I am learning how to use Looper and Handler class in android development
http://developer.android.com/reference/android/os/Looper.html
The example given in the android development is not clear to understand what is the usage and the how to use it.I dont know how to add Handler inside the Looper and how I can call the Looper to loop.
If it is available, can anyone give me a simple example to use it.
public class LooperTest extends Activity{
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
private class LooperTesting extends Thread
{
public Handler handler;
public void run()
{
Looper.prepare();
handler = new Handler()
{
public void handlerMessage(Message msg)
{
// do something
}
};
Looper.loop();
}
}
}
Hope this Links are helps to u
Link1
Link2
Link3
In your example you have only defined a Thread with a Looper. You need to start the Thread with the associated Looper before you can post any messages to it. I've added some code to your example to illustrate what has to be done:
public class LooperTest extends Activity{
LooperTesting mBgThread;
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
mBgThread = new mBgThread();
// Start the thread. When started, it will wait for incoming messages.
// Use the post* or send* methods of your handler-reference.
mBgThread.start();
}
public void onDestroy() {
// Don't forget to quit the Looper, so that the
// thread can finish.
mBgThread.handler.getLooper().quit();
}
private class LooperTesting extends Thread
{
public Handler handler;
public void run()
{
Looper.prepare();
handler = new Handler()
{
public void handlerMessage(Message msg)
{
// do something
}
};
Looper.loop();
}
}
}
Related
Can I create onClickListener() for a button in more than 1 threads that are executing simultaneously?
Would that listener be called individually in every thread?
No, a button has only one onClickListener. Setting a second one overwrites any listener set previously. And that function will only be called on the UI thread. You can pass messages to multiple threads from that function though.
There are a lot of ways to communicate between threads. Since you are wondering how to pass something over to the thread, here is a simple example:
public class MainActivity extends AppCompatActivity {
protected void onCreate(Bundle savedInstanceState) {
...
//Start a thread you need to
anotherThread = new AnotherThread();
anotherThread.start();
}
protected void onResume() {
...
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
anotherThread.sendData(message);
}
});
}
}
public class AnotherThread extends Thread {
//Instantiate handler to associate it with the current thread.
//Handler enqueues all tasks in the MessageQueue using Looper
//and execute them upon coming out of queue;
private Handler handler;
#Override
public void run() {
try {
//Here we create a unique Looper for this thread.
//The main purpose of which to keep thread alive looping through
//MessageQueue and send task to corresponding handler.
Looper.prepare();
handler = new Handler() {
#Override
public void handleMessage(Message msg) {
//manage incoming messages here
String value = msg.getData().getString("key");
}
};
Looper.loop();
} catch (Throwable e) {
e.printStackTrace();
}
}
public synchronized void sendData(Message message) {
handler.post(new Runnable() {
#Override
public void run() {
handler.sendMessage(message);
}
});
}
}
To get more about thread communication I'd recommend you the following:
1, 2, 3.
I just wanted to test Log.i() and look at the console in android studio. In the code below onResume should start the thread and run() should write an endless stream of "dings" with the tag "run" in the monitor. But the run method apparently only gets called once. Why?
public class MainActivity extends Activity implements Runnable {
Thread gameThread = null;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i("onCreate","getting started");
}
public void run() {
Log.i("run","ding");
}
#Override
public void onResume() {
super.onResume();
gameThread = new Thread(this);
gameThread.start();
}
}
You're missing the notion of what threading really does. It allows you to run a unit of work asynchronously. So, all the same normal rules apply. The reason it only runs once, is because the thread exits after run() returns. So just like any other method, you should put something like
while(true)
{
Log.i("run","ding");
}
inside of run(). Ideally you would actually check some condition so that you can exit the thread as needed.
Finally, it is probably a bad idea to have your MainActivity implement Runnable. Typically it is good style to have a thread implemented by its own class, for example DingThread implements Runnable.
You're missing while loop that why its run only once. Use below code. This is the better approach to use thread concept.
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i("onCreate","getting started");
}
#Override
public void onResume() {
super.onResume();
startThread();// create thread obj and start
}
private GameThread mGameThread = null;
private void startThread() {
stopThread();// if thread already running stop it then create new thread and start (for avoiding multi-threading).
mGameThread = new GameThread();
mGameThread.start();//start the thread.
}
//To stop the thread simply call this method.
private void stopThread() {
if(mGameThread != null) {
mGameThread.setStop();
mGameThread = null;
}
}
private class GameThread extends Thread {
private boolean mIsStop;// mIsStop is default false
#Override
public void run() {
while (!mIsStop) { // if mIsStop is false then only come inside loop.
Log.i("run","ding"); //log will print
}
}
public void setStop() {
mIsStop = true;// set mIStop variable to true.
}
}
}
I created a Looper thread class:
public class MyLooperThread extends Thread{
private Handler mHandler;
public void init(){
start(); //start the thread
synchronized (this) {
wait(5000); //wait for run()
}
Log.d("DEBUG","Init Done!");
//EXCEPTION: Can't create handler inside thread that has not called Looper.prepare()
MyObject obj = new MyObject(mHandler);
}
#Override
public void run() {
Looper.prepare();
mHandler = new Handler(){
#Override
public void handleMessage(Message msg){
//Check installed app package names, NOTHING RELATED WITH UI ...
}
};
synchronized (this) {
notify();
}
Looper.loop();
}//end of run()
}
In my Activity, I call above MyLooperThread 's init() method in onCreate(). Besides, I have a ToggleButton element, when ToggleButton is checked, I call MyLooperThread's init() method too.
public class MyActivity extends Activity implements OnCheckedChangeListener{
…
#Override
protected void onCreate(Bundle savedInstanceState){
…
myToggleButton.setOnCheckedChangeListener(this);
myToggleButton.setChecked(true);//checked by default
MyLooperThread myLooper = new MyLooperThread();
myLooper.init();
}
#Override
public void onCheckedChanged(CompoundButton button, boolean isChecked) {
if(isChecked){
MyLooperThread myLooper = new MyLooperThread();
myLooper.init();
}else{
...
}
}
}
When launch my app, it is fine. My toggle button is shown as checked by default. When I uncheck it & check it again, I got exception:
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
which is pointed to the init() method's last line of code MyObject obj = new MyObject(mHandler);
Why I got this exception? I don't understand, my mHandler is created after I called Looper.prepare() in run().
Pretty sure the error is saying you're trying to do something UI related when not on the UI thread.
As you haven't posted anything that you're doing inside the handleMessage, I'm gonna assume that you're trying to change one of the UI elements. Always use the UI thread to update the UI. It should go something like this:
handleMessage(Message msg) {
...
getActivity().runOnUiThread(new Runnable {
...
// update UI here
...
});
...
}
just use a HandlerThread():
ht = new HandlerThread();
ht.start();
h = new Handler(ht.getLooper());
I have read similar questions about this problem and tried to find a solution but I have not figure out what is the reason in my case. My program's code rough is below :
public class ReportSystem extends Activity implements SensorEventListener , Runnable{
ReportLocation reportObj = new ReportLocation(this); //my other class
Thread thread_send = new Thread(this);
Handler handler = new Handler() {
public void handleMessage(Message message) {
msg.setText("---"));
}
};
public void onCreate(Bundle savedInstanceState){
//something...
}
public void onSensorChanged(SensorEvent event){
if(event.values[0] > 10)
thread_send.start(); // thread is started..
}
public void run(){
reportObj.send(); //connect with server and send data by the help of RepotLocation class' send function
handler.sendEmptyMessage(0);
}
}//end class
What should I do to solve this problem ?
Try this:
public void run(){
Looper.prepare();
reportObj.send();
handler.sendEmptyMessage(0);
Looper.loop();
}
I have the following:-
public class resApp extends MapActivity implements Runnable {
public void run() {
searchImage.setVisibility(View.GONE);
}
}
I also have a background thread that runs before this but that seems to run ok.
When i run the app the run() never gets called.
Can you help?
This code did work about 6 months ago but the device was 2.1.
Thanks
Chris
edit
I had already implemented
private Handler handler;
handler = new Handler() {
#Override
public void handleMessage(Message msg) {
if (msg.toString().equalsIgnoreCase("1")) {
ad.dismiss();
} else {
pd.dismiss();
}
}
};
as an example and I already have an asynchronous task that runs in the back ground and in 2.1 I could have getters and setters in there. I have now had to pull these out and put them into the run() method as 2.2 doesn't like setting onclicklistener in an async task.
All I need to do is call the run() method on post execute but have tried everything:-
protected void onPostExecute(Object result) {
// Pass the result data back to the main activity
if (dialog != null) {
resApp.this.dialog.dismiss();
}
}
Could I just do:-
handler = new Handler() {
#Override
public void handleMessage(Message msg) {
this.resApp.run();
}
};
You can call the run() method by using Handler.
Handler myHandler = new Handler();
resApp myObj;
And call it by using myHandler.post(myObj);