Run on an iOS device, and watch console for the message "MOTION UPDATE AFTER stopDeviceMotionUpdates WAS CALLED". This indicates that a callback block for startDeviceMotionUpdatesToQueue:withHandler: was called after stopDeviceMotionUpdates was called.
###UIGravityBehavior's UIDynamicBehavior's dangling pointer
The dynamicAnimator property on all UIDynamicBehavior objects is unsafe. Any behavior that outlives it's animator is a crash waiting to happen.
The following will always crash, with a behavior of any type.
UIView *item = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
[self.view addSubview:item];
UIDynamicAnimator *animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
UIGravityBehavior *behavior = [[UIGravityBehavior alloc] initWithItems:@[item]];
[animator addBehavior:behavior];
double delayInSeconds = 0.4;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
NSTimeInterval time = behavior.dynamicAnimator.elapsedTime;
});
To see how easy it is to cause issues when combined w/ motion updates with block callbacks, you can try the following:
In SMLViewController.m change CAPTURE_GRAVITY to YES, then run on a device and wait. The app will eventually crash at SMLViewController:86 when trying to set the angle on the gravity behavior. The block is called under the same circumstances as above - the callback takes place after stopDeviceMotionUpdates is called. In this case, the view's UIDynamicAnimator has been deallocated, but the gravity behavior was captured by the block.