Fixed the debounce extension in order to update to the last action pa… (#4294)

## Fixes #4293

## PR Type

What kind of change does this PR introduce?

Bugfix

## What is the current behavior?

The debounce extension was incorrectly updating the action in the ConcurrentList.

According to documentation the value passed in the `updateValueFactory` function, of the `AddOrUpdate` method, is the current value in the dictionary.

https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.addorupdate?view=net-5.0

![image](https://user-images.githubusercontent.com/29243277/135733394-f7c4d682-9c24-45cd-af4d-69721a7c6383.png)

Therefore the current behavior is maintaining the old action and not updating with the new passed action.

```csharp
_debounceInstances.AddOrUpdate(timer, action, (k, v) => v);
```

## What is the new behavior?

Changed so that the action in the `_ debounceInstances` Dictionary is updated with the latest version.

## PR Checklist

Please check if your PR fulfills the following requirements: <!-- and remove the ones that are not applicable to the current PR -->

- [x] Tested code with current [supported SDKs](../#supported)
- [x] Contains **NO** breaking changes
- [x]  Tests for the changes have been added
- [x]  Header has been added to all new source files (run build/UpdateHeaders.bat)

## Other information

PS: The debuonce extension is referenced in the `CanvasPathGeometryPage` class (although this changes does not seem to break it).

Author: msftbot[bot]

Microsoft.Toolkit.Uwp/Extensions/DispatcherQueueTimerExtensions.cs

@@ -62,7 +62,7 @@ public static void Debounce(this DispatcherQueueTimer timer, Action action, Time
                 timer.Tick += Timer_Tick;
 
                 // Store/Update function
-                _debounceInstances.AddOrUpdate(timer, action, (k, v) => v);
+                _debounceInstances.AddOrUpdate(timer, action, (k, v) => action);
             }
 
             // Start the timer to keep track of the last call here.

UnitTests/UnitTests.UWP/Extensions/Test_DispatcherQueueTimerExtensions.cs

@@ -0,0 +1,49 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.Toolkit.Uwp.UI;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+using System.Threading.Tasks;
+
+namespace UnitTests.Extensions
+{
+    [TestClass]
+    public class Test_DispatcherQueueTimerExtensions
+    {
+        [TestCategory("DispatcherQueueTimerExtensions")]
+        [TestMethod]
+        public async Task Test_DispatcherQueueTimerExtensions_Debounce()
+        {
+            var debounceTimer = App.DispatcherQueue.CreateTimer();
+
+            var triggeredCount = 0;
+            string triggeredValue = null;
+
+            var value = "He";
+            debounceTimer.Debounce(
+                () =>
+                {
+                    triggeredCount++;
+                    triggeredValue = value;
+                },
+                TimeSpan.FromMilliseconds(60));
+
+            var value2 = "Hello";
+            debounceTimer.Debounce(
+                () =>
+                {
+                    triggeredCount++;
+                    triggeredValue = value2;
+                },
+                TimeSpan.FromMilliseconds(60));
+
+            await Task.Delay(TimeSpan.FromMilliseconds(110));
+
+            Assert.AreEqual(false, debounceTimer.IsRunning, "Expected to stop the timer.");
+            Assert.AreEqual(value2, triggeredValue, "Expected to execute the last action.");
+            Assert.AreEqual(1, triggeredCount, "Expected to postpone execution.");
+        }
+    }
+}

UnitTests/UnitTests.UWP/UnitTests.UWP.csproj

@@ -165,6 +165,7 @@
     <Compile Include="Converters\Test_StringFormatConverter.cs" />
     <Compile Include="Converters\Test_TypeToObjectConverter.cs" />
     <Compile Include="Extensions\Helpers\ObjectWithNullableBoolProperty.cs" />
+    <Compile Include="Extensions\Test_DispatcherQueueTimerExtensions.cs" />
     <Compile Include="Extensions\Test_StringExtensions.cs" />
     <Compile Include="Extensions\Test_UIElementExtensions_Coordinates.cs" />
     <Compile Include="Extensions\Test_VisualTreeExtensions.cs" />