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PORestoring animation where it left off when app resumes from background
 

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  1. PORestoring animation where it left off when app resumes from background
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    <p>I have an endlessly looping <code>CABasicAnimation</code> of a repeating image tile in my view: </p> <pre><code>a = [CABasicAnimation animationWithKeyPath:@"position"]; a.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionLinear]; a.fromValue = [NSValue valueWithCGPoint:CGPointMake(0, 0)]; a.toValue = [NSValue valueWithCGPoint:CGPointMake(image.size.width, 0)]; a.repeatCount = HUGE_VALF; a.duration = 15.0; [a retain]; </code></pre> <p>I have tried to "pause and resume" the layer animation as described in <a href="http://developer.apple.com/library/ios/#qa/qa1673/_index.html" rel="noreferrer">Technical Q&amp;A QA1673</a>. </p> <p>When the app enters background, the animation gets removed from the layer. To compensate I listen to <code>UIApplicationDidEnterBackgroundNotification</code> and call <code>stopAnimation</code> and in response to <code>UIApplicationWillEnterForegroundNotification</code> call <code>startAnimation</code>.</p> <pre><code>- (void)startAnimation { if ([[self.layer animationKeys] count] == 0) [self.layer addAnimation:a forKey:@"position"]; CFTimeInterval pausedTime = [self.layer timeOffset]; self.layer.speed = 1.0; self.layer.timeOffset = 0.0; self.layer.beginTime = 0.0; CFTimeInterval timeSincePause = [self.layer convertTime:CACurrentMediaTime() fromLayer:nil] - pausedTime; self.layer.beginTime = timeSincePause; } - (void)stopAnimation { CFTimeInterval pausedTime = [self.layer convertTime:CACurrentMediaTime() fromLayer:nil]; self.layer.speed = 0.0; self.layer.timeOffset = pausedTime; } </code></pre> <p>The problem is that it starts again at the beginning and there is ugly jump from last position, as seen on app snapshot the system took when application did enter background, back to the start of the animation loop.</p> <p>I can not figure out how to make it start at last position, when I re-add the animation. Frankly, I just don't understand how that code from QA1673 works: in <code>resumeLayer</code> it sets the layer.beginTime twice, which seems redundant. But when I've removed the first set-to-zero, it did not resume the animation where it was paused. This was tested with simple tap gesture recognizer, that did toggle the animation - this is not strictly related to my issues with restoring from background.</p> <p>What state should I remember before the animation gets removed and how do I restore the animation from that state, when I re-add it later?</p>
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    1. COany luck with this? I have animations pausing and resuming in my game pause. However when I come out of background any interrupted animations seem to be complete. I know I can do this by capturing the state of the presentation layer. Seems like a real pain though. There must be an easier way!
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    2. CONo luck. I also keep the `[layer presentationLayer]` as a workaround in my back pocket, if the need to fix this gets critical. As you say, it seems like a quite a PITA.
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    3. COI got the Apple QA1673 working and it was great for pausing and resuming the game. But what's the point when it doesn't work for backgrounding? For me the whole point of the QA1673 was to save the bother of doing it the presentationLayer way.
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