iOS：动画在observeValueForKeyPath中不起作用

Question

I used KVO to observe changes in a frame and set another frame accordingly

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{
    if ([keyPath isEqualToString:KeyPathForFrameBeingObserved]) {
         [UIView animateWithDuration:1 animations:^{
            CGRect frame = [self playWithFrame:viewBeingMonitored.frame];
            self.viewToAnimate.frame = frame;
         }];
    }
}

Code in the animation block is executed but animation is not working, I suspected that repeated calls to animation method could cause this but using log messages I found that animation is not working even for a single call, can anyone explain that ?

iPad 2/(iOS8.4)

• I have tried including the animation method call in dispatch_async(dispatch_get_main_queue(), ^{ ... }); but animation still not working.
• I have tried pushing the observed changes in queue and delaying the animation method for a second using dispatch_after to run it only using the last element in the queue to avoid repeated calls but didn't work.
• KVO is working perfectly (adding the observer is already handled), but the code in the animation block is executed without animation.

Answer 1

It seems like you're trying to observe the property of an UIKit object. Keep in mind that:

... the classes of the UIKit framework generally do not support KVO ...

Source: https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/KVO.html

So, you have to check if your observeValueForKeyPath:ofObject:change:context: is calling in fact.

Answer 2

This is an older question but I thought I would answer in case someone else is reading this. You will need to call setFrame to set the frame of the animation instead of the simple assignment.

CGRect frame = [self playWithFrame:viewBeingMonitored.frame]; [self.viewToAnimate setFrame:frame];