c# - Xamarin 表单后台任务仅在应用程序在 ios 上打开时运行

Question

1. 预期行为
   • ios
     • 当应用程序最小化并且每 5 秒收到一次通知时。
   • 安卓
     • 当应用程序最小化并且每 5 秒收到一次通知时。
2. 实际行为
   • ios
     • 应用程序最小化并且没有通知进来，打开应用程序会导致每 5 秒收到一次通知。
   • 安卓
     • 当应用程序最小化并且每 5 秒收到一次通知时。

       //from app.xaml.cs
       protected override void OnSleep()
           {
               void ScheduleNotification()
               {
                   // Start a timer that runs after 5 seconds.
                   Device.StartTimer(TimeSpan.FromSeconds(5), () =>
                   {
                       System.Threading.Tasks.Task.Factory.StartNew( () =>
                       {
                           // Do the actual request and wait for it to finish.
                            PerformNotification();
                           // Switch back to the UI thread to update the UI
                           Device.BeginInvokeOnMainThread(() =>
                           {
                               // Update the UI
                               // ...
                               // Now repeat by scheduling a new request
                               ScheduleNotification();
                               count++;
                           });
                       }, CancellationToken.None, TaskCreationOptions.DenyChildAttach, TaskScheduler.Default);
       
                       // Don't repeat the timer (we will start a new timer when the request is finished)
                       return false;
                   });
               }
              void PerformNotification()
               {
                   CrossLocalNotifications.Current.Show("title", "body"+ count.ToString());
       
               }
               ScheduleNotification();
           }
       

Answer 1

在 iOS 上，不要求在后台运行的应用程序在后台运行的时间不能超过 10 秒。为了使某些代码在后台运行，您可以在 iOS 和 android 上实现 DependencyService。您可以将以下代码放入 iOS 上的该服务中:

public async Task RunCodeInBackgroundMode(Func<Task> action, string name = "MyBackgroundTaskName")
    {
        nint taskId = 0;
        var taskEnded = false;
        taskId = UIApplication.SharedApplication.BeginBackgroundTask(name, () =>
        {
            //when time is up and task has not finished, call this method to finish the task to prevent the app from being terminated
            Console.WriteLine($"Background task '{name}' got killed");
            taskEnded = true;
            UIApplication.SharedApplication.EndBackgroundTask(taskId);
        });
        await Task.Factory.StartNew(async () =>
        {
            //here we run the actual task
            Console.WriteLine($"Background task '{name}' started");
            await action();
            taskEnded = true;
            UIApplication.SharedApplication.EndBackgroundTask(taskId);
            Console.WriteLine($"Background task '{name}' finished");
        });

        await Task.Factory.StartNew(async () =>
        {
            //Just a method that logs how much time we have remaining. Usually a background task has around 180 seconds to complete. 
            while (!taskEnded)
            {
                Console.WriteLine($"Background task '{name}' time remaining: {UIApplication.SharedApplication.BackgroundTimeRemaining}");
                await Task.Delay(1000);
            }
        });
    }

在 Android 上，您可以使用以下代码:

public async Task RunCodeInBackgroundMode(Func<Task> action, string name = "MyBackgroundTaskName")
        {
            var powerManager = (PowerManager)Application.Context.GetSystemService(Context.PowerService);
            var wakeLock = powerManager.NewWakeLock(WakeLockFlags.Partial,
                                                    name);
            //acquire a partial wakelock. This prevents the phone from going to sleep as long as it is not released.
            wakeLock.Acquire();
            var taskEnded = false;

            await Task.Factory.StartNew(async () =>
            {
                //here we run the actual code
                Console.WriteLine($"Background task '{name}' started");
                await action();
                Console.WriteLine($"Background task '{name}' finished");
                wakeLock.Release();
                taskEnded = true;
            });

            await Task.Factory.StartNew(async () =>
            {
                //just a method to keep track of how long the task runs
                var stopwatch = new Stopwatch();
                stopwatch.Start();
                while (!taskEnded)
                {
                    Console.WriteLine($"Background '{name}' task with wakelock still running ({stopwatch.Elapsed.TotalSeconds} seconds)");
                    await Task.Delay(1000);
                }
                stopwatch.Stop();
            });
        }

在共享项目中，您可以通过以下方式调用这些方法:

var deviceInfoServic = ServiceLocator.Instance.Get<YourService Interface here>();
await deviceInfoServic.RunCodeInBackgroundMode(async () =>
{
    //your code here
});