Silverlight多线程与网络编程实战

### Silverlight 多线程与网络编程实战 #### 多线程中的取消支持 在多线程编程中，为线程添加取消功能是很有必要的。为了实现取消功能，线程包装器需要一个字段来指示是否需要停止处理。以下是添加到 `ThreadWrapperBase` 中的代码： ```csharp // Flag that indicates a stop is requested. private bool cancelRequested = false; protected bool CancelRequested { get { return cancelRequested; } } // Call this to request a cancel. public void RequestCancel() { cancelRequested = true; } // When cancelling, the worker should call the OnCancelled() method // to raise the Cancelled event. public event EventHandler Cancelled; protected void OnCancelled() { if (Cancelled != null) Cancelled(this, EventArgs.Empty); } ``` 在 `FindPrimesThreadWrapper.DoWork()` 方法中，需要定期检查是否有取消请求： ```csharp int iteration = list.Length / 100; if (i % iteration == 0) { if (CancelRequested) { return; } } ``` 同时，需要修改 `ThreadWrapperBase.StartTaskAsync()` 方法，以识别操作结束的两种可能方式： ```csharp private void StartTaskAsync() { DoTask(); if (CancelRequested) { status = StatusState.Cancelled; OnCancelled(); } else { status = StatusState.Completed; OnCompleted(); } } ``` 要使用取消功能，只需为 `Cancelled` 事件添加事件处理程序，并添加一个取消按钮。以下是取消请求的代码： ```csharp private void cmdCancel_Click(object sender, RoutedEventArgs e) { threadWrapper.RequestCancel(); } ``` 取消完成后的事件处理程序： ```csharp private void threadWrapper_Cancelled(object sender, EventArgs e) { this.Dispatcher.BeginInvoke(delegate() { lblStatus.Text = "Search cancelled."; cmdFind.IsEnabled = true; cmdCancel.IsEnabled = false; }); } ``` 需要注意的是，Silverlight 线程不能使用 `Abort()` 方法停止，只能请求礼貌停止。 #### BackgroundWorker 组件 `BackgroundWorker` 是一种简单而安全的多线程方法，它在 .NET 2.0 中首次引入，用于简化 Windows Forms 应用程序中的线程处理。在 Silverlight 中同样适用。 ##### 创建 BackgroundWorker 要使用 `BackgroundWorker`，首先需要创建一个实例，并以编程方式附加事件处理程序。以下是初始化代码： ```csharp private BackgroundWorker backgroundWorker = new BackgroundWorker(); public BackgroundWorkerTest() { InitializeComponent(); backgroundWorker.WorkerReportsProgress = true; backgroundWorker.WorkerSupportsCancellation = true; backgroundWorker.DoWork += backgroundWorker_DoWork; backgroundWorker.ProgressChanged += backgroundWorker_ProgressChanged; backgroundWorker.RunWorkerCompleted += backgroundWorker_RunWorkerCompleted; } ``` ##### 运行 BackgroundWorker 以质数搜索为例，首先需要创建一个自定义类来传递输入参数： ```csharp public class FindPrimesInput { public int From { get; set; } public int To { get; set; } public FindPrimesInput(int from, int to) { From = from; To = to; } } ``` 当用户点击查找质数按钮时，启动 `BackgroundWorker`： ```csharp private void cmdFind_Click(object sender, RoutedEventArgs e) { // Disable this button and clear previous results. cmdFind.IsEnabled = false; cmdCancel.IsEnabled = true; lstPrimes.Items.Clear(); // Get the search range. int from, to; if (!Int32.TryParse(txtFrom.Text, out from)) { MessageBox.Show("Invalid From value."); return; } if (!Int32.TryParse(txtTo.Text, out to)) { MessageBox.Show("Invalid To value."); return; } // Start the search for primes on another thread. FindPrimesInput input = new FindPrimesInput(from, to); backgroundWorker.RunWorkerAsync(input); } ``` `BackgroundWorker` 开始执行时，会触发 `DoWork` 事件： ```csharp private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e) { // Get the input values. FindPrimesInput input = (FindPrimesInput)e.Argument; // Start the search for primes and wait. // This is the time-consuming part, but it won't freeze the // user interface because it takes place on another thread. int[] pri ```