BackgroundWorker原理剖析

BackgroundWorker類位于System.ComponentModel命名空間下，主要用來異步執行一個長時間的操作，然后，在完成事件中安全更新UI的控件屬性。UI中的控件是不允許非創建該控件的線程修改的。典型用法如下：

BackgroundWorker m_worker = new BackgroundWorker();// 設置支持進度報告、異步取消功能，默認都為falsem_worker.WorkerReportsPRogress = true;m_worker.WorkerSupportsCancellation = true;// 綁定事件m_worker.DoWork += m_worker_DoWork;m_worker.ProgressChanged += m_worker_ProgressChanged;m_worker.RunWorkerCompleted += m_worker_RunWorkerCompleted;void m_worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {    if (e.Cancelled == true) {         // 處理取消        return;    } else if (e.Error != null) {        // 處理異常        return;    }        // 在UI中顯示結果    // txtBox.Text = e.Result.ToString();}void m_worker_ProgressChanged(object sender, ProgressChangedEventArgs e) {    //progressBar.Value = e.ProgressPercentage;}void m_worker_DoWork(object sender, DoWorkEventArgs e) {    BackgroundWorker sendWorker = sender as BackgroundWorker;    for (int i = 0; i < 100; i++) {        // 做異步工作。。。。        // 報告進度        sendWorker.ReportProgress(i);        // 請求取消工作內容        if (sendWorker.CancellationPending == true) {            e.Cancel = true;            return;        }    }    // 可選，設置異步工作結果    e.Result = GetResultData();}

它的實現原理最重要的只有兩點：

一點是用異步委托間接使用線程池執行長時間的操作；

另外一點是通過AsyncOperationManager和AsyncOperation對調用RunWorkerAsync的線程SynchronizationContext進行抽象；

BackgroundWorker的源碼參見http://www.projky.com/dotnet/4.5.1/System/ComponentModel/BackgroundWorker.cs.html

首先從它的構造函數開始：

private delegate void WorkerThreadStartDelegate(object argument);private AsyncOperation                      asyncOperation = null;private readonly WorkerThreadStartDelegate  threadStart;private readonly SendOrPostCallback operationCompleted;private readonly SendOrPostCallback progressReporter;public BackgroundWorker(){    threadStart        = new WorkerThreadStartDelegate(WorkerThreadStart);    operationCompleted = new SendOrPostCallback(AsyncOperationCompleted);    progressReporter   = new SendOrPostCallback(ProgressReporter);}

定義了一個私有的委托類型WorkerThreadStartDelegate，以便于在該委托類型對象上直接調用BaginInvoke到線程池執行委托。SendOrPostCallback 是方便在UI線程（本質是調用RunWorkAsync時捕獲的當前線程同步上下文對象，為了容易理解，就叫它UI線程）上執行回調而創建的。而asyncOperation則通過Post方法在UI線程上異步來執行SendOrPostCallback委托。

在對DoWork添加事件后，需要調用RunWorkerAsync，有兩個重載，但我們只關注最后一個帶參數的：

public void RunWorkerAsync(object argument){    if (isRunning)    {        throw new InvalidOperationException(SR.GetString(SR.BackgroundWorker_WorkerAlreadyRunning));    }    isRunning = true;    cancellationPending = false;        asyncOperation = AsyncOperationManager.CreateOperation(null);    threadStart.BeginInvoke(argument,                            null,                            null);}

其實，asyncOperation = AsyncOperationManager.CreateOperation(null);這一行代碼，等同于下面的代碼：

if (SynchronizationContext.Current == null) {    SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());}SynchronizationContext currentContext = SynchronizationContext.Current;asyncOperation = AsyncOperation.CreateOperation(null, currentContext)

簡單來說，就是獲得當前的SynchronizationContext的對象，如果不存在，則創建一個默認的（基于線程池實現的）。并讓asyncOperation擁有SynchronizationContext的引用。

在.NET中，有很多種SynchronizationContext的子類，比如Winform里面的WindowsFormsSynchronizationContext類，WPF里面的DispatcherSynchronizationContext類，asp.net里面的AspNetSynchronizationContext類。重點是，當在Winform的UI線程中訪問SynchronizationContext.Current屬性，獲得的就是WindowsFormsSynchronizationContext的對象。

那么，最終，AsyncOperation的Post方法，就是直接調用SynchronizationContext的Post方法，來實現在UI中回調的目的。

public void Post(SendOrPostCallback d, object arg){    VerifyNotCompleted();    VerifyDelegateNotNull(d);    syncContext.Post(d, arg);}

還有一點，threadStart.BeginInvoke會用線程池中的線程執行類似如下的代碼：

object workerResult = null;Exception error = null;bool cancelled = false;try{    DoWorkEventArgs doWorkArgs = new DoWorkEventArgs(argument);    DoWorkEventHandler handler = (DoWorkEventHandler)(Events[doWorkKey]);    if (handler != null)    {        handler(this, doWorkArgs);    }    if (doWorkArgs.Cancel)    {        cancelled = true;    }    else    {        workerResult = doWorkArgs.Result;    }}catch (Exception exception){    error = exception;}RunWorkerCompletedEventArgs e =     new RunWorkerCompletedEventArgs(workerResult, error, cancelled); asyncOperation.PostOperationCompleted(operationCompleted, e);

其中，對DoWork事件的聲明如下：

private static readonly object doWorkKey = new object();public event DoWorkEventHandler DoWork{    add{        this.Events.AddHandler(doWorkKey, value);    }    remove{        this.Events.RemoveHandler(doWorkKey, value);    }}

Events是從Component下派生來的protected EventHandlerList對象。

從BackgroundWorker的實現可以看出，它的實現是普遍性的，并不一定要用在Winform或者WPF中。

本文引用的源碼參考列表可以從扣丁格魯上查看。

http://www.projky.com/dotnet/4.5.1/System/ComponentModel/BackgroundWorker.cs.html

http://www.projky.com/dotnet/4.5.1/System/ComponentModel/AsyncOperation.cs.html

http://www.projky.com/dotnet/4.5.1/System/ComponentModel/AsyncOperationManager.cs.html

http://www.projky.com/dotnet/4.5.1/System/Threading/SynchronizationContext.cs.html

http://www.projky.com/dotnet/4.5.1/System/Windows/Forms/WindowsFormsSynchronizationContext.cs.html

http://www.projky.com/dotnet/4.5.1/System/Windows/Threading/DispatcherSynchronizationContext.cs.html

http://www.projky.com/dotnet/4.5.1/System/Web/AspNetSynchronizationContext.cs.html

關于基于事件的異步編程設計模式EAP更多參考請見http://msdn.microsoft.com/zh-cn/library/hkasytyf(v=vs.110).aspx