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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