• iOS应用程序一共有五种状态：

  • Not Running 程序还没运行
  • Inactive 程序运行在foreground但没有接收事件
  • Active 程序运行在foreground接收事件
  • Background 程序运行在background正在执行代码
  • Suspended 程序运行在background没有执行代码

  • iOS应用程序状态变化会回调APPDelegate中的方法，但不是每一种状态变化都会有对应的方法(上图的红框的两个变化就没有对应的方法)

    • application:didFinishLaunchingWithOptions: Not Running -> Inactive
    • applicationDidBecomeActive: Inactive -> Active
    • applicationWillResignActive: Active -> Inactive
    • applicationDidEnterBackground: Background -> Suspended
    • applicationWillEnterForeground: Background -> Inactive
    • applicationWillTerminate: Suspended -> Not Running

  • 常见的应用状态变化场景

    • 程序第一次启动（或者被杀掉以后启动）：
      Not Running -> Inactive -> Active
    • 点击Home键（没有在Inof.plist中设置Application does not run in background）：
      Active -> Inactive -> Background -> Suspended
    • 点击Home键（在Inof.plist中设置Application does not run in background为YES，应用不能运行在后台，进入后台后会立即进入Not Running）：
      Active -> Inactive -> Background -> Suspended -> Not Running
    • 挂起重新运行
      Suspended -> Background -> Inactive -> Active
    • 内存清除（杀掉应用或删除应用）
      Suspended -> Not Running
    • 应用之间的切换 Active -> Inactive Inactive -> Active
    • 点击Home键（在Inof.plist中设置Application does not run in background为YES，应用不能运行在后台，进入后台后会立即进入Not Running）：
      Active -> Inactive -> Background -> Suspended -> Not Running

  • 可通过在APPDelegate的回调方法中打印数据，来查看应用状态变化

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    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        print("`application:didFinishLaunchingWithOptions:`  Not Running -> Inactive")
        return true
    }

    func applicationWillResignActive(_ application: UIApplication) {

        print("`applicationWillResignActive:`  Active -> Inactive")
    }

    func applicationDidEnterBackground(_ application: UIApplication) {
        
        print("`applicationDidEnterBackground:`  Background -> Suspended")
    }

    func applicationWillEnterForeground(_ application: UIApplication) {
        
        print("`applicationWillEnterForeground:` Background -> Inactive")
    }

    func applicationDidBecomeActive(_ application: UIApplication) {

        print("`applicationDidBecomeActive:`  Inactive -> Active")
    }

    func applicationWillTerminate(_ application: UIApplication) {

        print("`applicationWillTerminate:`  Suspended -> Not Running")
    }