有人给我发了电子邮件，询问我是否有WPF的WaitOneAndPump版本。目标是等待句柄（类似于WaitHandle.WaitOne）并在等待的同时在相同的堆栈帧上抽取WPF Dispatcher事件。
我真的不认为这样的API应该用于任何生产代码，无论是WinForms还是WPF （可能除了UI自动化之外）。 WPF没有公开WinForms'DoEvents的显式版本，这是一个非常好的设计决策，给定the fair share of abuse the DoEvents API has been taking。
尽管如此，这个问题本身很有意思，所以我将把它作为一个练习并发布任何我想出的答案。如果感兴趣，请随意发布您自己的版本。
答案 0 :(得分：6)
版本WaitOneAndPump我提出了使用DispatcherHooks Events和MsgWaitForMultipleObjectsEx，以避免运行busy-waiting loop。
同样，在生产代码中使用此WaitOneAndPump（或任何其他嵌套的消息循环变体）几乎总是一个糟糕的设计决策。我只能想到两个.NET合法使用嵌套消息循环的API：Window.ShowDialog和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
}
此代码尚未经过严格测试，可能包含错误，但我认为我的概念是正确的，就问题而言。
答案 1 :(得分：2)
我之前必须做类似的事情，用UI自动化测试UI的进程。实现是这样的
public static bool WaitOneAndPump(WaitHandle handle, int timeoutMillis)
{
bool gotHandle = false;
Stopwatch stopwatch = Stopwatch.StartNew();
while(!(gotHandle = waitHandle.WaitOne(0)) && stopwatch.ElapsedMilliseconds < timeoutMillis)
{
DispatcherFrame frame = new DispatcherFrame();
Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background,
new DispatcherOperationCallback(ExitFrame), frame);
Dispatcher.PushFrame(frame);
}
return gotHandle;
}
private static object ExitFrame(object f)
{
((DispatcherFrame)f).Continue = false;
return null;
}
我之前遇到过低于后台优先级的问题。我相信，问题是WPF命中测试的优先级更高，因此根据鼠标的位置，ApplicationIdle优先级可能永远不会运行。
<强>更新
所以看起来上面的方法会挂掉CPU。这是一种替代方法，在方法为消息提取时使用DispatcherTimer进行检查。
public static bool WaitOneAndPump2(this WaitHandle waitHandle, int timeoutMillis)
{
if (waitHandle.WaitOne(0))
return true;
DispatcherTimer timer = new DispatcherTimer(DispatcherPriority.Background)
{
Interval = TimeSpan.FromMilliseconds(50)
};
DispatcherFrame frame = new DispatcherFrame();
Stopwatch stopwatch = Stopwatch.StartNew();
bool gotHandle = false;
timer.Tick += (o, e) =>
{
gotHandle = waitHandle.WaitOne(0);
if (gotHandle || stopwatch.ElapsedMilliseconds > timeoutMillis)
{
timer.IsEnabled = false;
frame.Continue = false;
}
};
timer.IsEnabled = true;
Dispatcher.PushFrame(frame);
return gotHandle;
}