ios - 为什么当应用程序从后台返回时不会调用 viewWillAppear？

Question

我正在编写一个应用程序，如果用户在打电话时正在查看应用程序，我需要更改 View 。

我已经实现了以下方法:

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

但是当应用返回前台时它不会被调用。

我知道我可以实现:

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

但我不想这样做。我更愿意将我所有的布局信息放在 viewWillAppear: 方法中，让它处理所有可能的场景。

我什至尝试从 applicationWillEnterForeground: 调用 viewWillAppear:，但我似乎无法确定此时哪个是当前 View Controller 。

有人知道处理这个问题的正确方法吗？我确定我缺少一个明显的解决方案。

Answer 1

swift

简答

使用 NotificationCenter 观察者而不是 viewWillAppear。

override func viewDidLoad() {
    super.viewDidLoad()

    // set observer for UIApplication.willEnterForegroundNotification
    NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)

}

// my selector that was defined above
@objc func willEnterForeground() {
    // do stuff
}

---

长答案

要了解应用何时从后台返回，请使用 NotificationCenter 观察者而不是 viewWillAppear。这是一个示例项目，显示了哪些事件何时发生。 (这是对 this Objective-C answer 的改编。)

import UIKit
class ViewController: UIViewController {

    // MARK: - Overrides

    override func viewDidLoad() {
        super.viewDidLoad()
        print("view did load")

        // add notification observers
        NotificationCenter.default.addObserver(self, selector: #selector(didBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)

    }

    override func viewWillAppear(_ animated: Bool) {
        print("view will appear")
    }

    override func viewDidAppear(_ animated: Bool) {
        print("view did appear")
    }

    // MARK: - Notification oberserver methods

    @objc func didBecomeActive() {
        print("did become active")
    }

    @objc func willEnterForeground() {
        print("will enter foreground")
    }

}

第一次启动应用时，输出顺序为:

view did load
view will appear
did become active
view did appear

按下home键后，再将app拉回前台，输出顺序为:

will enter foreground
did become active 

因此，如果您最初尝试使用 viewWillAppear，那么 UIApplication.willEnterForegroundNotification 可能就是您想要的。

注意

从 iOS 9 及更高版本开始，您无需移除观察者。 documentation状态:

If your app targets iOS 9.0 and later or macOS 10.11 and later, you don't need to unregister an observer in its dealloc method.