Recently, I am tied up writing a custom WPF control called FolderPicker, and I come to the scenario that when I update the data source, I want to let the UI thread to update the control UI, basically, at this time, I should force the dispatcher to process the layout message, in Windows Forms application, If we want to process the windows message such as WM_SIZE, WM_PAINT messages in an event handler, we can call the Application.DoEvents(), then how about in WPF?
Actually, WPF doesn't provide such an API, the reason is a bit complicated, and really worth a new post contributing to it, then how to implement such kinda API in WPF, after some examination on this problem, and thanks to Kevin Moore's WPF samples code and Ian Griffiths helpful tips, I finally come up with my own implementation of Application.DoEvents logic.
using System;
using System.Windows;
using System.Windows.Threading;
namespace Sheva.Windows
{
/// <summary>
/// Designates a Windows Presentation Foundation application model with added functionalities.
/// </summary>
public class WpfApplication : Application
{
private static DispatcherOperationCallback exitFrameCallback = new
DispatcherOperationCallback(ExitFrame);
/// <summary>
/// Processes all UI messages currently in the message queue.
/// </summary>
public static void DoEvents()
{
// Create new nested message pump.
DispatcherFrame nestedFrame = new DispatcherFrame();
// Dispatch a callback to the current message queue, when getting called,
// this callback will end the nested message loop.
// note that the priority of this callback should be lower than the that of UI event messages.
DispatcherOperation exitOperation = Dispatcher.CurrentDispatcher.BeginInvoke(
DispatcherPriority.Background, exitFrameCallback, nestedFrame);
// pump the nested message loop, the nested message loop will
// immediately process the messages left inside the message queue.
Dispatcher.PushFrame(nestedFrame);
// If the "exitFrame" callback doesn't get finished, Abort it.
if (exitOperation.Status != DispatcherOperationStatus.Completed)
{
exitOperation.Abort();
}
}
private static Object ExitFrame(Object state)
{
DispatcherFrame frame = state as DispatcherFrame;
// Exit the nested message loop.
frame.Continue = false;
return null;
}
}
}
The theory behind the code shown above is that when pushing a nested message
loop, the nested message loop will immediately process the messages in the
queue, so the layout or other window messages can get processed. But you should
avoid using nested message loop if at all possible, because as Ian Griffths
says:"bad things happen when you nest message pumps."
what does this mean? let me save it for another day:)
Edit: Jessica Fosler has wrriten an awesome article on Application.DoEvents which is really worth a read.
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