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POCATransaction begin/commit in loop not being honored, all happens at once
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  1. POCATransaction begin/commit in loop not being honored, all happens at once
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    copied!<p>I'm working through the 4th ed. of the Hillegass/Preble Cocoa book and have hit a snag I can't understand in Core Animation.</p> <p>In particular, I have the following loop that presents a list of images, supposedly one-at-a-time, into a layer-hosting view. The expected result is that I should see each image fly out one-at-a-time as <code>presentImage</code> is called. The actual result is that the view stays empty until the loop is complete and then the images fly out all-at-once. During the delay, I see the multiple log messages indicating the multiple calls to <code>presentImage</code>. When those log messages stop, indicating the loop is done, then all the images fly out at once.</p> <pre><code>// loop to present each image in list of URLs // called at end of applicationDidFinishLaunching NSTimeInterval t0 = [NSDate timeIntervalSinceReferenceDate]; for(id url in urls) { NSImage *img = [[NSImage alloc] initWithContentsOfURL:url]; if(!img) continue; NSImage *thumbImg = [self thumbImageFromImage:img]; [self presentImage:thumbImg]; NSString *dt = [NSString stringWithFormat:@"%0.1fs", [NSDate timeIntervalSinceReferenceDate]-t0]; NSLog(@"Presented %@ at %@ sec",url,dt); } </code></pre> <p>The <code>thumbImageFromImage</code> method does just what you'd think (creates a smaller image). The code for <code>presentImage</code> is as follows:</p> <pre><code>- (void)presentImage:(NSImage *)image { CGRect superlayerBounds = view.layer.bounds; NSPoint center = NSMakePoint(CGRectGetMidX(superlayerBounds), CGRectGetMidY(superlayerBounds)); NSRect imageBounds = NSMakeRect(0, 0, image.size.width, image.size.height); CGPoint randomPoint = CGPointMake(CGRectGetMaxX(superlayerBounds)*(double)random()/(double)RAND_MAX, CGRectGetMaxY(superlayerBounds)*(double)random()/(double)RAND_MAX); CAMediaTimingFunction *tf = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut]; CABasicAnimation *posAnim = [CABasicAnimation animation]; posAnim.fromValue = [NSValue valueWithPoint:center]; posAnim.duration = 1.5; posAnim.timingFunction = tf; CABasicAnimation *bdsAnim = [CABasicAnimation animation]; bdsAnim.fromValue = [NSValue valueWithRect:NSZeroRect]; bdsAnim.duration = 1.5; bdsAnim.timingFunction = tf; CALayer *layer = [CALayer layer]; layer.contents = image; layer.actions = [NSDictionary dictionaryWithObjectsAndKeys:posAnim,@"position", bdsAnim, @"bounds", nil]; [CATransaction begin]; [view.layer addSublayer:layer]; layer.position = randomPoint; layer.bounds = NSRectToCGRect(imageBounds); [CATransaction commit]; } </code></pre> <p>The observed result is as if the <code>[CATransaction begin]</code> and <code>[CATransaction commit]</code> calls are bracketing the <code>for</code> loop rather than the call to <code>addSublayer</code> inside <code>presentImage</code>. In fact, I do observe the same all-at-once behavior if I remove the <code>[CATransaction begin]</code> and <code>[CATransaction commit]</code> from inside <code>presentImage</code> and instead bracket the <code>for</code> loop. Actually, I observe the same all-at-once behavior if I remove calls to <code>[CATransaction begin]</code> and <code>[CATransaction commit]</code> altogether.</p>
 

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