BackgroundWorker 组件用来执行诸如数据库事务、文件下载等耗时的异步操作。
开始
在应用程序中添加一个BackgroundWorker实例,如果用的是VS,可以从工具上直接拖到应用程序:
BackgroundWorker backgroundWorker1 = new BackgroundWorker();
为了开始在后台操作,必须调用BackgroundWorker的RunWorkerAsync()方法,当调用此方时,BackgroundWorker 通过触发DoWork 事件,开始执行后台操作,DoWork 事件的代码是在另一个线程里执行的。代码如下:
代码一:
BackgroundWorker backgroundWorker1 = new BackgroundWorker(); //在另一个线程里开始操作(btnStart是一个按钮控件) //也可以利用RunWokerAsync()方法传递参数, private void btnStart_Click(object sender, EventArgs e) { backgroundWorker1.RunWokerAsync(2000/*参数是可选的*/); } //DoWork事件在另一个线程里执行 private void backgroundWorker1_DoWork(objectsender,DoWorkeventArgs e) { //允许长时间的操作 int input = (int)e.Argument; Thread.Sleep(input); }
显示后台操作进度
为了显示后台操作的执行进度,首先要使WorkerReportsProgress 等于true,然后调用BackgroundWorker的ReportProgress()方法,通过它传递操作完成的进度值,此外,该方法触发ProgressChanged事件,在是此事件中,通过ProgressChangedEventArgs的实例,接收到主线程传递过来的参数。
代码二:
BackgroundWorker backgroundWorker1 = new BackgroundWorker(); backgroundWorker1.WorkerReportsProgress = true; private void btnStart_Click(object sender, EventArgs e) { backgroundWorker1.RunWokerAsync(); } private void backgroundWorker1_DoWork(object sender, DoWorkeventArgs e) { for (int i = 1; i < 11; i++) { Thread.Sleep(2000); backgroundWorker1.ReportProgress(i*10); } } private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e) { //progressBar1是ProgressBar控件 progressBar1.Value = e.ProgressPercentage; }
取消后台操作
为了使 BackgroundWorker 可以取消后台正在执行的操作,首先要把属性WorkerSupportsCancellation 的值设置为 true。接着调用CancelAsync()方法,该方法使得属性CancellationPending 为true,利用CancellationPending 属性,可以判断是否取消后台异步操作。
代码三:
BackgroundWorker backgroundWorker1 = new BackgroundWorker(); backgroundWorker1.WorkerReportsProgress = true; backgroundWorker1.WorkerSupportsCancellation = true; private void btnStart_Click(object sender, EventArgs e) { backgroundWorker1.RunWokerAsync(); } private void btnCancel_Click(object sender, EventArgs e) { backgroundWorker1.CancelAsync(); } private void backgroundWorker1_DoWork(object sender, DoWorkeventArgs e) { for (int i = 1; i < 11; i++) { Thread.Sleep(2000); backgroundWorker1.ReportProgress(i*10); if(backgroundWorker1.CancellationPending) { e.Cancel = true; return; } } } private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e) { progressBar1.Value = e.ProgressPercentage; }
后台操作完成后,反馈给用户
当后台操作完成以后,无论是completed 还是cancelled,则RunWorkerCompleted 事件被触发,通过此方法可以将后台操作的完成结果反馈给用户;
另外,通过RunWorkerCompletedEventArgs实例的Cancelled 属性,以判断是否是Cancel操作使得后台操作终止;
代码片段:
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) { if(e.Cancelled) { MessageBox.Show("Operation Cancelled"); } else { MessageBox.Show("OperationCompleted"); } }
从后台操作返回值
在执行DoWork 事件时,DoWorkEventArgs 实例的Result 属性,返回值到用户;在RunWorkerCompleted 事件里,RunWorkerCompletedEventArgs 实例的Result 属性接收值;
代码片段:
private void backgroundWorker1_DoWork(object sender, DoWorkeventArgs e) { Thread.Sleep(2000); //在此处设置返回值 e.Result = 10; } private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) { //在此处接收传递回来的值 int returnValue = (int)e.Result; }