WPF Progressbar

参考：http://www.codeproject.com/KB/WPF/WpfProgressBar.aspx

Introduction

This article demonstrates how to use a WPF ProgressBar in a "tight code loop".

Background

For years, progress bars have been very useful and very easy to use in regular Windows Forms applications. All that was necessary was to set the Minimum and Maximum properties, then incrementally modify the Value property to report the progress. When necessary, DoEvents() was inserted to allow the form to refresh and update the progress bar.

WPF progress bars are conceptually the same as Windows progress bars; however, a very noticeable difference is that using standard coding techniques, the WPF Progress Bars do not update correctly while the application is processing. This creates a very undesirable effect, especially since the whole purpose of the progress bar is to... report the progress.

There are many situations where "tight loops" are required in code, such as when data is being read from a file. A standard coding approach is to open the file, then loop through the entire file, reading either lines, characters, or bytes in each pass. A progress bar may be used to report the progress of this operation.

Tight loops present a bit of a challenge for WPF progress bars, and the standard coding techniques do not produce the same results as they do in Windows Forms. Therefore, this article specifically addresses this issue, and demonstrates how to use a WPF ProgressBar in a "tight code loop".

Using the code

The WPF ProgressBar has a "SetValue" method that can be used to update the progress of the ProgressBar. However, this method will not cause the ProgresssBar to refresh when there is a tight coding loop. Therefore, it is necessary to use the Invoke method of the Dispatcher class to update the progress of the ProgressBar and cause it to refresh correctly on the form.

The Dispatcher class provides services for managing the queue of work items for a thread.

One of the arguments of the Dispatcher.Invoke method is a delegate. Since a delegate can be created and used to point to a specific method to invoke, we will create and use one that will point to the ProgressBar's SetValue method, so it will have the exact same signature as well.

Here's the complete example in Visual Basic and C#:

下面是测试代码：

private void button1_Click(object sender, RoutedEventArgs e)
        {
            Action<DependencyProperty, double> UpdateProgressbarValue = (dp, value) => progressBar1.SetValue(dp, value);

            //Configure the ProgressBar
            progressBar1.Minimum = 0;
            progressBar1.Maximum = short.MaxValue;
            progressBar1.Value = 0;

            //Stores the value of the ProgressBar
            double myvalue = 0;

            //Tight Loop: Loop until the ProgressBar.Value reaches the max
            do
            {
                myvalue += 1;

                /*Update the Value of the ProgressBar:
                    1) Pass the "updatePbDelegate" delegate
                       that points to the ProgressBar1.SetValue method
                    2) Set the DispatcherPriority to "Background"
                    3) Pass an Object() Array containing the property
                       to update (ProgressBar.ValueProperty) and the new value */
                Dispatcher.Invoke(UpdateProgressbarValue,
                    System.Windows.Threading.DispatcherPriority.Background,
                    ProgressBar.ValueProperty, myvalue);
            }
            while (progressBar1.Value != progressBar1.Maximum);
        }