ios - 为什么 (isMovingToParentViewController) 中的代码不在 viewDidLayoutSubviews() 中运行

Question

我有一个 viewController，我正在使用 anchor ，我想获取按钮的框架。我只需要 View 被推送时的框架，因此我使用 isMovingToParentViewController 。像这样:

if isMovingToParentViewController {

    let myButtonFrame = myButton.convert(myButton.bounds, to: self.view)
}

当我将其添加到 viewWillAppear 中时，获取按钮框架的代码会运行，但是当我将相同的代码添加到 viewDidLayoutSubviews 中时，它不会运行。

这是为什么？

要清楚的是，当我在viewDidLayoutSubviews中添加断点时，它确实会命中if isMovingToParentViewController，但其中的代码内部永远不会被命中。

当 View 在 isMovingToParentViewController 上推送时会被击中，当它弹出 isMovingToParentViewController 不会被击中。

override func viewDidLoad() {
    super.viewDidLoad()

    // anchors are set here but the frames haven't been set yet
}

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    if isMovingToParentViewController {

        // this code runs
        let myButtonFrame = myButton.convert(myButton.bounds, to: self.view)
    }
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    if isMovingToParentViewController {

        // this code NEVER runs
        let myButtonFrame = myButton.convert(myButton.bounds, to: self.view)
    }
}

Answer 1

header 文档指出 isMovingToParentViewController 和其他 3 个方法仅在 viewWillAppear 等外观回调中有效，因此在 viewDidLayoutSubviews 等布局回调中无效.

/*
  These four methods can be used in a view controller's appearance callbacks to determine if it is being
  presented, dismissed, or added or removed as a child view controller. For example, a view controller can
  check if it is disappearing because it was dismissed or popped by asking itself in its viewWillDisappear:
  method by checking the expression ([self isBeingDismissed] || [self isMovingFromParentViewController]).
*/

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly, getter=isBeingPresented) BOOL beingPresented NS_AVAILABLE_IOS(5_0);
@property(nonatomic, readonly, getter=isBeingDismissed) BOOL beingDismissed NS_AVAILABLE_IOS(5_0);

@property(nonatomic, readonly, getter=isMovingToParentViewController) BOOL movingToParentViewController NS_AVAILABLE_IOS(5_0);
@property(nonatomic, readonly, getter=isMovingFromParentViewController) BOOL movingFromParentViewController NS_AVAILABLE_IOS(5_0);
#else
- (BOOL)isBeingPresented NS_AVAILABLE_IOS(5_0);
- (BOOL)isBeingDismissed NS_AVAILABLE_IOS(5_0);

- (BOOL)isMovingToParentViewController NS_AVAILABLE_IOS(5_0);
- (BOOL)isMovingFromParentViewController NS_AVAILABLE_IOS(5_0);
#endif