How to reliably block duplicate animation calls in Objective-C? - ios

Question

A user can initiate an animation with a swipe gesture. I want to block duplicate calls to the animation, to make sure that once the animation has started, it cannot be initiated again until it has completed -- which may happen if the user accidentally swipes multiple times.
I imagine that most people achieve this control using a boolean flag (BOOL isAnimatingFlag) in the manner shown at bottom. I've done things like this before in apps many times -- but I never feel 100% certain as to whether my flag is guaranteed to have the value I intend, since the animation uses blocks and it's unclear to me what thread my animation completion block is being run on.
Is this way (of blocking duplicate animations) reliable for multi-thread execution?
/* 'atomic' doesn't
* guarantee thread safety
* I've set up my flag as follows:
* Does this look correct for the intended usage?
*/
#property (assign, nonatomic) BOOL IsAnimatingFlag;
//…
#synthesize IsAnimatingFlag
//…
-(void)startTheAnimation{
// (1) return if IsAnimatingFlag is true
if(self.IsAnimatingFlag == YES)return;
/* (2) set IsAnimatingFlag to true
* my intention is to prevent duplicate animations
* which may be caused by an unwanted double-tap
*/
self.etiIsAnimating = YES;
// (3) start a new animation
[UIView animateWithDuration:0.75 delay:0.0 options:nil animations:^{
// animations would happen here...
} completion:^(BOOL finished) {
// (4) reset the flag to enable further animations
self.IsAnimatingFlag = NO;
}];
}

Answer 1

Disable the gesture if you don't want the user triggering it multiple times
- (void)startTheAnimation:(id)sender
{
[sender setEnabled:NO];
[UIView animateWithDuration:0.75 delay:0.0 options:nil animations:^{
// animations would happen here...
} completion:^(BOOL finished) {
[sender setEnabled:YES];
}];
}
Update
Gestures also have an enabled property so you could use the same idea as if it were a button and change it' enabled state

Answer 2

Animation completion block will always run on the main thread.
In the example in the UIView Class Reference you can see that [view removeFromSuperview] is called directly from the block. That's mean a completion block runs on the main thread as it's the only thread safe to call UI-releated methods.
So you are all good if you calling startTheAnimation only from the main thread. If you not you need to dispatch it on the main thread anyway because you call UI-releated methods in it.
If you need to call startTheAnimation from other threads than main thread you can do something like this:
-(void)startTheAnimation{
dispatch_async(dispatch_get_main_queue(), ^{
// Your code here
});
}
Of course, it's better from user experience point of view to, for example, disable a button or modify the UI in other ways to indicate that an animation is in progress. However, it's all the same code. Whatever you need to do you first need to disable it before an animation starts and the re-enable after it's finished.

Answer 3

Where you call this method, you could try using dispatch_once GCD function:
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
[myThingy startTheAnimation];
});