namespace System.Windows.Threading
{
public sealed class DispatcherHooks
{
// boring stuff elided
public event EventHandler DispatcherInactive { add; remove; }
public event DispatcherHookEventHandler OperationPosted { add; remove; }
public event DispatcherHookEventHandler OperationCompleted { add; remove; }
public event DispatcherHookEventHandler OperationPriorityChanged { add; remove; }
public event DispatcherHookEventHandler OperationAborted { add; remove; }
}
}
The names of these events are self-explanatory. DispatcherInactive doesn't need to provide any additional information, but the other four events need to identify the affected operation.
namespace System.Windows.Threading
{public delegate void DispatcherHookEventHandler(object sender, DispatcherHookEventArgs e);
public sealed class DispatcherHookEventArgs : EventArgs
{
public DispatcherHookEventArgs(DispatcherOperation operation) { ... }
public Dispatcher Dispatcher { get; }
public DispatcherOperation Operation { get; }
}
}
Now that we are familiar with the DispatcherOperation class, we know what information is easily available to us once we receive a set of DispatcherHookEventArgs. Again, I find it odd that DispatcherOperation doesn't expose its Name property, but we can use reflection to easily get around that.
Starting with a standard WPF application, I edited my App.xaml.cs file to hook into the UI Dispatcher and log events to standard output.
public partial class App : Application
{
private PropertyInfo namePropertyInfo;
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
namePropertyInfo = typeof (DispatcherOperation).
BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.NonPublic);
var hooks = Dispatcher.Hooks;
hooks.DispatcherInactive += HandleDispatcherInactive;
hooks.OperationAborted += HandleOperationAborted;
hooks.OperationCompleted += HandleOperationCompleted;
hooks.OperationPosted += HandleOperationPosted;
hooks.OperationPriorityChanged += HandleOperationPriorityChanged
}
protected override void OnExit(ExitEventArgs e)
{
var hooks = Dispatcher.Hooks;
hooks.DispatcherInactive -= HandleDispatcherInactive;
hooks.OperationAborted -= HandleOperationAborted;
hooks.OperationCompleted -= HandleOperationCompleted;
hooks.OperationPosted -= HandleOperationPosted;
hooks.OperationPriorityChanged -= HandleOperationPriorityChanged
base.OnExit(e);
}
private void HandleDispatcherInactive(objec
{
LogOperationEvent("Inactive", null);
}
private void HandleOperationAborted(object sender, DispatcherHookEventArgs e)
{
LogOperationEvent("Aborted", e.Operation);
}
private void HandleOperationCompleted(objec
{
LogOperationEvent("Completed", e.Operation);
}
private void HandleOperationPosted(object sender, DispatcherHookEventArgs e)
{
LogOperationEvent("Posted", e.Operation);
}
private void HandleOperationPriorityChanged
{
LogOperationEvent("
}
private void LogOperationEvent(string eventName, DispatcherOperation operation)
{
var builder = new StringBuilder();
builder.AppendFormat("{0,-16}"
if (operation != null)
{
var operationName = namePropertyInfo.GetValue(
builder.AppendFormat("{0,-16}"
builder.Append(operationName);
}
Console.WriteLine(builder.
}
}
When you run this example, interact with the main window by clicking on it, typing characters, moving it, resizing it, etc. The output window will show you a stream of operations posted to the Dispatcher's work queue to respond to your requests:
Posted Input System.Windows.Input.
Posted Background System.Windows.Input.
Inactive
Posted Input System.Windows.Interop.
Completed Input System.Windows.Interop.
Inactive
Posted Input System.Windows.Interop.
Posted Render System.Windows.Media.
Posted Inactive System.Windows.Input.
Posted Inactive System.Windows.Threading.
Completed Render System.Windows.Media.
Completed Input System.Windows.Interop.
Inactive
PriorityChanged Inactive System.Windows.Threading.
PriorityChanged Inactive System.Windows.Input.
Completed Background System.Windows.Threading.
Completed Input System.Windows.Input.
Inactive
Posted Normal System.Windows.Interop.
Posted Input System.Windows.Input.
Aborted Normal System.Windows.Interop.
Completed Input System.Windows.Input.
Inactive
Posted Input System.Windows.Interop.
Posted Input System.Windows.Input.
Posted Render System.Windows.Media.
Posted Input System.Windows.Input.
Posted Input System.Windows.Input.
Posted Loaded System.Windows.
Posted Background System.Windows.Input.
Posted Normal System.Windows.Application.
Posted Normal MS.Win32.HwndWrapper.
I considered putting this information in a separate WPF Window, but I was worried about how that would affect the Dispatcher queue! I hope to play with multiple UI Dispatchers shortly and should then be able to work up a separately-dispatched "spy" window. For now, this output gives us good feedback on what our main Dispatcher is doing for us; naturally, you could apply the same technique to non-UI Dispatchers.
[Wolfgang Gartner: Illmerica -- YouTube]
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