iOS NSNotificationCenter 奇怪的延迟

Question

在我的应用程序中，我需要检测 ImagePicker 是否准备好拍照。我在这里找到了解决方案:How to know if iPhone camera is ready to take picture?

所以我在 viewDidLoad:

中得到了这段代码

[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(showButton:)
name:AVCaptureSessionDidStartRunningNotification object:nil];

选择器看起来像这样:

- (void)showButton:(NSNotification *)notification{

    NSLog(@"---CAMERA READY");

    [button setHidden:NO];
    [button setAlpha:0.0];
    [button setTransform:CGAffineTransformScale(CGAffineTransformIdentity, 1.5, 1.5)];

    [UIView animateWithDuration:.2 delay:0.0 options:UIViewAnimationOptionCurveEaseOut animations:^{
        [button setAlpha:1.0];
        [button setTransform:CGAffineTransformScale(CGAffineTransformIdentity, 1.0, 1.0)];
    } completion:^(BOOL finished) {

    }];

    NSLog(@"---CAMERA READY");

}

这里发生了一些奇怪的事情，因为两个 NSLog 都会立即显示，但整个按钮动画甚至会在 30 秒后触发。

UIView 是否单独更新？如何同步日志和动画？

Answer 1

打算将其放入答案框中以供将来的用户查看..

您遇到的情况使您的通知看起来没有在主线程上被调用，并且在后台线程上更新 UI 是未定义的行为。解决这个问题的方法很简单，只需将方法的内容包装在 dispatch_async() 调用中并将所有内容发送回主队列即可。

- (void)showButton:(NSNotification *)notification {

    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"---CAMERA READY");

        [button setHidden:NO];
        [button setAlpha:0.0];
        [button setTransform:CGAffineTransformScale(CGAffineTransformIdentity, 1.5, 1.5)];

        [UIView animateWithDuration:.2 delay:0.0 options:UIViewAnimationOptionCurveEaseOut animations:^{
            [button setAlpha:1.0];
            [button setTransform:CGAffineTransformScale(CGAffineTransformIdentity, 1.0, 1.0)];
        } completion:nil];

        NSLog(@"---CAMERA READY");
    });
}

作为替代方案，您可以只使用不同的 NSNotificationCenter 实例方法。具体来说，-addObserverForName:object:queue:usingBlock: 允许您指定应在哪个操作队列上执行回调 block 。在这种情况下，您需要传递 [NSOperationQueue mainQueue];。

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html#//apple_ref/occ/instm/NSNotificationCenter/addObserverForName:object:queue:usingBlock :

NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
self.localeChangeObserver = [center addObserverForName:NSCurrentLocaleDidChangeNotification object:nil
    queue:mainQueue usingBlock:^(NSNotification *note) {

        NSLog(@"The user's locale changed to: %@", [[NSLocale currentLocale] localeIdentifier]);
    }];