c#如何使用BackgroundWorker的ProgressChanged事件在线程中间做UI动作

Question

Here is simple code which has UI actions in the middle of backgroundworker. There is a form and a label "lbProgress" on it. When ProgressPercentage = 2, label is not changed but the action does.

    private void Form1_Load(object sender, EventArgs e)
    {
        bw.WorkerSupportsCancellation = true;
        bw.WorkerReportsProgress = true;
        bw.DoWork += bw_DoWork;
        bw.ProgressChanged += bw_ProgressChanged;
        bw.RunWorkerCompleted += bw_RunWorkerCompleted;
    }
    int ii;
    private void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        ii = 0;

        bw.ReportProgress(1);
        ii += 10;
        Thread.Sleep(1000);

        bw.ReportProgress(2); // here intended to do UI actions

        bw.ReportProgress(3);
        ii += 20;
        Thread.Sleep(1000);
    }
    private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        MessageBox.Show(ii.ToString());
    }
    private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        lbProgress.Text = e.ProgressPercentage.ToString();
        if (e.ProgressPercentage == 2)
        {
            this.Text += ", 00";
            ii += 100;
            Thread.Sleep(1000);
        }
    }

    private void btRun_Click(object sender, EventArgs e)
    {
        bw.RunWorkerAsync();
    }

Is it even possible to do large UI actions in the middle of backgroundworker thread using ProgressChanged event?

Answer 1

This is my answer to your question "Why do I see this behaviour?":

From the documentation of ReportProgress :

The call to the ReportProgress method is asynchronous and returns immediately.

So what happens is:

• The BackgroundWorker calls ReportProgress(2)
• The ProgressChanged event handler for progress 2 is started on the UI thread and blocks. The UI is not updated because the UI thread is blocked . (Hint: the screen is redrawn usually only when the UI thread is idle)
• In the meantime, the BackgroundWorker has called ReportProgress(3)
• The Blocked event handler finally finishes
• The ProgressChanged event handler for progress 3 is run updating lbProgress.Text to "3" before your eyes had a chance to notice that it was "2" just a microsecond before (if it was rendered at all).