Question

我有一个ViewControllers层次结构，以UINavigationViewController为根我怎样才能知道某些ViewController是第一次启动还是由于导航堆栈的展开而启动了？

Answer 1

假设您想知道在首次显示视图控制器时是否正在调用viewWillAppear:（或viewDidAppear:），或者由于另一个视图控制器已被关闭而显示它是否正在显示，您可以轻松地执行以下操作：

更新的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

当你在导航堆栈上推送新的viewcontroller时，它首次实例化，当你从堆栈中弹出它时，它会被释放或释放。所以当你推或前进时它是第一次，但是当你回到之前的任何一个视图控制器时，当前的vc已经在内存而不是第一次!!!

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
}

如何确定第一次启动UIViewController？

4 个答案: