From a0c9f16aaee4e3622f4c688bd70b41344eea6390 Mon Sep 17 00:00:00 2001
From: Sipke Schoorstra <sipkeschoorstra@outlook.com>
Date: Wed, 30 Dec 2020 12:00:53 +0100
Subject: [PATCH] Add generic BackgroundWorker for running tasks asynchronously
 in the background

---
 .../Services/IBackgroundWorker.cs             | 13 +++++++++
 src/core/Elsa.Core/Elsa.Core.csproj           |  1 +
 .../ElsaServiceCollectionExtensions.cs        |  2 ++
 .../HostedServices/StartBackgroundWorker.cs   | 14 +++++++++
 .../Elsa.Core/Services/BackgroundWorker.cs    | 29 +++++++++++++++++++
 5 files changed, 59 insertions(+)
 create mode 100644 src/core/Elsa.Abstractions/Services/IBackgroundWorker.cs
 create mode 100644 src/core/Elsa.Core/HostedServices/StartBackgroundWorker.cs
 create mode 100644 src/core/Elsa.Core/Services/BackgroundWorker.cs

diff --git a/src/core/Elsa.Abstractions/Services/IBackgroundWorker.cs b/src/core/Elsa.Abstractions/Services/IBackgroundWorker.cs
new file mode 100644
index 000000000..d7a7ebb44
--- /dev/null
+++ b/src/core/Elsa.Abstractions/Services/IBackgroundWorker.cs
@@ -0,0 +1,13 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Elsa.Services
+{
+    public interface IBackgroundWorker
+    {
+        Task ScheduleTask(Func<ValueTask> task, CancellationToken cancellationToken = default);
+        Task ScheduleTask(Action task, CancellationToken cancellationToken = default);
+        Task RunAsync(CancellationToken cancellationToken = default);
+    }
+}
\ No newline at end of file
diff --git a/src/core/Elsa.Core/Elsa.Core.csproj b/src/core/Elsa.Core/Elsa.Core.csproj
index da3351923..e23689e01 100644
--- a/src/core/Elsa.Core/Elsa.Core.csproj
+++ b/src/core/Elsa.Core/Elsa.Core.csproj
@@ -37,6 +37,7 @@
         <PackageReference Include="Scrutor" Version="3.3.0" />
         <PackageReference Include="Storage.Net" Version="9.2.7" />
         <PackageReference Include="System.Linq.Async" Version="5.0.0" />
+        <PackageReference Include="System.Threading.Channels" Version="4.7.1" />
         <PackageReference Include="YamlDotNet" Version="9.1.0" />
         <PackageReference Include="Rebus.ServiceProvider" Version="6.0.0" />
     </ItemGroup>
diff --git a/src/core/Elsa.Core/Extensions/ElsaServiceCollectionExtensions.cs b/src/core/Elsa.Core/Extensions/ElsaServiceCollectionExtensions.cs
index b61044d63..e0fc9987c 100644
--- a/src/core/Elsa.Core/Extensions/ElsaServiceCollectionExtensions.cs
+++ b/src/core/Elsa.Core/Extensions/ElsaServiceCollectionExtensions.cs
@@ -36,6 +36,7 @@ namespace Microsoft.Extensions.DependencyInjection
             this IServiceCollection services,
             Action<ElsaOptions>? configure = default)
         {
+            services.AddHostedService<StartBackgroundWorker>();
             services.AddStartupRunner();
             
             var options = new ElsaOptions(services);
@@ -95,6 +96,7 @@ namespace Microsoft.Extensions.DependencyInjection
                 .AddSingleton<IActivityFactory, ActivityFactory>()
                 .AddSingleton<IWorkflowBlueprintMaterializer, WorkflowBlueprintMaterializer>()
                 .AddSingleton<IWorkflowBlueprintReflector, WorkflowBlueprintReflector>()
+                .AddSingleton<IBackgroundWorker, BackgroundWorker>()
                 .AddScoped<IWorkflowSelector, WorkflowSelector>()
                 .AddScoped<IWorkflowPublisher, WorkflowPublisher>()
                 .AddScoped<IWorkflowContextManager, WorkflowContextManager>()
diff --git a/src/core/Elsa.Core/HostedServices/StartBackgroundWorker.cs b/src/core/Elsa.Core/HostedServices/StartBackgroundWorker.cs
new file mode 100644
index 000000000..61578f263
--- /dev/null
+++ b/src/core/Elsa.Core/HostedServices/StartBackgroundWorker.cs
@@ -0,0 +1,14 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Elsa.Services;
+using Microsoft.Extensions.Hosting;
+
+namespace Elsa.HostedServices
+{
+    public class StartBackgroundWorker : BackgroundService
+    {
+        private readonly IBackgroundWorker _backgroundWorker;
+        public StartBackgroundWorker(IBackgroundWorker backgroundWorker) => _backgroundWorker = backgroundWorker;
+        protected override async Task ExecuteAsync(CancellationToken stoppingToken) => await _backgroundWorker.RunAsync(stoppingToken);
+    }
+}
\ No newline at end of file
diff --git a/src/core/Elsa.Core/Services/BackgroundWorker.cs b/src/core/Elsa.Core/Services/BackgroundWorker.cs
new file mode 100644
index 000000000..7fc54b296
--- /dev/null
+++ b/src/core/Elsa.Core/Services/BackgroundWorker.cs
@@ -0,0 +1,29 @@
+using System;
+using System.Threading;
+using System.Threading.Channels;
+using System.Threading.Tasks;
+
+namespace Elsa.Services
+{
+    public class BackgroundWorker : IBackgroundWorker
+    {
+        private readonly Channel<Func<ValueTask>> _channel;
+        public BackgroundWorker() => _channel = Channel.CreateBounded<Func<ValueTask>>(10);
+
+        public async Task ScheduleTask(Func<ValueTask> task, CancellationToken cancellationToken) => await _channel.Writer.WriteAsync(task, cancellationToken);
+
+        public async Task ScheduleTask(Action task, CancellationToken cancellationToken) =>
+            await _channel.Writer.WriteAsync(() =>
+            {
+                task();
+                return new ValueTask();
+            }, cancellationToken);
+
+        public async Task RunAsync(CancellationToken cancellationToken)
+        {
+            while (await _channel.Reader.WaitToReadAsync(cancellationToken))
+            while (_channel.Reader.TryRead(out var task))
+                await task();
+        }
+    }
+}
\ No newline at end of file