I am quite new to Android and Java. Basically, I would like to realize an encapsulation of a background thread of Android, and inside this background thread, I have an infinite loop which will periodically take some operations of getting data(like from Internet, or from some hardware devices).
The encapsulated class must provide only a function like getData() for others to get data. But everytime when i call this getData() function from other classes, it never gives me the changing values, but only the initialized values.
I've studied both of the AsyncTask, Handler and Message ways to realize multithread. And both of them give me the initialized values.
Here is the encapsulated class of Handler and Message:
public class getDataFromUSB{
private int usb_data;
private Handler handler = new Handler(){
#Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(msg.what==1){
usb_data=msg.arg1;
}
}
};
private Thread thread = new Thread(){
#Override
public void run(){
while(!Thread.currentThread().isInterrupted())
{
int a=read_usb();
Message msg = new Message();
msg.arg1 = a;
msg.what = 1;
handler.sendMessage(msg);
try {
Thread.sleep(15);
} catch (InterruptedException e) {
System.err.println("");
this.interrupt();
}
}
}
};
public void start(){
thread.start();
}
public int get_data(){
return usb_data;
}
public int read_usb()
{
int a=10;
return a;
}
}
And then in another class, here is the code of calling getDataFromUSB:
getDataFromUSB usb1= new getDataFromUSB();
usb1.start();
int a=usb1.getData();
Log.e(TAG,"a = " +a);
Then everytime i call this usb1.getData(), the value is always 0. I don't understand why.
Now I proceed to do some more realistic things. I add an object of random in my getDataFrom USB class to provide different numbers, I also change the way of assigning values to usb_data, I think it's better to do it just in the background thread, there is no need to move it to the handlemessage. So it becomes:
public class getDataFromUSB{
private int usb_data;
private Random random = new Random(555L);
private Handler handler = new Handler(){
#Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
};
private Thread thread = new Thread(){
#Override
public void run(){
while(!Thread.currentThread().isInterrupted())
{
int a=read_usb();
usb_data=a;
Message msg = new Message();
handler.sendMessage(msg);
}
}
};
public void start(){
thread.start();
}
public int get_data(){
return usb_data;
}
public int read_usb()
{
return random.nextInt();
}
}
Then I call it from another class like what Nikita suggested:
Handler h = new Handler();
for (int i=0;i<20;i++){
h.postDelayed(new Runnable() {
public void run() {
int data=usb1.get_data();
Log.e(TAG,"data= " +data);
}
},500);
}
The strange thing is that it then gives sometimes all the same numbers, sometimes several some numbers, sometimes all different numbers. As I understand, the usb_data has always been changed inside the background thread, so we are not obliged to wait for handlemessage to proceed. Everytime I call getData(), it should give me the newest value. Isn't that right?
The problem might be following: you run your code from main UI thread. When you new start thread - it sends message to handler. This message is added to main thread's queue and will be processed in main thread when it finishes it's current job. Currently main thread runs your code and there is no chance that message will be processed before you call usb.getData().
To check whether your update thread works properly you can post delayed runnable that will print value of usb1.getData():
final getDataFromUSB usb1= new getDataFromUSB();
usb1.start();
Handler h = new Handler();
h.postDelayed(new Runnable() {
public void run() {
int a=usb1.getData();
Log.e(TAG,"a = " +a);
}
}, 500); // Waits 500 milliseconds and runs runnable on current thread.
Related
My app receives messages and has to schedule multiple replies one after another each with a different delay. What is the best way to have about 2000 delayed tasks with different delays? It seems like Android can't run that much handlers at the same time due to limited ressources.
Any ideas?
Thanks in advance.
Code simplified:
class TimedTask extends TimerTask { //runs every 10 seconds and checks for new contacts in queue to reply to (contacts can go up to 500 ones)
SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(SendMessagesService.this);
String contacts = sp.getString("contacts", ""); //queued by another Thread
String contacts_array[] = contacts.split(",");
for (String contact : contacts_array) {
Handler handler = new Handler();
handler.postDelayed(new Runnable() {
#Override
public void run() {
String message = sp.getString("message1",""); //there are different messages with different delays, depending on which were already sent
ReceiveContactsAdapter mDbHelper = new ReceiveContactsAdapter(SendMessagesService.this);
mDbHelper.open();
mDbHelper.sendMessages(message, contact);
mDbHelper.close();
}
}, delay); //delay depends on which message will be sent
}
}
You can use thread.sleep(milisec) instead. It delays the thread for the particular time.
#Override
public void run()
{
while (!Thread.interrupted())
try
{
Thread.sleep(1000);
MainActivity.this.runOnUiThread(new Runnable()
{
#Override
public void run()
{
////
}
});
}
catch (InterruptedException e)
{
}
}
})).start();
I am trying to load an object from a server in Android. This object is loaded in a thread. When loading is finished, an _objectHandler is called to get some key - values from the object, for example, the _filename key. Every time a filename is retrieved, I want to display it. For this reason, I am looping over the element of the loaded object in a second thread, and calling a _handler every time a value is loaded. What I want to get is all the _filename values, but what I am getting is only the last value of the _fielName. what I am doing wrong?
ArrayList <myObject> object;
String filename;
Thread thread = new Thread (MyActivity.this);
thread.start();
public void run() {
Looper.prepare();
try {
object = getObjectFromServer();
} catch (Exception e) {
e.printStackTrace();
}
_objectHandler.sendEmptyMessage(0);
Looper.loop();
}
Handler _objectHandler = new Handler() {
#Override
public void handleMessage(Message msg) {
for (int i = 0; i < object.size(); i++) {
myObject obj= object(i);
new Thread(new Runnable() {
public void run() {
filename= obj.getFileName();
Message msg = new Message();
_handler.sendEmptyMessage(0);
}
}).start();
}
}
};
Handler _handler = new Handler() {
#Override
public void handleMessage(Message msg) {
Log.i("The fielname is ", " filename" + filename
}
};
you can use android.os.Handler class. This will provide you a mechanism for enqueue an action to be performed on a different thread than your own.
I am learning via a book and it gives me this example:
Handler handler=new Handler() {
#Override
public void handleMessage(Message msg) {
bar.incrementProgressBy(5);
}
};
and
Thread background=new Thread(new Runnable() {
public void run() {
try {
for (int i=0;i<20 && isRunning.get();i++) {
Thread.sleep(500);
handler.sendMessage(handler.obtainMessage());
}
} catch (Throwable t) {
// just end the background thread
}
}
});
Which works out great. But, further down in the book it says:
If you would rather not fuss with Message objects, you can also pass
Runnable objects to the Handler, which will run those Runnable
objects on the activity UI thread. ...you can use those same methods
on any View (i.e., any widget or container). This slightly simplifies
your code, in that you can then skip the Handler object.
But there are no examples given of how to do this via a Runnable object. Does anyone have an example?
Something like this:
Handler h = new Handler();
Thread background=new Thread(new Runnable() {
public void run() {
try {
for (int i=0;i<20 && isRunning.get();i++) {
Thread.sleep(500);
handler.post(new Runnable() {
public void run() {
bar.incrementProgressBy(5);
}
});
}
}
catch (Throwable t) {
// just end the background thread
}
}
});
As per the Android Docs for Handler:
public final boolean post (Runnable r)
Since: API Level 1 Causes the Runnable r to be added to the message
queue. The runnable will be run on the thread to which this handler is
attached. Parameters
r -- The Runnable that will be executed. Returns
Returns true if the Runnable was successfully placed in to the message
queue. Returns false on failure, usually because the looper processing
the message queue is exiting.
Ive created a new thread for a file browser. The thread reads the contents of a directory. What I want to do is update the UI thread to draw a graphical representation of the files and folders. I know I can't update the UI from within a new thread so what I want to do is:
whilst the file scanning thread iterates through a directories files and folders pass a file path string back to the UI thread. The handler in the UI thread then draws the graphical representation of the file passed back.
public class New_Project extends Activity implements Runnable {
private Handler handler = new Handler() {
#Override
public void handleMessage(Message msg) {
Log.d("New Thread","Proccess Complete.");
Intent intent = new Intent();
setResult(RESULT_OK, intent);
finish();
}
};
public void fileScanner(){
//if (!XMLEFunctions.canReadExternal(this)) return;
pd = ProgressDialog.show(this, "Reading Directory.",
"Please Wait...", true, false);
Log.d("New Thread","Called");
Thread thread = new Thread(this);
thread.start();
}
public void run() {
Log.d("New Thread","Reading Files");
getFiles();
handler.sendEmptyMessage(0);
}
public void getFiles() {
for (int i=0;i<=allFiles.length-1;i++){
//I WANT TO PASS THE FILE PATH BACK TU A HANDLER IN THE UI
//SO IT CAN BE DRAWN.
**passFilePathBackToBeDrawn(allFiles[i].toString());**
}
}
}
It seems passing simple messages is int based... What I needed to do was pass a Bundle
using Message.setData(Bundle) and Message.getData(Bundle)
So Happy =0)
//Function From Within The Thread
public void newProjectCreation() {
Message msg = new Message();
Bundle bundle = new Bundle();
bundle.putString("Test", "test value");
msg.setData(bundle);
handler2.sendMessage(msg);
}
//Handler in The UI Thread Retreieves The Data
//And Can Update the GUI as Required
private Handler handler2 = new Handler() {
#Override
public void handleMessage(Message msg) {
Bundle bundle = msg.getData();
Toast.makeText(New_Project.this,bundle.getString("Test"),Toast.LENGTH_SHORT).show();
}
};
Check out AsyncTask for this kind of stuff. It's really much more elegant than rolling your own handler and passing messages back and forth.
http://developer.android.com/reference/android/os/AsyncTask.html
I am developing an Android app and I am doing some heavy work (bringing data from an online web page and parsing it to store in database) in a service. Currently, it is taking about 20+ mins and for this time my UI is stuck. I was thinking of using a thread in service so my UI doesn't get stuck but it is giving error. I am using the following code:
Thread thread = new Thread()
{
#Override
public void run() {
try {
while(true) {
sleep(1000);
Toast.makeText(getBaseContext(), "Running Thread...", Toast.LENGTH_LONG).show();
}
} catch (InterruptedException e) {
Toast.makeText(getBaseContext(), e.toString(), Toast.LENGTH_LONG).show();
}
}
};
thread.start();
This simple code is giving run time error. Even If I take out the while loop, it is still not working.
Please, can any one tell me what mistake I am doing. Apparently, I copied this code directly from an e-book. It is suppose to work but its not.
Android commandment: thou shall not interact with UI objects from your own threads
Wrap your Toast Display into runOnUIThread(new Runnable() { });
Example of new thread creation taken from Android samples (android-8\SampleSyncAdapter\src\com\example\android\samplesync\client\NetworkUtilities.java):
public static Thread performOnBackgroundThread(final Runnable runnable) {
final Thread t = new Thread() {
#Override
public void run() {
try {
runnable.run();
} finally {
}
}
};
t.start();
return t;
}
runnable is the Runnable that contains your Network operations.
You can use HandlerThread and post to it, here is an example to service that has one.
public class NetworkService extends Service {
private HandlerThread mHandlerThread;
private Handler mHandler;
private final IBinder mBinder = new MyLocalBinder();
#Override
public void onCreate() {
super.onCreate();
mHandlerThread = new HandlerThread("LocalServiceThread");
mHandlerThread.start();
mHandler = new Handler(mHandlerThread.getLooper());
}
public void postRunnable(Runnable runnable) {
mHandler.post(runnable);
}
public class MyLocalBinder extends Binder {
public NetworkService getService() {
return NetworkService.this;
}
}
#Override
public IBinder onBind(Intent intent) {
return mBinder;
}
}
You may define your jobs in a runnable object, use a thread object for running it and start this thread in your service's onStartCommand() function. Here is my notes:
In your service class:
define your main loop in an Runnable object
create Thread object with the runnable object as parameter
In your service class's onStartCommand method():
call thread object's start function()
my code :
private Runnable busyLoop = new Runnable() {
public void run() {
int count = 1;
while(true) {
count ++;
try {
Thread.sleep(100);
} catch (Exception ex) {
;
}
ConvertService.running.sendNotification("busyLoop" + count);
}
}
};
public int onStartCommand(Intent intent, int flags, int startId) {
sendNotification("onStartCommand");
if (! t.isAlive()) {
t.start();
}
return START_STICKY;
}