Ios 如何进行键值观察并在 UIView 的框架上获取 KVO 回调

通常存在不支持KVO的通知或其他可观察到的事件.尽管文件上说是'no'，但从表面上看，CALayer支持UIView是安全的.观察CALayer在实践中是有效的，因为它广泛使用了KVO和适当的访问器(而不是ivar操作).这并不能保证它会在future 发挥作用.

无论如何，视图的框架只是其他属性的产物.因此，我们需要注意以下几点:

[self.view addObserver:self forKeyPath:@"frame" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"bounds" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"transform" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"position" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"zPosition" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"anchorPoint" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"anchorPointZ" options:0 context:NULL];
[self.view.layer addObserver:self forKeyPath:@"frame" options:0 context:NULL];

请参阅此处的完整示例 https://gist.github.com/hfossli/7234623个

NOTE: This is not said to be supported in the docs, but it works as of today with all iOS versions this far (currently iOS 2 -> iOS 11)

注意:请注意，在最终值结算之前，您将收到多个回调.例如，更改视图或图层的边框将导致图层更改position和bounds(按该顺序).

使用ReactiveCocoa，您可以

RACSignal *signal = [RACSignal merge:@[
  RACObserve(view, frame),
  RACObserve(view, layer.bounds),
  RACObserve(view, layer.transform),
  RACObserve(view, layer.position),
  RACObserve(view, layer.zPosition),
  RACObserve(view, layer.anchorPoint),
  RACObserve(view, layer.anchorPointZ),
  RACObserve(view, layer.frame),
  ]];

[signal subscribeNext:^(id x) {
    NSLog(@"View probably changed its geometry");
}];

如果你只想知道什么时候能做bounds次改变

@weakify(view);
RACSignal *boundsChanged = [[signal map:^id(id value) {
    @strongify(view);
    return [NSValue valueWithCGRect:view.bounds];
}] distinctUntilChanged];

[boundsChanged subscribeNext:^(id ignore) {
    NSLog(@"View bounds changed its geometry");
}];

如果你只想知道什么时候能做frame次改变

@weakify(view);
RACSignal *frameChanged = [[signal map:^id(id value) {
    @strongify(view);
    return [NSValue valueWithCGRect:view.frame];
}] distinctUntilChanged];

[frameChanged subscribeNext:^(id ignore) {
    NSLog(@"View frame changed its geometry");
}];