Updating the GUI Before a Method Completes
April 28, 2008
As a Web developer whose formal programming training focused only on console applications and simple GUI apps, it was not immediately obvious to me how to update a (Windows Forms) GUI while a method was still running. Once I decided it was worth the effort to learn how to do so, I was not surprised to find that it is rather easy, using BeginInvoke().
Problem: I have a long-running method and want to update the mouse cursor asynchronously. More accurately, I need to span a couple of methods. There is an image in a PictureBox from which the user needs to select a slice, and the user wants to be able to move the selection window around the image without redrawing the box ("redrawing" is of course from the user's perspective; from the programmer's perspective, the box is redrawn each time it is moved). I want to use this.Cursor = Cursors.Hand in the MouseDown event, leave it in this state during the MouseMove event, and restore the cursor to Cursors.Default in the MouseUp event.
Solution: Create a delegate method, i.e. public delegate void InvokeDelegate();. Create a couple of functions that handle the cursor change:
/// <summary>
/// Changes the cursor to a hand
/// </summary>
private void cursorMove()
{
this.Cursor = Cursors.Hand;
}
/// <summary>
/// Changes the cursor back to the default
/// </summary>
private void cursorDefault()
{
this.Cursor = Cursors.Default;
}
Finally, path these method names into InvokeDelegate whilst using BeginInvoke(Delegate method):
private void pbxMain_MouseDown(object sender, MouseEventArgs e)
{
…
this.BeginInvoke(new InvokeDelegate(cursorMove));
…
}
private void pbxMain_MouseUp(object sender, MouseEventArgs e)
{
…
this.BeginInvoke(new InvokeDelegate(cursorDefault));
…
}