異步函數async await在wpf都做了什麼？

首先我們來看一段控制台應用代碼：

 class Program
 {
     static async Task Main(string[] args)
     {
        System.Console.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
        var result = await ExampleTask(2);
        System.Console.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
        System.Console.WriteLine(result);
        Console.WriteLine("Async Completed");
     }

     private static async Task<string> ExampleTask(int Second)
     {
        await Task.Delay(TimeSpan.FromSeconds(Second));
        return $"It's Async Completed in {Second} seconds";
     }
 }

輸出結果

Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:4,Is Thread Pool:True
It's Async Completed in 2 seconds
Async Completed

如果這段代碼在WPF運行，猜猜會輸出啥？

      private async void Async_Click(object sender, RoutedEventArgs e)
      {
          Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
          var result= await ExampleTask(2);
          Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
          Debug.WriteLine(result);
          Debug.WriteLine("Async Completed");   
      }

      private async Task<string> ExampleTask(int Second)
      {
          await Task.Delay(TimeSpan.FromSeconds(Second));
          return $"It's Async Completed in {Second} seconds";
      }

輸出結果：

Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:1,Is Thread Pool:False
It's Async Completed in 2 seconds
Async Completed

這時候你肯定是想說，小朋友，你是否有很多問號？？？？，我們接下看下去

一.SynchronizationContext(同步上下文)

首先我們知道async await 異步函數本質是狀態機,我們通過反編譯工具dnspy，看看反編譯的兩段代碼是否有不同之處：

控制台應用：

internal class Program
{
    [DebuggerStepThrough]
	private static Task Main(string[] args)
	{
		Program.<Main>d__0 <Main>d__ = new Program.<Main>d__0();
		<Main>d__.args = args;
		<Main>d__.<>t__builder = AsyncTaskMethodBuilder.Create();
		<Main>d__.<>1__state = -1;
		<Main>d__.<>t__builder.Start<Program.<Main>d__0>(ref <Main>d__);
		return <Main>d__.<>t__builder.Task;
	}
    
	[DebuggerStepThrough]
	private static Task<string> ExampleTask(int Second)
	{
		Program.<ExampleTask>d__1 <ExampleTask>d__ = new Program.<ExampleTask>d__1();
		<ExampleTask>d__.Second = Second;
		<ExampleTask>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
		<ExampleTask>d__.<>1__state = -1;
		<ExampleTask>d__.<>t__builder.Start<Program.<ExampleTask>d__1>(ref <ExampleTask>d__);
		return <ExampleTask>d__.<>t__builder.Task;
	}

	[DebuggerStepThrough]
	private static void <Main>(string[] args)
	{
	        Program.Main(args).GetAwaiter().GetResult();
	}
}

WPF:

public class MainWindow : Window, IComponentConnector
{

	public MainWindow()
	{
	       this.InitializeComponent();
	}

	[DebuggerStepThrough]
	private void Async_Click(object sender, RoutedEventArgs e)
	{
		MainWindow.<Async_Click>d__1 <Async_Click>d__ = new MainWindow.<Async_Click>d__1();
		<Async_Click>d__.<>4__this = this;
		<Async_Click>d__.sender = sender;
		<Async_Click>d__.e = e;
		<Async_Click>d__.<>t__builder = AsyncVoidMethodBuilder.Create();
		<Async_Click>d__.<>1__state = -1;
		<Async_Click>d__.<>t__builder.Start<MainWindow.<Async_Click>d__1>(ref <Async_Click>d__);
	}

	[DebuggerStepThrough]
	private Task<string> ExampleTask(int Second)
	{
	        MainWindow.<ExampleTask>d__3 <ExampleTask>d__ = new MainWindow.<ExampleTask>d__3();
		<ExampleTask>d__.<>4__this = this;
		<ExampleTask>d__.Second = Second;
		<ExampleTask>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
		<ExampleTask>d__.<>1__state = -1;
		<ExampleTask>d__.<>t__builder.Start<MainWindow.<ExampleTask>d__3>(ref <ExampleTask>d__);
		return <ExampleTask>d__.<>t__builder.Task;
	}

	[DebuggerNonUserCode]
	[GeneratedCode("PresentationBuildTasks", "4.8.1.0")]
	public void InitializeComponent()
	{
		bool contentLoaded = this._contentLoaded;
		if (!contentLoaded)
		{
		     this._contentLoaded = true;
		     Uri resourceLocater = new Uri("/WpfApp1;component/mainwindow.xaml", UriKind.Relative);
		     Application.LoadComponent(this, resourceLocater);
		}
	}
	private bool _contentLoaded;
}

我們可以看到完全是一致的，沒有任何區別，為什麼編譯器生成的代碼是一致的，卻會產生不一樣的結果，我們看看創建和啟動狀態機代碼部分的實現：

public static AsyncVoidMethodBuilder Create()
{
	SynchronizationContext synchronizationContext = SynchronizationContext.Current;
	if (synchronizationContext != null)
	{
		synchronizationContext.OperationStarted();
	}
	return new AsyncVoidMethodBuilder
	{
		_synchronizationContext = synchronizationContext
	};
}

[DebuggerStepThrough]
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public void Start<[Nullable(0)] TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
	AsyncMethodBuilderCore.Start<TStateMachine>(ref stateMachine);
}

[DebuggerStepThrough]
public static void Start<TStateMachine>(ref TStateMachine stateMachine) where TStateMachine : IAsyncStateMachine
{
	if (stateMachine == null)
	{
		ThrowHelper.ThrowArgumentNullException(ExceptionArgument.stateMachine);
	}
	Thread currentThread = Thread.CurrentThread;
	Thread thread = currentThread;
	ExecutionContext executionContext = currentThread._executionContext;
	ExecutionContext executionContext2 = executionContext;
	SynchronizationContext synchronizationContext = currentThread._synchronizationContext;
	try
	{
	     stateMachine.MoveNext();//狀態機執行代碼
	}
	finally
	{
	     SynchronizationContext synchronizationContext2 = synchronizationContext;
	     Thread thread2 = thread;
	     if (synchronizationContext2 != thread2._synchronizationContext)
	     {
		  thread2._synchronizationContext = synchronizationContext2;
	     }
	     ExecutionContext executionContext3 = executionContext2;
	     ExecutionContext executionContext4 = thread2._executionContext;
	     if (executionContext3 != executionContext4)
	     {
		 ExecutionContext.RestoreChangedContextToThread(thread2, executionContext3, executionContext4);
	     }
	}
}

在這裏總結下：

• 創建狀態機的Create函數通過SynchronizationContext.Current獲取到當前同步執行上下文
• 啟動狀態機的Start函數之後通過MoveNext函數執行我們的異步方法
• 這裏還有一個小提示，不管async函數裏面有沒有await，都會生成狀態機，只是MoveNext函數執行同步方法,因此沒await的情況下避免將函數標記為async，會損耗性能

同樣的這裏貌似沒能獲取到原因，但是有個很關鍵的地方，就是Create函數為啥要獲取當前同步執行上下文，之後我從MSDN找到關於SynchronizationContext
的介紹，有興趣的朋友可以去閱讀以下，以下是各個.NET框架使用的SynchronizationContext：

SynchronizationContext	默認
WindowsFormsSynchronizationContext	WindowsForm
DispatcherSynchronizationContext	WPF/Silverlight
AspNetSynchronizationContext	ASP.NET

我們貌似已經一步步接近真相了，接下來我們來看看DispatcherSynchronizationContext

二.DispatcherSynchronizationContext

首先來看看DispatcherSynchronizationContext類的比較關鍵的幾個函數實現：

public DispatcherSynchronizationContext(Dispatcher dispatcher, DispatcherPriority priority)
{
     if (dispatcher == null)
     {
         throw new ArgumentNullException("dispatcher");
     }
     Dispatcher.ValidatePriority(priority, "priority");
     _dispatcher = dispatcher;
     _priority = priority;
     SetWaitNotificationRequired();
 }

//同步執行
public override void Send(SendOrPostCallback d, object state)
{
     if (BaseCompatibilityPreferences.GetInlineDispatcherSynchronizationContextSend() && _dispatcher.CheckAccess())
     {
         _dispatcher.Invoke(DispatcherPriority.Send, d, state);
     }
     else
     {
          _dispatcher.Invoke(_priority, d, state);
     }
}

//異步執行
public override void Post(SendOrPostCallback d, object state)
{
     _dispatcher.BeginInvoke(_priority, d, state);
}

我們貌似看到了熟悉的東西了，Send函數調用Dispatcher的Invoke函數，Post函數調用Dispatcher的BeginInvoke函數，那麼是否WPF執行異步函數之後會調用這裏的函數嗎？我用dnspy進行了調試：

我通過調試之後發現，當等待執行完整個狀態機的之後，也就是兩秒后跳轉到該Post函數，那麼，我們可以將之前的WPF那段代碼大概可以改寫成如此：

private async void Async_Click(object sender, RoutedEventArgs e)
{
    //async生成狀態機的Create函數。獲取到UI主線程的同步執行上下文
    DispatcherSynchronizationContext synchronizationContext = (DispatcherSynchronizationContext)SynchronizationContext.Current;
    
    //UI主線程執行
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
    
    //開始在狀態機的MoveNext執行該異步操作
    var result= await ExampleTask(2);
    
    //等待兩秒，異步執行完成，再在同步上下文異步執行
    synchronizationContext.Post((state) =>
    {
         //模仿_dispatcher.BeginInvoke
         Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
         Debug.WriteLine(result);
         Debug.WriteLine("Async Completed");  
     },"Post");           
 }

輸出結果：

Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:1,Is Thread Pool:False
It's Async Completed in 2 seconds
Async Completed

也就是asyn負責生成狀態機和執行狀態機，await將代碼分為兩部分，一部分是異步執行狀態機部分，一部分是異步執行完之後，通過之前拿到的DispatcherSynchronizationContext，再去異步執行接下來的部分。我們可以通過dnspy調試DispatcherSynchronizationContext的 _dispatcher字段的Thread屬性，知道Thread為UI主線程，而同步界面UI控件的時候，也就是通過Dispatcher的BeginInvoke函數去執行同步的

三.Task.ConfigureAwait

Task有個ConfigureAwait方法，是可以設置是否對Task的awaiter的延續任務執行原始上下文，也就是為true時，是以一開始那個UI主線程的DispatcherSynchronizationContext執行Post方法,而為false，則以await那個Task裏面的DispatcherSynchronizationContext執行Post方法，我們來驗證下：

我們將代碼改為以下：

private async void Async_Click(object sender, RoutedEventArgs e)
{
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
    var result= await ExampleTask(2).ConfigureAwait(false);
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
    Debug.WriteLine(result);
    Debug.WriteLine($"Async Completed");
}

輸出：

Thread Id is Thread:1,Is Thread Pool:False
Thread Id is Thread:4,Is Thread Pool:True
It's Async Completed in 2 seconds
Async Completed

結果和控制台輸出的一模一樣，且通過dnspy斷點調試依舊進入到DispatcherSynchronizationContext的Post方法,因此我們也可以證明我們上面的猜想，而且默認ConfigureAwait的參數是為true的,我們還可以將異步結果賦值給UI界面的Text block：

private async void Async_Click(object sender, RoutedEventArgs e)
{
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
    var result= await ExampleTask(2).ConfigureAwait(false);
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");
    this.txt.Text = result;//修改部分
    Debug.WriteLine($"Async Completed");
}

拋出異常：

調用線程無法訪問此對象，因為另一個線程擁有該對象

補充
推薦林大佬的一篇文章，也講的也簡潔透徹C# dotnet 自己實現一個線程同步上下文

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