Someone emailed me and asked if I have a version of WaitOneAndPump
for WPF. The goal is to wait for a handle (similar to WaitHandle.WaitOne
) and pump WPF Dispatcher events while waiting, on the same stack frame.
I really don't think an API like this should be used in any production code, either for WinForms or WPF (perhaps, besides for UI Automation). WPF doesn't expose an explicit version of WinForms' DoEvents
, which is a very good design decision, given the fair share of abuse the DoEvents
API has been taking.
Nevertheless, the question itself is interesting, so I'm going to take it as an exercise and post whatever I may come up with as the answer. Feel free to post your own version too, if interested.
The version of WaitOneAndPump
I've come up with uses DispatcherHooks
Events and MsgWaitForMultipleObjectsEx
, to avoid running a busy-waiting loop.
Again, using this WaitOneAndPump
(or any other nested message loop variants) in the production code is almost always will be a bad design decision. I can think of only two .NET APIs which legitimately use a nested message loop: Window.ShowDialog
and Form.ShowDialog
.
using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Threading;
namespace Wpf_21642381
{
#region MainWindow
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
this.Loaded += MainWindow_Loaded;
}
// testing
async void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
await Dispatcher.Yield(DispatcherPriority.ApplicationIdle);
try
{
Func<Task> doAsync = async () =>
{
await Task.Delay(6000);
};
var task = doAsync();
var handle = ((IAsyncResult)task).AsyncWaitHandle;
var startTick = Environment.TickCount;
handle.WaitOneAndPump(5000);
MessageBox.Show("Lapse: " + (Environment.TickCount - startTick));
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
}
#endregion
#region WaitExt
// WaitOneAndPump
public static class WaitExt
{
public static bool WaitOneAndPump(this WaitHandle handle, int millisecondsTimeout)
{
using (var operationPendingMre = new ManualResetEvent(false))
{
var result = false;
var startTick = Environment.TickCount;
var dispatcher = Dispatcher.CurrentDispatcher;
var frame = new DispatcherFrame();
var handles = new[] {
handle.SafeWaitHandle.DangerousGetHandle(),
operationPendingMre.SafeWaitHandle.DangerousGetHandle() };
// idle processing plumbing
DispatcherOperation idleOperation = null;
Action idleAction = () => { idleOperation = null; };
Action enqueIdleOperation = () =>
{
if (idleOperation != null)
idleOperation.Abort();
// post an empty operation to make sure that
// onDispatcherInactive will be called again
idleOperation = dispatcher.BeginInvoke(
idleAction,
DispatcherPriority.ApplicationIdle);
};
// timeout plumbing
Func<uint> getTimeout;
if (Timeout.Infinite == millisecondsTimeout)
getTimeout = () => INFINITE;
else
getTimeout = () => (uint)Math.Max(0, millisecondsTimeout + startTick - Environment.TickCount);
DispatcherHookEventHandler onOperationPosted = (s, e) =>
{
// this may occur on a random thread,
// trigger a helper event and
// unblock MsgWaitForMultipleObjectsEx inside onDispatcherInactive
operationPendingMre.Set();
};
DispatcherHookEventHandler onOperationCompleted = (s, e) =>
{
// this should be fired on the Dispather thread
Debug.Assert(Thread.CurrentThread == dispatcher.Thread);
// do an instant handle check
var nativeResult = WaitForSingleObject(handles[0], 0);
if (nativeResult == WAIT_OBJECT_0)
result = true;
else if (nativeResult == WAIT_ABANDONED_0)
throw new AbandonedMutexException(-1, handle);
else if (getTimeout() == 0)
result = false;
else if (nativeResult == WAIT_TIMEOUT)
return;
else
throw new InvalidOperationException("WaitForSingleObject");
// end the nested Dispatcher loop
frame.Continue = false;
};
EventHandler onDispatcherInactive = (s, e) =>
{
operationPendingMre.Reset();
// wait for the handle or a message
var timeout = getTimeout();
var nativeResult = MsgWaitForMultipleObjectsEx(
(uint)handles.Length, handles,
timeout,
QS_EVENTMASK,
MWMO_INPUTAVAILABLE);
if (nativeResult == WAIT_OBJECT_0)
// handle signalled
result = true;
else if (nativeResult == WAIT_TIMEOUT)
// timed out
result = false;
else if (nativeResult == WAIT_ABANDONED_0)
// abandonded mutex
throw new AbandonedMutexException(-1, handle);
else if (nativeResult == WAIT_OBJECT_0 + 1)
// operation posted from another thread, yield to the frame loop
return;
else if (nativeResult == WAIT_OBJECT_0 + 2)
{
// a Windows message
if (getTimeout() > 0)
{
// message pending, yield to the frame loop
enqueIdleOperation();
return;
}
// timed out
result = false;
}
else
// unknown result
throw new InvalidOperationException("MsgWaitForMultipleObjectsEx");
// end the nested Dispatcher loop
frame.Continue = false;
};
dispatcher.Hooks.OperationCompleted += onOperationCompleted;
dispatcher.Hooks.OperationPosted += onOperationPosted;
dispatcher.Hooks.DispatcherInactive += onDispatcherInactive;
try
{
// onDispatcherInactive will be called on the new frame,
// as soon as Dispatcher becomes idle
enqueIdleOperation();
Dispatcher.PushFrame(frame);
}
finally
{
if (idleOperation != null)
idleOperation.Abort();
dispatcher.Hooks.OperationCompleted -= onOperationCompleted;
dispatcher.Hooks.OperationPosted -= onOperationPosted;
dispatcher.Hooks.DispatcherInactive -= onDispatcherInactive;
}
return result;
}
}
const uint QS_EVENTMASK = 0x1FF;
const uint MWMO_INPUTAVAILABLE = 0x4;
const uint WAIT_TIMEOUT = 0x102;
const uint WAIT_OBJECT_0 = 0;
const uint WAIT_ABANDONED_0 = 0x80;
const uint INFINITE = 0xFFFFFFFF;
[DllImport("user32.dll", SetLastError = true)]
static extern uint MsgWaitForMultipleObjectsEx(
uint nCount, IntPtr[] pHandles,
uint dwMilliseconds, uint dwWakeMask, uint dwFlags);
[DllImport("kernel32.dll")]
static extern uint WaitForSingleObject(IntPtr hHandle, uint dwMilliseconds);
}
#endregion
}
This code hasn't been heavily tested and may contain bugs, but I think I've got the concept right, as far as the question goes.
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