异步函数async await在wpf都做了什么？

首先我们来看一段控制台应用代码：

 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";
     }
 }

输出结果

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

如果这段代码在WPF运行，猜猜会输出啥？

      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";
      }

输出结果：

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

一.SynchronizationContext(同步上下文)

首先我们知道async await 异步函数本质是状态机,我们通过反编译工具dnspy，看看反编译的两段代码是否有不同之处：

控制台应用：

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;
}

我们可以看到完全是一致的，没有任何区别，为什么编译器生成的代码是一致的，却会产生不一样的结果，我们看看创建和启动状态机代码部分的实现：

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();//状态机执行代码
	}
	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);
	     }
	}
}

在这里总结下：

• 创建状态机的Create函数通过SynchronizationContext.Current获取到当前同步执行上下文
• 启动状态机的Start函数之后通过MoveNext函数执行我们的异步方法
• 这里还有一个小提示，不管async函数里面有没有await，都会生成状态机，只是MoveNext函数执行同步方法,因此没await的情况下避免将函数标记为async，会损耗性能

同样的这里貌似没能获取到原因，但是有个很关键的地方，就是Create函数为啥要获取当前同步执行上下文，之后我从MSDN找到关于SynchronizationContext

的介绍，有兴趣的朋友可以去阅读以下，以下是各个.NET框架使用的SynchronizationContext：

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

我们貌似已经一步步接近真相了，接下来我们来看看DispatcherSynchronizationContext

二.DispatcherSynchronizationContext

首先来看看DispatcherSynchronizationContext类的比较关键的几个函数实现：

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

//同步执行
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);
     }
}

//异步执行
public override void Post(SendOrPostCallback d, object state)
{
     _dispatcher.BeginInvoke(_priority, d, state);
}

我们貌似看到了熟悉的东西了，Send函数调用Dispatcher的Invoke函数，Post函数调用Dispatcher的BeginInvoke函数，那么是否WPF执行异步函数之后会调用这里的函数吗？我用dnspy进行了调试：

我通过调试之后发现，当等待执行完整个状态机的之后，也就是两秒后跳转到该Post函数，那么，我们可以将之前的WPF那段代码大概可以改写成如此：

private async void Async_Click(object sender, RoutedEventArgs e)
{
    //async生成状态机的Create函数。获取到UI主线程的同步执行上下文
    DispatcherSynchronizationContext synchronizationContext = (DispatcherSynchronizationContext)SynchronizationContext.Current;

    //UI主线程执行
    Debug.WriteLine($"Thread Id is Thread:{Thread.CurrentThread.ManagedThreadId},Is Thread Pool:{Thread.CurrentThread.IsThreadPoolThread}");

    //开始在状态机的MoveNext执行该异步操作
    var result= await ExampleTask(2);

    //等待两秒，异步执行完成，再在同步上下文异步执行
    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");
 }

输出结果：

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负责生成状态机和执行状态机，await将代码分为两部分，一部分是异步执行状态机部分，一部分是异步执行完之后，通过之前拿到的DispatcherSynchronizationContext，再去异步执行接下来的部分。我们可以通过dnspy调试DispatcherSynchronizationContext的 _dispatcher字段的Thread属性，知道Thread为UI主线程，而同步界面UI控件的时候，也就是通过Dispatcher的BeginInvoke函数去执行同步的

三.Task.ConfigureAwait

Task有个ConfigureAwait方法，是可以设置是否对Task的awaiter的延续任务执行原始上下文，也就是为true时，是以一开始那个UI主线程的DispatcherSynchronizationContext执行Post方法,而为false，则以await那个Task里面的DispatcherSynchronizationContext执行Post方法，我们来验证下：

我们将代码改为以下：

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");
}

输出：

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

结果和控制台输出的一模一样，且通过dnspy断点调试依旧进入到DispatcherSynchronizationContext的Post方法,因此我们也可以证明我们上面的猜想，而且默认ConfigureAwait的参数是为true的,我们还可以将异步结果赋值给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");
}

抛出异常：

调用线程无法访问此对象，因为另一个线程拥有该对象