Stopping GCD when replacing scene? - ios

Question

I know its impossible to stop a GCD action that started, but i wonder what happens when it is trying to execute after i have already replace a view, means that i have a few GCD functions that loading images to scrollView, and sometimes the user wants to replace the scene while they are working, how should i stop it than ? or shouldn't i ?
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^
{
NSData *imgdata = [NSData dataWithContentsOfURL:url];
UIImage *image=[UIImage imageWithData:imgdata scale:1];
dispatch_async(dispatch_get_main_queue(), ^
{
[UIView transitionWithView:backView
duration:0.55f
options:UIViewAnimationOptionTransitionCrossDissolve
animations:^
{
backView.image=image;
}
completion:nil];
});
});
}

Answer 1

You could switch to NSOperation to get the more precise cancel mechanism. But it cannot guarantee that it will cancel if the operation is already in progress. As an option you can check if your backView still has superview.
If you removed a view from superview no animation work occurred but every step of transition lifecycle happens:
- (void)testUIViewOffScreen
{
UIWindow *aWindow = [[UIApplication sharedApplication] windows][0];
UIView *aView = [[UIView alloc] initWithFrame:CGRectMake(10.0, 10.0, 10.0, 10.0)];
aView.backgroundColor = [UIColor redColor];
[aWindow addSubview:aView];
[aWindow layoutIfNeeded];
dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[UIView transitionWithView:aView duration:1.0 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
NSLog(#"Transition will start now");
aView.frame = CGRectMake(100.0, 100.0, 100.0, 100.0);
} completion:^(BOOL finished) {
NSLog(#"finished: %#", finished ? #"YES" : #"NO");
}];
});
});
[[NSRunLoop mainRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1.0]];
[aView removeFromSuperview];
[[NSRunLoop mainRunLoop] runUntilDate:[NSDate distantFuture]];
}
The output:
23.941 TestCase[70489:60b] Transition will start now
23.942 TestCase[70489:60b] finished: NO