Question

我有一个viewController，我使用的是定位点，我想获取按钮的框架。我只在按下视图时才需要框架，所以我使用isMovingToParentViewController。像这样：

if isMovingToParentViewController {

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

当我将其添加到viewWillAppear时，用于获取按钮框架的代码将运行，但是当我将相同的代码添加至viewDidLayoutSubviews时，它将无法运行。

为什么？

只需在viewDidLayoutSubviews中添加断点，它确实会命中if isMovingToParentViewController，但其中的代码内部永远不会命中。

在视图上按下isMovingToParentViewController 做时会被命中，在弹出isMovingToParentViewController 不会命中时会被命中。

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

标题文档指出isMovingToParentViewController和其他3种方法仅在外观回调如viewWillAppear中有效，因此在布局回调如viewDidLayoutSubviews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

为什么（isMovingToParentViewController）中的代码未在viewDidLayoutSubviews（）中运行

1 个答案: