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    <p>After quite a lot of searching and talks with iOS development gurus, it appears that <a href="http://developer.apple.com/library/ios/#qa/qa1673/_index.html#//apple_ref/doc/uid/DTS40010053" rel="noreferrer">QA1673</a> doesn't help when it comes to pausing, backgrounding, then moving to foreground. My experimentation even shows that delegate methods that fire off from animations, such as<code>animationDidStop</code>become unreliable.</p> <p>Sometimes they fire, sometimes they don't. </p> <p>This creates a lot of problems because it means that, not only are you looking at a different screen that you were when you paused, but also the sequence of events currently in motion can be disrupted.</p> <p>My solution thus far has been as follows:</p> <p>When the animation starts, I get the start time:</p> <p><code>mStartTime = [layer convertTime:CACurrentMediaTime() fromLayer:nil];</code></p> <p>When the user hits the pause button, I remove the animation from the<code>CALayer</code>:</p> <p><code>[layer removeAnimationForKey:key];</code></p> <p>I get the absolute time using<code>CACurrentMediaTime()</code>: </p> <p><code>CFTimeInterval stopTime = [layer convertTime:CACurrentMediaTime() fromLayer:nil];</code></p> <p>Using the<code>mStartTime</code>and<code>stopTime</code>I calculate an offset time:</p> <pre><code>mTimeOffset = stopTime - mStartTime; </code></pre> <p>I also set the model values of the object to be that of the<code>presentationLayer</code>. So, my<code>stop</code>method looks like this:</p> <pre><code>//-------------------------------------------------------------------------------------------------- - (void)stop { const CALayer *presentationLayer = layer.presentationLayer; layer.bounds = presentationLayer.bounds; layer.opacity = presentationLayer.opacity; layer.contentsRect = presentationLayer.contentsRect; layer.position = presentationLayer.position; [layer removeAnimationForKey:key]; CFTimeInterval stopTime = [layer convertTime:CACurrentMediaTime() fromLayer:nil]; mTimeOffset = stopTime - mStartTime; } </code></pre> <p>On resume, I recalculate what's left of the paused animation based upon the<code>mTimeOffset</code>. That's a bit messy because I'm using<code>CAKeyframeAnimation</code>. I figure out what keyframes are outstanding based on the<code>mTimeOffset</code>. Also, I take into account that the pause may have occurred mid frame, e.g. halfway between<code>f1</code>and<code>f2</code>. That time is deducted from the time of that keyframe.</p> <p>I then add this animation to the layer afresh:</p> <p><code>[layer addAnimation:animationGroup forKey:key];</code></p> <p>The other thing to remember is that you will need to check the flag in<code>animationDidStop</code>and only remove the animated layer from the parent with<code>removeFromSuperlayer</code>if the flag is<code>YES</code>. That means that the layer is still visible during the pause.</p> <p>This method does seem very laborious. It does work though! I'd love to be able to simply do this using <a href="http://developer.apple.com/library/ios/#qa/qa1673/_index.html#//apple_ref/doc/uid/DTS40010053" rel="noreferrer">QA1673</a>. But at the moment for backgrounding, it doesn't work and this seems to be the only solution.</p>
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    1. USMax MacLeod
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    1. COI'm curious on how you did your recalculation to resume the animation. Supposing the animation is linear I think you can just calculate the distance between your offset and the end point and that's it.
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    2. COwill try to dig out some code this weekend!
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