
result.putExtra("name", item);
try {
data.send(PendingIntentService.this,200,result);
} catch (PendingIntent.CanceledException e) {
e.printStackTrace();
}
SystemClock.sleep(400);
}
}
}
}
}

My method:
Class to manage send and receive message from/to service/activity:
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;
import java.util.ArrayList;
import java.util.List;
public class MessageManager {
public interface IOnHandleMessage{
// Messages
int MSG_HANDSHAKE = 0x1;
void onHandleMessage(Message msg);
}
private static final String LOGCAT = MessageManager.class.getSimpleName();
private Messenger mMsgSender;
private Messenger mMsgReceiver;
private List<Message> mMessages;
public MessageManager(IOnHandleMessage callback, IBinder target){
mMsgReceiver = new Messenger(new MessageHandler(callback, MessageHandler.TYPE_ACTIVITY));
mMsgSender = new Messenger(target);
mMessages = new ArrayList<>();
}
public MessageManager(IOnHandleMessage callback){
mMsgReceiver = new Messenger(new MessageHandler(callback, MessageHandler.TYPE_SERVICE));
mMsgSender = null;
mMessages = new ArrayList<>();
}
/* START Getter & Setter Methods */
public Messenger getMsgSender() {
return mMsgSender;
}
public void setMsgSender(Messenger sender) {
this.mMsgSender = sender;
}
public Messenger getMsgReceiver() {
return mMsgReceiver;
}
public void setMsgReceiver(Messenger receiver) {
this.mMsgReceiver = receiver;
}
public List<Message> getLastMessages() {
return mMessages;
}
public void addMessage(Message message) {
this.mMessages.add(message);
}
/* END Getter & Setter Methods */
/* START Public Methods */
public void sendMessage(int what, int arg1, int arg2, Bundle msgData){
if(mMsgSender != null && mMsgReceiver != null) {
try {
Message msg = Message.obtain(null, what, arg1, arg2);
msg.replyTo = mMsgReceiver;
if(msgData != null){
msg.setData(msgData);
}
mMsgSender.send(msg);
} catch (RemoteException rE) {
onException(rE);
}
}
}
public void sendHandshake(){
if(mMsgSender != null && mMsgReceiver != null){
sendMessage(IOnHandleMessage.MSG_HANDSHAKE, 0, 0, null);
}
}
/* END Public Methods */
/* START Private Methods */
private void onException(Exception e){
Log.e(LOGCAT, e.getMessage());
e.printStackTrace();
}
/* END Private Methods */
/** START Private Classes **/
private class MessageHandler extends Handler {
// Types
final static int TYPE_SERVICE = 0x1;
final static int TYPE_ACTIVITY = 0x2;
private IOnHandleMessage mCallback;
private int mType;
public MessageHandler(IOnHandleMessage callback, int type){
mCallback = callback;
mType = type;
}
#Override
public void handleMessage(Message msg){
addMessage(msg);
switch(msg.what){
case IOnHandleMessage.MSG_HANDSHAKE:
switch(mType){
case TYPE_SERVICE:
setMsgSender(msg.replyTo);
sendHandshake();
break;
case TYPE_ACTIVITY:
Log.v(LOGCAT, "HERE");
break;
}
break;
default:
if(mCallback != null){
mCallback.onHandleMessage(msg);
}
break;
}
}
}
/** END Private Classes **/
}
In Activity Example:
public class activity extends AppCompatActivity
implements ServiceConnection,
MessageManager.IOnHandleMessage {
[....]
private MessageManager mMessenger;
private void initMyMessenger(IBinder iBinder){
mMessenger = new MessageManager(this, iBinder);
mMessenger.sendHandshake();
}
private void bindToService(){
Intent intent = new Intent(this, TagScanService.class);
bindService(intent, mServiceConnection, Context.BIND_AUTO_CREATE);
/* START THE SERVICE IF NEEDED */
}
private void unbindToService(){
/* UNBIND when you want (onDestroy, after operation...)
if(mBound) {
unbindService(mServiceConnection);
mBound = false;
}
}
/* START Override MessageManager.IOnHandleMessage Methods */
#Override
public void onHandleMessage(Message msg) {
switch(msg.what){
case Constants.MSG_SYNC_PROGRESS:
Bundle data = msg.getData();
String text = data.getString(Constants.KEY_MSG_TEXT);
setMessageProgress(text);
break;
case Constants.MSG_START_SYNC:
onStartSync();
break;
case Constants.MSG_END_SYNC:
onEndSync(msg.arg1 == Constants.ARG1_SUCCESS);
mBound = false;
break;
}
}
/* END Override MessageManager.IOnHandleMessage Methods */
/** START Override ServiceConnection Methods **/
private class BLEScanServiceConnection implements ServiceConnection {
#Override
public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
initMyMessenger(iBinder);
mBound = true;
}
#Override
public void onServiceDisconnected(ComponentName componentName) {
mMessenger = null;
mBound = false;
}
}
/** END Override ServiceConnection Methods **/
In Service Example:
public class Blablabla extends Service
implements MessageManager.IOnHandleMessage {
[...]
private MessageManager mMessenger;
#Nullable
#Override
public IBinder onBind(Intent intent) {
super.onBind(intent);
initMessageManager();
return mMessenger.getMsgReceiver().getBinder();
}
private void initMessageManager(){
mMessenger = new MessageManager(this);
}
/* START Override IOnHandleMessage Methods */
#Override
public void onHandleMessage(Message msg) {
/* Do what you want when u get a message looking the "what" attribute */
}
/* END Override IOnHandleMessage Methods */
Send a message from Activity / Service:
mMessenger.sendMessage(what, arg1, arg2, dataBundle);
How this works:
on the activity you start or bind the service.
The service "OnBind" methods return the Binder to his MessageManager, the in the Activity through the "Service Connection" interface methods implementation "OnServiceConnected" you get this IBinder and init you MessageManager using it.
After the Activity has init his MessageManager the MessageHandler send and Handshake to the service so it can set his "MessageHandler" sender ( the "private Messenger mMsgSender;" in MessageManager ). Doing this the service know to who send his messages.
You can also implement this using a List/Queue of Messenger "sender" in the MessageManager so you can send multiple messages to different Activities/Services or you can use a List/Queue of Messenger "receiver" in the MessageManager so you can receive multiple message from different Activities/Services.
In the "MessageManager" instance you have a list of all messages received.
As you can see the connection between "Activity's Messenger" and "Service Messenger" using this "MessageManager" instance is automatic, it is done through the "OnServiceConnected" method and through the use of the "Handshake".
Hope this is helpful for you :) Thank you very much!
Bye :D

To follow up on #MrSnowflake answer with a code example.
This is the XABBER now open source Application class. The Application class is centralising and coordinating Listeners and ManagerInterfaces and more. Managers of all sorts are dynamically loaded. Activity´s started in the Xabber will report in what type of Listener they are. And when a Service start it report in to the Application class as started. Now to send a message to an Activity all you have to do is make your Activity become a listener of what type you need. In the OnStart() OnPause() register/unreg. The Service can ask the Application class for just that listener it need to speak to and if it's there then the Activity is ready to receive.
Going through the Application class you'll see there's a loot more going on then this.

As mentioned by Madhur, you can use a bus for communication.
In case of using a Bus you have some options:
Otto event Bus library (deprecated in favor of RxJava)
http://square.github.io/otto/
Green Robot’s EventBus
http://greenrobot.org/eventbus/
NYBus (RxBus, implemented using RxJava. very similar to the EventBus)
https://github.com/MindorksOpenSource/NYBus