如何在进度栏中显示图像加载进度c＃

Question

I want to know how to show image loading progress in progress-bar in win form.

my code is

private void Form2_Load(object sender, EventArgs e)//load form and get profile and avatar
    {
        backgroundWorker1.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker1_ProgressChanged);
        backgroundWorker1.RunWorkerCompleted += new RunWorkerCompletedEventHandler(backgroundWorker1_RunWorkerCompleted);
        backgroundWorker1.RunWorkerAsync();
    }


  private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        label2.Text = "Avatar Loaded"; 
    }




private void backgroundWorker1_ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)//showing progress
    {
        if (InvokeRequired)
        {
            BeginInvoke(new Action(() => backgroundWorker1_ProgressChanged(sender, e)));
        }
        else
        {
            if (progressBarX1.Value != e.ProgressPercentage)
            {
                progressBarX1.Value = e.ProgressPercentage;
                progressBarX1.Refresh();
            }
        }
    }

 private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)//loading avatar
    {
        WebClient wc = new WebClient();
        Stream stream = wc.OpenRead("http://avatar.nimbuzz.com/getAvatar?jid=" + textBox1.Text);
        pictureBox1.Image = (Image.FromStream(stream));
    }

The image loads successfully but do not show any progress in progress-bar

Answer 1

The problem with your code is that backgroundWorker1_ProgressChanged() is called in the context of the worker thread but UI updates can only be done within the UI thread (main thread). In your scenario you must call:

progressBarX1.Value = e.ProgressPercentage;

within the UI thread. There are many ways how to do this but a simple one is to use InvokeReqzired() to check if you are in the UI thread and you are not then call BeginInvoke() to send the execution of that command to the UI-/main-thread. The following should do the trick:

private void backgroundWorker1_ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
  if (InvokeRequired)
  {
    BeginInvoke(new Action(() => backgroundWorker1_ProgressChanged(sender, e)));
  }
  else
  {
    progressBarX1.Value = e.ProgressPercentage;
  }
}

Update: I would also change:

    backgroundWorker1.RunWorkerAsync();
    backgroundWorker1.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker1_ProgressChanged);

to:

    backgroundWorker1.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker1_ProgressChanged);
    backgroundWorker1.RunWorkerAsync();

so the progress event is assigned (and therefore in place) before starting the asynchronous work. Otherwise you may miss a progress call.

Update 2:

You also need the following line to execute in the UI thread:

pictureBox1.Image = (Image.FromStream(stream));

To do so use the Completed event of BackgroundWorker and bass the image data using the result parameter. Then in the eventhandler use InvokeRequired() and BeginInvoke() just like in the Progress event).

Uodate 3:

Remeins unchanged is good as you don't get a crash because of not calling UI stuff in the UI thread ;-)

Try to force a repaint on the control by calling:

progressBarX1.Refresh();

(just after you assigned a new value to it).

So the code in the progress event looks like this:

private void backgroundWorker1_ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
if (InvokeRequired)
{
  BeginInvoke(new Action(() => backgroundWorker1_ProgressChanged(sender, e)));
}
else
{
  if (progressBarX1.Value != e.ProgressPercentage)
  {
    progressBarX1.Value = e.ProgressPercentage;
    progressBarX1.Refresh();
  }
}

}

If that still doesn't work check that your main/UI thread is not blocked or totally busy.

Update 4:

Just to make sure: you need to enable progress-reporting on the background worker and call its ReportProgress() method from time to time within the backgroundWorker1_DoWork(). See the following MS-Tutorial for more information on usage of the BackgroundWorker: http://msdn.microsoft.com/en-us/library/cc221403(v=vs.95).aspx

Enabling progress reporting on a backgroundworker:

backgroundWorker1.WorkerReportsProgress = true;

or use the form-designer and set the property WorkerReportsProgress to True. Then you still need to call the backgroundWorker1.ReportProgress() from time to time.

Update 5:

Ok, lets give it a complete try. I've checked some reference docs from MS so in case of the backgroundworker ProgressChanged and Completed events you don't need to BeginInvoke() them as MS does this already for you.

private void Form2_Load(object sender, EventArgs e)
{
    getto();

    backgroundWorker1.WorkerReportsProgress = true;
    backgroundWorker1.ProgressChanged -= backgroundWorker1_ProgressChanged;
    backgroundWorker1.ProgressChanged += backgroundWorker1_ProgressChanged;
    backgroundWorker1.RunWorkerCompleted -= backgroundWorker1_Completed;
    backgroundWorker1.RunWorkerCompleted += backgroundWorker1_Completed;
    backgroundWorker1.RunWorkerAsync();
}

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    using (var wc = new WebClient())
    {
        wc.DownloadProgressChanged += (sender, e) => backgroundWorker1.ReportProgress(sender, e);
        using (var stream = wc.OpenRead("http://avatar.nimbuzz.com/getAvatar?jid=" + textBox1.Text))
        {
            e.Result = (Image.FromStream(stream));
        }
    }
}

private void backgroundWorker1_ProgressChanged(object sender, System.ComponentModel.ProgressChangedEventArgs e)
{
    progressBarX1.Value = e.ProgressPercentage;
    progressBarX1.Refresh();
}

private void backgroundWorker1_Completed(object sender, DoWorkEventArgs e)
{
    pictureBox1.Image = e.Result;
}

Answer 2

Well, i would use the ProgressChanged Event from a WebClient to get the result of how far the download progress actually is. Then you can't save it from the thread of the backgroundworker, so you gotta do this when you have the file.

UPDATED

    private BackgroundWorker bgw;

    private void Form1_Load(object sender, EventArgs e)
    {
        bgw = new BackgroundWorker();
        bgw.DoWork += bgw_DoWork;
        bgw.WorkerReportsProgress = true;
        bgw.RunWorkerCompleted += bgw_RunWorkerCompleted;
        bgw.ProgressChanged += bgw_ProgressChanged;
        bgw.RunWorkerAsync();
    }

    void bgw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        progressBar1.Value = e.ProgressPercentage;
    }

    void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        pictureBox1.Image = (Image)e.Result;
    }

    void bgw_DoWork(object sender, DoWorkEventArgs e)
    {
        WebClient wc = new WebClient();
        wc.DownloadProgressChanged += wc_DownloadProgressChanged;
        Stream stream = wc.OpenRead("" + textBox1.Text);
        e.Result = (Image.FromStream(stream));
    }

    void wc_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        bgw.ReportProgress(e.ProgressPercentage);
    }