为什么当应用程序从后台返回时不会调用 viewWillAppear?

Why does viewWillAppear not get called when an app comes back from the background?(为什么当应用程序从后台返回时不会调用 viewWillAppear?)

问题描述

I'm writing an app and I need to change the view if the user is looking at the app while talking on the phone.

I've implemented the following method:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"viewWillAppear:");
    _sv.frame = CGRectMake(0.0, 0.0, 320.0, self.view.bounds.size.height);
}

But it's not being called when the app returns to the foreground.

I know that I can implement:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(statusBarFrameChanged:) name:UIApplicationDidChangeStatusBarFrameNotification object:nil];

but I don't want to do this. I'd much rather put all my layout information in the viewWillAppear: method, and let that handle all possible scenarios.

I've even tried to call viewWillAppear: from applicationWillEnterForeground:, but I can't seem to pinpoint which is the current view controller at that point.

Does anybody know the proper way to deal with this? I'm sure I'm missing an obvious solution.

The method viewWillAppear should be taken in the context of what is going on in your own application, and not in the context of your application being placed in the foreground when you switch back to it from another app.

In other words, if someone looks at another application or takes a phone call, then switches back to your app which was earlier on backgrounded, your UIViewController which was already visible when you left your app 'doesn't care' so to speak -- as far as it is concerned, it's never disappeared and it's still visible -- and so viewWillAppear isn't called.

I recommend against calling the viewWillAppear yourself -- it has a specific meaning which you shouldn't subvert! A refactoring you can do to achieve the same effect might be as follows:

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self doMyLayoutStuff:self];
}

- (void)doMyLayoutStuff:(id)sender {
    // stuff
}

Then also you trigger doMyLayoutStuff from the appropriate notification:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doMyLayoutStuff:) name:UIApplicationDidChangeStatusBarFrameNotification object:self];

There's no out of the box way to tell which is the 'current' UIViewController by the way. But you can find ways around that, e.g. there are delegate methods of UINavigationController for finding out when a UIViewController is presented therein. You could use such a thing to track the latest UIViewController which has been presented.

Update

If you layout out UIs with the appropriate autoresizing masks on the various bits, sometimes you don't even need to deal with the 'manual' laying out of your UI - it just gets dealt with...

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