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POAndroid Threading - running a thread from another thread and waiting for a handler message
 

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  1. POAndroid Threading - running a thread from another thread and waiting for a handler message
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    <p>My application uses an Executor to create threads to perform tasks.</p> <p>I have a thread which loads and returns bitmaps to the main thread when needed.</p> <p>If the bitmap is not found locally, another thread is used to do the download of the file.</p> <p>All threads create a Looper to ensure that Handlers can be created from that thread (in case it needs to talk to another thread by sending messages to the Handler)</p> <pre><code>public class LooperThread extends Thread { @Override public void run() { Looper.prepare(); super.run(); Looper.loop(); } } </code></pre> <p>The main thread asks Thread-1 to get the bitmap. If the bitmap exists, it is returned successfully to the main thread. If the bitmap doesnt exist, Thread-1 asks Thread-2 to download it. A Handler is created on Thread-1 so Thread-2 can use it to tell Thread-1 when the download is complete.</p> <p>The problem is that Thread-1 completes its execution and exits before the download is complete and so when Thread-2 sends a message through the handler, there is nothing to process the message. </p> <pre><code>final RequestHandler handler = new RequestHandler() { @Override protected void onComplete(Object response) { //This never happens as Thead-1 is already finished when download is complete DownloadCompleteData data = (DownloadCompleteData)response; onDownloadComplete(data.success, data.downloadId, data.filePath); } }; myExecutor.execute(new Runnable() { public void run() { //blocking download call DownloadCompleteData data = download(downloadRequest); //Thread-2 now sends message back to Thread-1 handler.sendMessage(handler.obtainMessage(COMPLETE, data)); } }); </code></pre> <p>I think I need to run an infinite loop in Thread-1 so it can stay alive and keep processing messages until it is interrupted, but I'm not sure how to do this properly.</p> <p>Anyone know the correct way to do it?</p>
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    1. COHow does Thread-1 stop? Loopers don't stop unless you tell them to do so.
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    2. COI assumed that once the thread had exited its run() method, the thread would no longer process messages on its message loop. I dont explicitly stop the thread or the looper - threads are managed by the Executors. If the message loop is still running, then the real question is why doesn't the message sent from Thread-2 get received by Thread-1?
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      1. USJeffG
    3. CO`Looper.loop()` is an infinite loop. Messages from the internal messagequeue are processed in that loop. So you can't wait for messages get delivered (because you wait on yourself). Not sure if that is what you do. Also have a look at [HandlerThread](http://developer.android.com/reference/android/os/HandlerThread.html) - they implement the looper logic already so you don't have to.
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