iOS 前台观察器未被调用

Question

我已经为后台和前台事件添加了一个观察者。

当双击主页按钮时，后台观察器被调用。但是当我立即选择我的应用程序时，我的前台观察器没有被调用。请检查此 gif 演示 https://raw.githubusercontent.com/billionlaughs/Demo/master/resources/foreground_issue.gif .

测试的 iOS 版本:13.1.2。

代码片段:

class ViewController: UIViewController {

    @IBOutlet weak var uiLabelToShow: UILabel!
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        //Will be invoked when app goes to background
        NotificationCenter.default.addObserver(self, selector: #selector(myBGObserver), name: .UIApplicationWillResignActive, object: nil)
        //Will be invoked when app goes to foreground
        NotificationCenter.default.addObserver(self, selector: #selector(myFGObserver), name: .UIApplicationWillEnterForeground, object: nil)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @objc func myBGObserver(){
        uiLabelToShow.text = "Background";
    }
    @objc func myFGObserver(){
        uiLabelToShow.text = "Foreground";
    }
}

我的演示项目的 Github 链接:https://github.com/billionlaughs/Demo

我找不到解决此问题的方法/原因。请引导我走上正确的道路。

Answer 1

UIApplicationWillEnterForeground 当您的应用程序在您单击主页按钮两次后隐藏某些方式时调用(不会保持显示为您的 gif)，您需要使用 UIApplicationDidBecomeActive的 UIApplicationWillEnterForeground

drawBack : initially text will contain Foreground

解决做

class ViewController: UIViewController {

    @IBOutlet weak var uiLabelToShow: UILabel!
    var once = true 
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        //Will be invoked when app goes to background
        NotificationCenter.default.addObserver(self, selector: #selector(myBGObserver), name: .UIApplicationWillResignActive, object: nil)
        //Will be invoked when app goes to foreground
        NotificationCenter.default.addObserver(self, selector: #selector(myFGObserver), name: .UIApplicationDidBecomeActive, object: nil)

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @objc func myBGObserver(){
        uiLabelToShow.text = "Background";
    }
    @objc func myFGObserver(){
        uiLabelToShow.text = once ? "Normal" : "Foreground";
        once = false
    }
}