BackgroundWorker原理剖析

BackgroundWorker类位于System.ComponentModel命名空间下，主要用来异步执行一个长时间的操作，然后，在完成事件中安全更新UI的控件属性。UI中的控件是不允许非创建该控件的线程修改的。典型用法如下：

BackgroundWorker m_worker = new BackgroundWorker();// 设置支持进度报告、异步取消功能，默认都为falsem_worker.WorkerReportsPRogress = true;m_worker.WorkerSupportsCancellation = true;// 绑定事件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) {         // 处理取消        return;    } else if (e.Error != null) {        // 处理异常        return;    }        // 在UI中显示结果    // 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++) {        // 做异步工作。。。。        // 报告进度        sendWorker.ReportProgress(i);        // 请求取消工作内容        if (sendWorker.CancellationPending == true) {            e.Cancel = true;            return;        }    }    // 可选，设置异步工作结果    e.Result = GetResultData();}

它的实现原理最重要的只有两点：

一点是用异步委托间接使用线程池执行长时间的操作；

另外一点是通过AsyncOperationManager和AsyncOperation对调用RunWorkerAsync的线程SynchronizationContext进行抽象；

BackgroundWorker的源码参见http://www.projky.com/dotnet/4.5.1/System/ComponentModel/BackgroundWorker.cs.html

首先从它的构造函数开始：

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);}

定义了一个私有的委托类型WorkerThreadStartDelegate，以便于在该委托类型对象上直接调用BaginInvoke到线程池执行委托。SendOrPostCallback 是方便在UI线程（本质是调用RunWorkAsync时捕获的当前线程同步上下文对象，为了容易理解，就叫它UI线程）上执行回调而创建的。而asyncOperation则通过Post方法在UI线程上异步来执行SendOrPostCallback委托。

在对DoWork添加事件后，需要调用RunWorkerAsync，有两个重载，但我们只关注最后一个带参数的：

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);}

其实，asyncOperation = AsyncOperationManager.CreateOperation(null);这一行代码，等同于下面的代码：

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

简单来说，就是获得当前的SynchronizationContext的对象，如果不存在，则创建一个默认的（基于线程池实现的）。并让asyncOperation拥有SynchronizationContext的引用。

在.NET中，有很多种SynchronizationContext的子类，比如Winform里面的WindowsFormsSynchronizationContext类，WPF里面的DispatcherSynchronizationContext类，asp.net里面的AspNetSynchronizationContext类。重点是，当在Winform的UI线程中访问SynchronizationContext.Current属性，获得的就是WindowsFormsSynchronizationContext的对象。

那么，最终，AsyncOperation的Post方法，就是直接调用SynchronizationContext的Post方法，来实现在UI中回调的目的。

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

还有一点，threadStart.BeginInvoke会用线程池中的线程执行类似如下的代码：

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，对DoWork事件的声明如下：

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

Events是从Component下派生来的protected EventHandlerList对象。

从BackgroundWorker的实现可以看出，它的实现是普遍性的，并不一定要用在Winform或者WPF中。

本文引用的源码参考列表可以从扣丁格鲁上查看。

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

关于基于事件的异步编程设计模式EAP更多参考请见http://msdn.microsoft.com/zh-cn/library/hkasytyf(v=vs.110).aspx