import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate{
    
  var bgTask: UIBackgroundTaskIdentifier!
    
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }

    
    //app进入后台
    override func applicationDidEnterBackground(_ application: UIApplication){
        //开启后台任务保活
        comeToBackgroundMode()
    }
   
    func comeToBackgroundMode() {
        //初始化一个后台任务BackgroundTask，这个后台任务的作用就是告诉系统当前app在后台有任务处理，需要时间
        let app = UIApplication.shared
        bgTask = app.beginBackgroundTask(expirationHandler: { [self] in
            app.endBackgroundTask(bgTask)
            bgTask = UIBackgroundTaskIdentifier.invalid
        })
        //开启定时器 不断向系统请求后台任务执行的时间
        let timer = Timer.scheduledTimer(timeInterval: 30.0, target: self, selector: #selector(applyForMoreTime), userInfo: nil, repeats: true)
        timer.fire()
    }

    @objc func applyForMoreTime() {
        //如果系统给的剩余时间小于60秒 就终止当前的后台任务，再重新初始化一个后台任务，重新让系统分配时间，这样一直循环下去，保持APP在后台一直处于active状态。
        if UIApplication.shared.backgroundTimeRemaining < 60 {
            UIApplication.shared.endBackgroundTask(bgTask)
            bgTask = UIApplication.shared.beginBackgroundTask(expirationHandler: { [self] in
                UIApplication.shared.endBackgroundTask(bgTask)
                bgTask = UIBackgroundTaskIdentifier.invalid
            })
        }
    }
    
}