readme of singleassignment

Author: neuecc

README.md

@@ -609,6 +609,30 @@ void Close()
 }
 ```
 
+```csharp
+IDisposable disposable;
+
+void OnInitialize(ISubscriber<int> subscriber)
+{
+    var bag = DisposableBag.CreateBuilder();
+
+    // calling once(or x count), you can use DisposableBag.CreateSingleAssignment to hold subscription reference.
+    var d = DisposableBag.CreateSingleAssignment();
+    
+    // you can invoke Dispose in handler action.
+    // assign disposable, you can use `SetTo` and `AddTo` bag.
+    // or you can use d.Disposable = subscriber.Subscribe();
+    subscriber.Subscribe(_ => { d.Dispose(); }).SetTo(d).AddTo(bag);
+
+    disposable = bag.Build();
+}
+
+void Close()
+{
+    disposable?.Dispose();
+}
+```
+
 The returned `IDisposable` value **must** be handled. If it is ignored, it will leak. However Weak reference, which is widely used in WPF, is an anti-pattern. All subscriptions should be managed explicitly.
 
 You can monitor subscription count by `MessagePipeDiagnosticsInfo`. It can get from service provider(or DI).

tests/MessagePipe.Tests/DisaposableBagTest.cs

@@ -1,5 +1,7 @@
 using FluentAssertions;
+using Microsoft.Extensions.DependencyInjection;
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using Xunit;
 
@@ -141,6 +143,30 @@ public void SingleAssignment()
             }
         }
 
+        [Fact]
+        public void CallOnce()
+        {
+            var provider = TestHelper.BuildServiceProvider();
+
+            var p = provider.GetRequiredService<IPublisher<int>>();
+            var s = provider.GetRequiredService<ISubscriber<int>>();
+
+            var d = DisposableBag.CreateSingleAssignment();
+
+            var list = new List<int>();
+            d.Disposable = s.Subscribe(x =>
+            {
+                list.Add(x);
+                d.Dispose();
+            });
+
+            p.Publish(100);
+            p.Publish(200);
+            p.Publish(300);
+
+            list.Should().Equal(new[] { 100 });
+        }
+
         public class Disposable : IDisposable
         {
             public int Number { get; }