Requirements: some time ago, when developing a project, there was a requirement. Because some information of the APP is sensitive, if the user presses the home key to return to the background during the operation of the APP, after the specified time, and then switches to the foreground, it is necessary to verify the fingerprint or gesture set by the user to authenticate the user

Scheme I:

When the APP enters the front desk, it will   rootViewController   Set as the specified verification interface

But there is a problem: when the user authentication is successful? How to return to the user's previous interface? I don't have a good solution for the time being. If you think of a good idea, we can discuss it together~

Scheme II:

When the APP enters the foreground, it overwrites a UIWindow on the current interface and sets UIWindowLevel. After the user's authentication is successful, it can hide or remove the Window object

The code is as follows:

    func applicationDidEnterBackground(_ application: UIApplication) {
        leaveTime = Date()
    }
    func applicationWillEnterForeground(_ application: UIApplication) {
        let interval = Int(Date().timeIntervalSince(leaveTime))
        
        //Here, if it takes more than 20 seconds, let the user verify it
        if interval > 20
        {
            let validateVC = ValidationVC()
            
            //After the user authentication is successful, you can reset the next windowslevel;
            validateVC.closeCallBack = {
                self.gestureWindow?.windowLevel = -1
            }
            let window = UIWindow(frame: UIScreen.main.bounds)
            window.windowLevel = UIWindowLevelAlert + 1
            window.rootViewController = validateVC
            window.makeKeyAndVisible()
            self.gestureWindow = window
        }
    }
    

UIWindowLevel

about   UIWindowLevel   I want to say a few more words,   UIWindow   It will be displayed according to   UIWindowLevel   To sort, i.e   Level   The high one will be at the top. The default is 0

open var windowLevel: UIWindowLevel // default = 0.0

The system defines three window levels for us, namely:

public let UIWindowLevelNormal: UIWindowLevel
public let UIWindowLevelAlert: UIWindowLevel
public let UIWindowLevelStatusBar: UIWindowLevel

Let's print out the values of these three level s to see the following:

STWLog("UIWindowLevelNormal: \(UIWindowLevelNormal)")
STWLog("UIWindowLevelStatusBar: \(UIWindowLevelStatusBar)")
STWLog("UIWindowLevelAlert: \(UIWindowLevelAlert)")

Print results:

AppDelegate.swift:(43)----------UIWindowLevelNormal: 0.0
AppDelegate.swift:(44)----------UIWindowLevelStatusBar: 1000.0
AppDelegate.swift:(45)----------UIWindowLevelAlert: 2000.0

So we can see their level at a glance;

Let's write a small Demo to test:

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    var normalWindow: UIWindow!
    var alertLevelWindow: UIWindow!
    var statusLevelWindow: UIWindow!
    var alertLevelWindow2: UIWindow!
   
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        window = UIWindow(frame:  CGRect(x: 0, y: 0, width: 100, height: 100))
        window?.tag = 10
        window?.backgroundColor = .red
        window?.rootViewController = ViewController()
        window?.makeKeyAndVisible()
        
        //Customize the Window and set the windowLevel to UIWindowLevelNormal
        normalWindow = UIWindow(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
        normalWindow?.tag = 100
        normalWindow.backgroundColor = .green
        normalWindow.windowLevel = UIWindowLevelNormal
        normalWindow.rootViewController = ViewController()
        normalWindow.makeKeyAndVisible()
        
       
        
        //Customize the Window and set the windowLevel to UIWindowLevelNormal
        alertLevelWindow = UIWindow(frame: CGRect(x: 150, y: 150, width: 100, height: 100))
        alertLevelWindow?.tag = 10000
        alertLevelWindow.backgroundColor = .blue
        alertLevelWindow.windowLevel = UIWindowLevelAlert
        alertLevelWindow.rootViewController = ViewController()
        alertLevelWindow.makeKeyAndVisible()
        
        
        //Customize the Window and set the windowLevel to UIWindowLevelStatusBar
        statusLevelWindow = UIWindow(frame: CGRect(x: 100, y: 100, width: 100, height: 100))
        statusLevelWindow?.tag = 1000
        statusLevelWindow.backgroundColor = .yellow
        statusLevelWindow.windowLevel = UIWindowLevelStatusBar
        statusLevelWindow.rootViewController = ViewController()
        statusLevelWindow.makeKeyAndVisible()
        
        //Customize the Window and set the windowLevel to UIWindowLevelStatusBar
        alertLevelWindow2 = UIWindow(frame: CGRect(x: 200, y: 200, width: 100, height: 100))
        alertLevelWindow2?.tag = 100000
        alertLevelWindow2.backgroundColor = .white
        alertLevelWindow2.windowLevel = UIWindowLevelAlert + 10 //Set a Level at will
        alertLevelWindow2.rootViewController = ViewController()
        alertLevelWindow2.makeKeyAndVisible()
        
        print("current keyWindow: \(String(describing: UIApplication.shared.keyWindow!.tag))")
        
       return true
    }
    
    func applicationWillEnterForeground(_ application: UIApplication) {
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 2.0, execute: {
            self.alertLevelWindow2 = nil
        })
        
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 4.0, execute: {
            print("here keyWindow: \(String(describing: UIApplication.shared.keyWindow!.tag))")
            
            for window in UIApplication.shared.windows
            {
                print("current window of tag Value is:\(window.tag)")
            }
        })
    }
}

The operation results are shown in the figure below:

Print result display:

current keyWindow: 100000
 here keyWindow: 10000
 current window of tag Value is:0
 current window of tag Value is:10
 current window of tag Value is:100
 current window of tag Value is:1000
 current window of tag Value is:10000

What is?   keWindow  ? Let's see how the official documents describe:

That is to say   windows   Array, the most recent time called   makeKeyAndVisible   The way is   keyWindow;

Summary:

1. UIWindowLevel   The value of is not only   UIWindowLevelNormal  ,  UIWindowLevelAlert,   UIWindowLevelStatusBar   These three can be user-defined arbitrary values, even negative numbers
2. UIWindow   The display of can indeed be passed   UIWindowLevel   To distinguish priorities, all windows will be added to the interface, but will be listed by priority, uiwindowlevel   The large one is displayed above, uiwindowlevel   The small ones are shown below.
3. UIWindowLevel   When the priorities are equal, it depends on who instantiates later, who instantiates later, and who displays first
4. If the current   Keywindow object set to   nil   The object is removed from the   Removed from the Windows array, and the last instantiated Window object will become   KeyWindow  , But still follow the description in summary 2, UIWindowLevel   The large one is displayed above, UIWindowLevel   The small ones are shown below.