如何确定 UIViewController 是第一次启动？

Question

我有一个 ViewControllers 层次结构，以 UINavigationViewController 作为根。
如何确定某个 ViewController 是第一次启动还是由于导航堆栈展开而启动？

Answer 1

假设您想知道viewWillAppear: （或viewDidAppear: ）是否在视图控制器第一次显示时被调用，或者是否因为另一个视图控制器已被解除而被显示，您可以轻松地执行以下操作：

较新的 Swift 版本：

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

    if isBeingPresented || isMovingToParent {
        // This is the first time this instance of the view controller will appear
    } else {
        // This controller is appearing because another was just dismissed
    }
}

较旧的 Swift 版本：

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

    if isBeingPresented() || isMovingToParentViewController() {
        // This is the first time this instance of the view controller will appear
    } else {
        // This controller is appearing because another was just dismissed
    }
}

Answer 2

当您在导航堆栈上推送新的视图控制器时，它是第一次实例化，当您从堆栈中弹出它时，它会被释放或释放。 因此，当您推动或前进时，这是第一次，但是当您从前一个返回到任何视图控制器时，当前 vc 已经在内存中，而不是第一次！！！

Answer 3

在 Objective-C 上，它看起来像：

-(void) viewWillAppear:(BOOL)animated {

    [super viewWillAppear:animated];

    if ([self isBeingPresented] || [self isMovingToParentViewController]) {
        // This is the first time
    } else {
        // This is the NOT first time
    }
}

Answer 4

isBeingPresented 和 isMovingToParent 是一个棘手的问题。

我的方法是创建一个计数器并在 viewWillAppear 中增加它。

var viewWillAppearCounter = 0

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

    if viewWillAppearCounter == 0 {
        print("viewWillAppear will be executed for the first time")
    } else {
        print("viewWillAppear was already executed \(viewWillAppearCounter) times")
    }
    viewWillAppearCounter += 1
}