Performance issues with accelerometer - ios

Question

I have an accelerometer that I use to shift layers of UIImageviews slightly, to get some depth perception.
Here is the code I use to instantiate the animations and accelerometer inside my viewdidload method.
UIAccelerometer *accelerator = [UIAccelerometer sharedAccelerometer];
accelerator.delegate = self;
accelerator.updateInterval = 0.1f;
animateLayer0 = [CABasicAnimation animationWithKeyPath:#"position"];
animateLayer1 = [CABasicAnimation animationWithKeyPath:#"position"];
animateLayer2 = [CABasicAnimation animationWithKeyPath:#"position"];
animateLayer0.duration = 0.1;
animateLayer0.fillMode = kCAFillModeForwards;
animateLayer0.removedOnCompletion = false;
animateLayer1.duration = 0.1;
animateLayer1.fillMode = kCAFillModeForwards;
animateLayer1.removedOnCompletion = false;
animateLayer2.duration = 0.1;
animateLayer2.fillMode = kCAFillModeForwards;
animateLayer2.removedOnCompletion = false;
Here is the accelerometer function's code:
-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration{
self.difference = 0 - acceleration.y;
if (fabsf(self.difference) > 0.01) {
for (int i = 0; i < self.accLayerPoints0.count; i++) {
NSValue *val = [self.accLayerPoints0 objectAtIndex:i];
CGPoint origin = [val CGPointValue];
float x = origin.x + (acceleration.y * ACC_LAYER0_THRESHOLD);
[animateLayer0 setToValue:[NSValue valueWithCGPoint:CGPointMake(x, origin.y)]];
UIImageView *layer0 = [self.accLayerObjects0 objectAtIndex:i];
[layer0.layer addAnimation:animateLayer0 forKey:nil];
}
for (int i = 0; i < self.accLayerPoints1.count; i++) {
NSValue *val = [self.accLayerPoints1 objectAtIndex:i];
CGPoint origin = [val CGPointValue];
float x = origin.x + (acceleration.y * ACC_LAYER1_THRESHOLD);
[animateLayer1 setToValue:[NSValue valueWithCGPoint:CGPointMake(x, origin.y)]];
UIImageView *layer0 = [self.accLayerObjects1 objectAtIndex:i];
[layer0.layer addAnimation:animateLayer1 forKey:nil];
}
for (int i = 0; i < self.accLayerPoints2.count; i++) {
NSValue *val = [self.accLayerPoints2 objectAtIndex:i];
CGPoint origin = [val CGPointValue];
float x = origin.x + (acceleration.y * ACC_LAYER2_THRESHOLD);
[animateLayer2 setToValue:[NSValue valueWithCGPoint:CGPointMake(x, origin.y)]];
UIImageView *layer0 = [self.accLayerObjects2 objectAtIndex:i];
[layer0.layer addAnimation:animateLayer2 forKey:nil];
}
}
}
My problem is after a while the ipad starts having performance issues, starts lagging.
I've used the Allocations tool to verify that it is the code in the accelerometer function that's causing the issue.
Is there a way to release the objects thats not in use anymore or clean the code?
I'm using ARC so I'm not sure how the cleaning works.

Answer 1

I don't think allocations are your problem. I'll bet if you use the time profiler, you'll see you're blocking the main thread with all that accelerometer activity. You are doing all this in a number of very tight loops for every single accelerometer event, which is probably WAY more than you realize.
Consider putting the position computations in a background queue and updating the positions of the UI objects on the main thread (this part is required, since it's generally not safe to update UI from a background queue). Do this by dispatching the actual UI update commands to the main queue. Maybe create a funnel point (-updateUIWithDictionaryContainingComputedPositionsForEachLayer:, called on the main queue with your background-computed values).
It's doubtful also that you need every single accelerometer event. Throttle the processing (Grand Central Dispatch can help there too, or a simple countdown-restart-if-new-event NSTimer) so not every single event is processed. You'll need to experiment to find the perceptual 'sweet spot' as a balance between smooth-looking animation and efficient responsiveness, but I'll bet the number of updates needed for a smooth visual effect is MUCH LOWER than the number of accelerometer events you're processing.