异步函数async await在wpf都做了什么？-白红宇的个人博客

发布日期：2021-05-09 06:18:02 浏览次数：18 分类：博客文章

本文共 9850 字，大约阅读时间需要 32 分钟。

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
   
     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:FalseThread Id is Thread:4,Is Thread Pool:TrueIt's Async Completed in 2 secondsAsync 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
   
     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:FalseThread Id is Thread:1,Is Thread Pool:FalseIt's Async Completed in 2 secondsAsync Completed

这时候你肯定是想说，小朋友，你是否有很多问号？？？？，我们接下看下去

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.
   
    d__0 
    
     d__ = new Program.
     
      d__0();		
      
       d__.args = args;		
       
        d__.<>t__builder = AsyncTaskMethodBuilder.Create();		
        
         d__.<>1__state = -1; 
         
          d__.<>t__builder.Start
          
           
            d__0>(ref 
            
             d__); return 
             
              d__.<>t__builder.Task; } [DebuggerStepThrough] private static Task
              
                ExampleTask(int Second) { Program.
               
                d__1 
                
                 d__ = new Program.
                 
                  d__1(); 
                  
                   d__.Second = Second; 
                   
                    d__.<>t__builder = AsyncTaskMethodBuilder
                    
                     .Create(); 
                     
                      d__.<>1__state = -1; 
                      
                       d__.<>t__builder.Start
                       
                        
                         d__1>(ref 
                         
                          d__); return 
                          
                           d__.<>t__builder.Task; } [DebuggerStepThrough] private static void 
                           
                            (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.
   
    d__1 
    
     d__ = new MainWindow.
     
      d__1();		
      
       d__.<>4__this = this;		
       
        d__.sender = sender;		
        
         d__.e = e; 
         
          d__.<>t__builder = AsyncVoidMethodBuilder.Create(); 
          
           d__.<>1__state = -1; 
           
            d__.<>t__builder.Start
            
             
              d__1>(ref 
              
               d__); } [DebuggerStepThrough] private Task
               
                 ExampleTask(int Second) { MainWindow.
                
                 d__3 
                 
                  d__ = new MainWindow.
                  
                   d__3(); 
                   
                    d__.<>4__this = this; 
                    
                     d__.Second = Second; 
                     
                      d__.<>t__builder = AsyncTaskMethodBuilder
                      
                       .Create(); 
                       
                        d__.<>1__state = -1; 
                        
                         d__.<>t__builder.Start
                         
                          
                           d__3>(ref 
                           
                            d__); return 
                            
                             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
   
    (ref stateMachine);}[DebuggerStepThrough]public static void Start
    
     (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

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:FalseThread Id is Thread:1,Is Thread Pool:FalseIt's Async Completed in 2 secondsAsync 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:FalseThread Id is Thread:4,Is Thread Pool:TrueIt's Async Completed in 2 secondsAsync 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

补充

推荐林大佬的一篇文章，也讲的也简洁透彻

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一.SynchronizationContext(同步上下文)

二.DispatcherSynchronizationContext

三.Task.ConfigureAwait

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