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PONon-lazy image loading in iOS
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copied!<p>I'm trying to load UIImages in a background thread and then display them on the iPad. However, there's a stutter when I set the imageViews' view property to the image. I soon figured out that image loading is lazy on iOS, and found a partial solution in this question:</p> <p><a href="https://stackoverflow.com/questions/1815476/cgimage-uiimage-lazily-loading-on-ui-thread-causes-stutter">CGImage/UIImage lazily loading on UI thread causes stutter</a></p> <p>This actually forces the image to be loaded in the thread, but there's still a stutter when displaying the image.</p> <p>You can find my sample project here: <a href="http://www.jasamer.com/files/SwapTest.zip" rel="nofollow noreferrer">http://www.jasamer.com/files/SwapTest.zip</a> (edit: <a href="http://www.jasamer.com/files/SwapTest-Fixed.zip" rel="nofollow noreferrer">fixed version</a>), check the SwapTestViewController. Try dragging the picture to see the stutter.</p> <p>The test-code I created that stutters is this (the forceLoad method is the one taken from the stack overflow question I posted above):</p> <pre><code>NSArray* imagePaths = [NSArray arrayWithObjects: [[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil], [[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil], nil]; NSOperationQueue* queue = [[NSOperationQueue alloc] init]; [queue addOperationWithBlock: ^(void) { int imageIndex = 0; while (true) { UIImage* image = [[UIImage alloc] initWithContentsOfFile: [imagePaths objectAtIndex: imageIndex]]; imageIndex = (imageIndex+1)%2; [image forceLoad]; //What's missing here? [self performSelectorOnMainThread: @selector(setImage:) withObject: image waitUntilDone: YES]; [image release]; } }]; </code></pre> <p>There are two reasons why I know the stuttering can be avoided: </p> <p>(1) Apple is able to load images without stuttering in the Photos app</p> <p>(2) This code does not cause stutter after placeholder1 and placeholder2 have been displayed once in this modified version of the above code: </p> <pre><code> UIImage* placeholder1 = [[UIImage alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil]]; [placeholder1 forceLoad]; UIImage* placeholder2 = [[UIImage alloc] initWithContentsOfFile:[[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil]]; [placeholder2 forceLoad]; NSArray* imagePaths = [NSArray arrayWithObjects: [[NSBundle mainBundle] pathForResource: @"a.png" ofType: nil], [[NSBundle mainBundle] pathForResource: @"b.png" ofType: nil], nil]; NSOperationQueue* queue = [[NSOperationQueue alloc] init]; [queue addOperationWithBlock: ^(void) { int imageIndex = 0; while (true) { //The image is not actually used here - just to prove that the background thread isn't causing the stutter UIImage* image = [[UIImage alloc] initWithContentsOfFile: [imagePaths objectAtIndex: imageIndex]]; imageIndex = (imageIndex+1)%2; [image forceLoad]; if (self.imageView.image==placeholder1) { [self performSelectorOnMainThread: @selector(setImage:) withObject: placeholder2 waitUntilDone: YES]; } else { [self performSelectorOnMainThread: @selector(setImage:) withObject: placeholder1 waitUntilDone: YES]; } [image release]; } }]; </code></pre> <p>However, I can't keep all my images in memory.</p> <p>This implies that forceLoad doesn't do the complete job - there's something else going on before the images are actually displayed. Does anyone know what that is, and how I can put that into the background thread?</p> <p>Thanks, Julian</p> <p><strong>Update</strong></p> <p>Used a few of Tommys tips. What I figured out is that it's CGSConvertBGRA8888toRGBA8888 that's taking so much time, so it seems it's a color conversion that's causing the lag. Here's the (inverted) call stack of that method.</p> <pre><code>Running Symbol Name 6609.0ms CGSConvertBGRA8888toRGBA8888 6609.0ms ripl_Mark 6609.0ms ripl_BltImage 6609.0ms RIPLayerBltImage 6609.0ms ripc_RenderImage 6609.0ms ripc_DrawImage 6609.0ms CGContextDelegateDrawImage 6609.0ms CGContextDrawImage 6609.0ms CA::Render::create_image_by_rendering(CGImage*, CGColorSpace*, bool) 6609.0ms CA::Render::create_image(CGImage*, CGColorSpace*, bool) 6609.0ms CA::Render::copy_image(CGImage*, CGColorSpace*, bool) 6609.0ms CA::Render::prepare_image(CGImage*, CGColorSpace*, bool) 6609.0ms CALayerPrepareCommit_(CALayer*, CA::Transaction*) 6609.0ms CALayerPrepareCommit_(CALayer*, CA::Transaction*) 6609.0ms CALayerPrepareCommit_(CALayer*, CA::Transaction*) 6609.0ms CALayerPrepareCommit_(CALayer*, CA::Transaction*) 6609.0ms CALayerPrepareCommit 6609.0ms CA::Context::commit_transaction(CA::Transaction*) 6609.0ms CA::Transaction::commit() 6609.0ms CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) 6609.0ms __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ 6609.0ms __CFRunLoopDoObservers 6609.0ms __CFRunLoopRun 6609.0ms CFRunLoopRunSpecific 6609.0ms CFRunLoopRunInMode 6609.0ms GSEventRunModal 6609.0ms GSEventRun 6609.0ms -[UIApplication _run] 6609.0ms UIApplicationMain 6609.0ms main </code></pre> <p>The last bit-mask changes he proposed didn't change anything, sadly.</p>

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