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POnull Canvas in SurfaceView Thread, despite stopping Thread in surfaceDestroyed() - only on Android 4 / ICS
 

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  1. POnull Canvas in SurfaceView Thread, despite stopping Thread in surfaceDestroyed() - only on Android 4 / ICS
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    <p>I have a SurfaceView extension where the bare bones of it are implemented as in the <a href="http://developer.android.com/resources/samples/LunarLander/index.html" rel="noreferrer">Lunar Lander</a> example. That is, the <code>run()</code> method of the drawing <code>Thread</code> is essentially:</p> <pre><code>public void run() { while (mRun) { Canvas c; try { c = mSurfaceHolder.lockCanvas(); synchronized (mSurfaceHolder) { doDraw(c); // Main drawing method - not included in this code snippet } } finally { // do this in a finally so that if an exception is thrown // during the above, we don't leave the Surface in an // inconsistent state if (c != null) { mSurfaceHolder.unlockCanvasAndPost(c); } } } } } </code></pre> <p>And the <code>Thread</code> is properly stopped when the surface is destroyed:</p> <pre><code>public void surfaceDestroyed(SurfaceHolder holder) { // we have to tell thread to shut down &amp; wait for it to finish, or else // it might touch the Surface after we return and explode boolean retry = true; thread.setRunning(false); while (retry) { try { thread.join(); retry = false; } catch (InterruptedException e) { } } } </code></pre> <p>On devices I have usually tested on to date (HTC Desire, Desire HD, and Archos 101 which between them have OS 2.2 and 2.3.3 if I remember right) there has never been a problem with the above. That is, when the surface is destroyed because the user backs out of the <code>Activity</code> or another <code>Activity</code> is invoked on top, the code within <code>surfaceDestroyed()</code> always ensures that <code>mSurfaceHolder.lockCanvas()</code> would never be called for it to return <code>null</code>. </p> <p>The difference I've found on my new HTC One X which is running Android 4 / ICS, however, is that during the call to method <code>surfaceDestroyed()</code> (i.e. code within that method is still executing) my drawing <code>Thread</code> would be given a <code>null</code> canvas from <code>mSurfaceHolder.lockCanvas()</code>. This would of course cause an application crash. On my One X, this will happen <em>every</em> single time the surface is destroyed - whether it be due to rotating the phone, backing out of the <code>Activity</code>, etc. </p> <p>I'm confused about this because I was under the impression that <code>mSurfaceHolder.lockCanvas()</code> should return a non-<code>null</code> <code>Canvas</code> until <code>surfaceDestroyed()</code> has actually <em>exited</em>. Indeed, this is what the Javadoc says:</p> <p><code>This is called immediately before a surface is being destroyed. After returning from this call, you should no longer try to access this surface. If you have a rendering thread that directly accesses the surface, you must ensure that thread is no longer touching the Surface before returning from this function.</code></p> <p>My solution for now is to just check for <code>null</code>. This works fine:</p> <pre><code>if(c != null){ doDraw(c); // Main drawing method - not included in this code snippet } </code></pre> <p>But, any ideas why I'm suddenly having to do this for Android 4 / ICS?</p>
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    1. COReturning `null` is a way to prevent you from touching the surface too so what you do should be fine. Idk why or what changed in the underlying implementation though.
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    2. COThanks, zapl. Yeah, it's really bizarre. Unless there is something odd somewhere I am doing to cause this apparent change in implementation, surely lots of SurfaceView-based apps are going to die on ICS.
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    3. CODoes anyone find out an "official" why because I got this error also on ICS devices or emulators.
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