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false
m_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