异步编程入门指南

### 异步编程入门指南 #### 1. BackgroundWorker 类的使用 BackgroundWorker 类虽为 GUI 编程设计，但我们先通过控制台程序来了解它。此程序会创建一个后台线程对一系列数字求和，过程中会多次检查是否被取消。若被取消，会清理并退出；若完成，会将总和存储在 Result 字段后退出。同时，主线程也会对自己的数字序列求和，并报告自身总和以及后台线程的结果。 以下是完整代码： ```csharp using System; using System.ComponentModel; // Must have this namespace using System.Threading; // Must have this namespace namespace ConsoleBackgroundWorker { class DoBackgroundwork { BackgroundWorker bgWorker = new BackgroundWorker(); public long BackgroundTotal { get; private set; } public bool CompletedNormally { get; private set; } // Constructor public DoBackgroundwork() { // Set BackgroundWorker properties bgWorker.WorkerReportsProgress = true; bgWorker.WorkerSupportsCancellation = true; // Connect handlers to BackgroundWorker object. bgWorker.DoWork += DoWork_Handler; bgWorker.ProgressChanged += ProgressChanged_Handler; bgWorker.RunWorkerCompleted += RunWorkerCompleted_Handler; } public void StartWorker() { if ( !bgWorker.IsBusy ) bgWorker.RunWorkerAsync(); } // This just calculates the sum of the integers from 0 to the input value. public static long CalculateTheSequence( long value ) { long total = 0; for ( int i=0; i < value; i++ ) total += i; return total; } public void DoWork_Handler( object sender, DoWorkEventArgs args ) { BackgroundWorker worker = sender as BackgroundWorker; // Do the background calculation long total = 0; for ( int i = 1; i <= 5; i++ ) { // Each time through the loop, check to see if we've been cancelled if ( worker.CancellationPending ) { args.Cancel = true; worker.ReportProgress( -1 ); break; } else { // If we haven't been cancelled, then continue the calculation. total += CalculateTheSequence( i * 10000000 ); worker.ReportProgress( i * 20 ); // Slow the program down to a more comfortable output rate // just for this demo. Thread.Sleep( 300 ); } } args.Result = total; // Store the result and exit. } // Handle input from background thread. private void ProgressChanged_Handler ( object sender, ProgressChangedEventArgs args ) { string output = args.ProgressPercentage == -1 ? " Cancelled" : string.Format(" {0}%", args.ProgressPercentage ); Console.WriteLine( output ); } // On completion of background thread, summarize and store the result. private void RunWorkerCompleted_Handler ( object sender, RunWorkerCompletedEventArgs args ) { CompletedNormally = !args.Cancelled; BackgroundTotal = args.Cancelled ? 0 : (long) args.Result; // Cast from object } public void Cancel() { if( bgWorker.IsBusy ) bgWorker.CancelAsync(); } } class Program { static void Main() { GiveInstructionsToTheUser(); OutputTheSummaryHeaders(); // Create and Start the background worker DoBackgroundwork bgw = new DoBackgroundwork(); bgw.StartWorker(); // Start the computation on the main thread. Each time through the loop, // check to see whether the user has cancelled the background thread. // After the calculation, add a short sleep, just to slow the program // down enough so the main thread doesn't run faster than the background. long mainTotal = 0; for ( int i = 0; i < 5; i++ ) { if ( Program.CheckForCancelInput() ) ```