Background tasks support

Also bump from 1.5.9999 to 1.5.99999 to distinguish
if it includes background task support.

Fixes xamarin#409

.github/workflows/ci.yml

@@ -3,6 +3,8 @@ on: [push, pull_request]
 
 jobs:
   build-deploy:
+    env:
+      BASE_VERSION: 1.5.99999
     runs-on: windows-latest
     steps:
     - uses: actions/checkout@v1
@@ -11,7 +13,7 @@ jobs:
         dotnet tool install --global Cake.Tool
 
         git clone https://github.com/nblockchain/fsx
-        $env:NUGET_VERSION = `.\fsx\Tools\fsi.bat fsx\Tools\nugetPush.fsx --output-version 1.5.9999`
+        $env:NUGET_VERSION = `.\fsx\Tools\fsi.bat fsx\Tools\nugetPush.fsx --output-version $env:BASE_VERSION`
         dotnet cake
 
     - name: Upload

Xamarin.Essentials/Background/Background.android.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Android.Content;
+
+namespace Xamarin.Essentials.Background
+{
+    public static partial class Background
+    {
+        public static void StartBackgroundService(ContextWrapper context)
+        {
+            var intent = new Intent(context, typeof(BackgroundService));
+            context.StartService(intent);
+        }
+    }
+}

Xamarin.Essentials/Background/Background.android.service.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Android.App;
+using Android.Content;
+using Android.OS;
+
+namespace Xamarin.Essentials.Background
+{
+    [Service]
+    public class BackgroundService : Service
+    {
+        static bool isRunning;
+
+        public override IBinder OnBind(Intent intent)
+        {
+            return null;
+        }
+
+        public override StartCommandResult OnStartCommand(Intent intent, StartCommandFlags flags, int startId)
+        {
+            if (!isRunning)
+            {
+                Background.StartJobs();
+
+                isRunning = true;
+            }
+
+            return StartCommandResult.Sticky;
+        }
+
+        public override void OnDestroy()
+        {
+        }
+    }
+}

Xamarin.Essentials/Background/Background.gtk.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Xamarin.Essentials.Background
+{
+    public partial class Background
+    {
+        public static void StartBackgroundService()
+        {
+            Background.StartJobs();
+        }
+    }
+}

Xamarin.Essentials/Background/Background.ios.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Xamarin.Essentials.Background
+{
+    public static partial class Background
+    {
+        public static void StartBackgroundService()
+        {
+            BackgroundService.Start();
+        }
+    }
+}

Xamarin.Essentials/Background/Background.ios.service.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using UIKit;
+
+namespace Xamarin.Essentials.Background
+{
+    public static class BackgroundService
+    {
+        static nint taskId;
+        static bool isRunning;
+
+        /// <summary>
+        /// Start the execution of background service
+        /// </summary>
+        public static void Start()
+        {
+            if (isRunning)
+                return;
+
+            taskId = UIApplication.SharedApplication.BeginBackgroundTask("BackgroundTask", Stop);
+            Background.StartJobs();
+
+            isRunning = true;
+        }
+
+        public static void Stop()
+        {
+            UIApplication.SharedApplication.EndBackgroundTask(taskId);
+        }
+    }
+}

Xamarin.Essentials/Background/Background.shared.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Xamarin.Essentials.Background
+{
+    public static partial class Background
+    {
+        static Dictionary<string, IBackgroundTask> schedules = new Dictionary<string, IBackgroundTask>();
+
+        public static void Add<T>(Func<T> schedule)
+            where T : IBackgroundTask
+        {
+#if NETSTANDARD1_0
+            var typeName = schedule.GetType().GenericTypeArguments[0]?.Name;
+#else
+            var typeName = schedule.GetType().GetGenericArguments()[0]?.Name;
+#endif
+
+            if (typeName != null && !schedules.ContainsKey(typeName))
+                schedules.Add(typeName, schedule());
+        }
+
+        internal static void StartJobs()
+        {
+            Task.WhenAll(schedules.Values.Select(x => x.StartJob()));
+        }
+    }
+
+    public interface IBackgroundTask
+    {
+        Task StartJob();
+    }
+}

Xamarin.Essentials/Background/Background.tizen.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Xamarin.Essentials.Background
+{
+    public partial class Background
+    {
+        public static void StartBackgroundService()
+        {
+            // TODO: https://docs.tizen.org/application/native/guides/applications/service-app/
+            throw new NotImplementedException();
+        }
+    }
+}

Xamarin.Essentials/Background/Background.uwp.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using Windows.ApplicationModel.Background;
+
+namespace Xamarin.Essentials.Background
+{
+    public static partial class Background
+    {
+        const string backServiceName = "Xamarin.Essentials.Background.BackgroundService";
+
+        public static void StartBackgroundService()
+        {
+            Task.Run(StartBackgroundServiceAsync);
+        }
+
+        static async Task StartBackgroundServiceAsync()
+        {
+            var access = await BackgroundExecutionManager.RequestAccessAsync();
+
+            switch (access)
+            {
+                case BackgroundAccessStatus.Unspecified:
+                case BackgroundAccessStatus.DeniedByUser:
+                case BackgroundAccessStatus.DeniedBySystemPolicy:
+                    return;
+            }
+
+            var task = new BackgroundTaskBuilder
+            {
+                Name = backServiceName,
+                TaskEntryPoint = backServiceName
+            };
+
+            var trigger = new ApplicationTrigger();
+            task.SetTrigger(trigger);
+
+            task.Register();
+
+            await trigger.RequestAsync();
+        }
+    }
+}

Xamarin.Essentials/Background/Background.uwp.service.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Windows.ApplicationModel.Background;
+
+namespace Xamarin.Essentials.Background
+{
+    public class BackgroundService : Windows.ApplicationModel.Background.IBackgroundTask
+    {
+        public void Run(IBackgroundTaskInstance taskInstance)
+        {
+            Background.StartJobs();
+        }
+    }
+}