聊聊WPF中的Dispatcher

DispatcherObject，Dispatcher，Thread之间的关系

我们都知道WPF中的控件类都是从System.Windows.Threading.DispatcherObject继承而来，而DispatcherObject又在构造时与当前线程的Dispatcher关联起来，CurrentDispatcher如果为null则会主动new一个Dispatcher并且在构造时和当前创建它的线程关联起来了。因此整个链为DispatcherObject <- Dispatcher <- Thread. 具体我们一起看看反编译的红色代码：

public abstract class DispatcherObject

{

private Dispatcher _dispatcher;

[EditorBrowsable(EditorBrowsableState.Advanced)]

public Dispatcher Dispatcher

{

get

{

return this ._dispatcher;

}

protected DispatcherObject()

{

base .\u002Ector();

this ._dispatcher = Dispatcher.CurrentDispatcher;

}

........................................................

}

public sealed class Dispatcher

{

public static Dispatcher CurrentDispatcher

{

get

{

return Dispatcher.FromThread(Thread.CurrentThread) ?? new Dispatcher();

}

private Dispatcher()

{

.............................

Dispatcher._tlsDispatcher = this ;

this ._dispatcherThread = Thread.CurrentThread;

.............................

}

..............................

}

这样设计的原则就保证：界面元素只有被创建它的线程访问

Dispatcher中Invoke，BeginInvoke和Win32中SendMessage，PostMessage的关系

上面我们提到wpf的界面元素只有被创建它的线程来访问，如果我们想在后台或者其他线程里该怎么办？

答案就是利用Dispatcher的Invoke和BeginInvoke，作用就是把委托放到界面元素关联的Dispatcher里的工作项里，然后此Dispatcher关联的线程进行执行。

所不同的是Invoke是在关联的线程里同步执行委托，而BeginInvoke是在关联的线程里异步执行委托。

做过Win32或者MFC编程的童鞋们都知道win32中有SendMessage和PostMessage类似的概念，这些概念有什么内在关系呢？

其实Dispatcher的Invoke和BeginInvoke都是调用Win32的PostMessage传递窗体句柄和消息号，反编译代码如下：

private bool RequestForegroundProcessing()

{

if (this._postedProcessingType >= 2)

return true;

if (this._postedProcessingType == 1)

SafeNativeMethods.KillTimer(new HandleRef((object) this, this._window.Value.Handle), 1);

this._postedProcessingType = 2;

return MS.Win32.UnsafeNativeMethods.TryPostMessage(new HandleRef((object) this, this._window.Value.Handle), Dispatcher._msgProcessQueue, IntPtr.Zero, IntPtr.Zero);

}

只不过Invoke在PostMessage后调用了内部返回值DispatcherOperation的wait方法，在执行结束后才返回。反编译代码如下：

DispatcherOperation dispatcherOperation = this.BeginInvokeImpl(priority, method, args, isSingleParameter);

if (dispatcherOperation != null)

{

int num = (int) dispatcherOperation.Wait(timeout);

if (dispatcherOperation.Status == DispatcherOperationStatus.Completed)

obj = dispatcherOperation.Result;

else if (dispatcherOperation.Status == DispatcherOperationStatus.Aborted)

obj = (object) null;

else

dispatcherOperation.Abort();

}

WPF的消息泵和Win32的消息泵之间的关系

WPF的窗体程序都必须隐式或者显式调用Application.Run()来初始化WPF窗体。当Application.Run()调用时，会在其内部调用Dispatcher.Run()方法。最终会在PushFrame()方法内初始化消息泵。

具体为：Application.Run() -> Dispatcher.Run() -> Dispatcher.PushFrame() -> Dispatcher.PushFrameImpl()

private void PushFrameImpl(DispatcherFrame frame)

{

MSG msg = new MSG();

++this._frameDepth;

try

{

SynchronizationContext current = SynchronizationContext.Current;

bool flag = current != this._dispatcherSynchronizationContext;

try

{

if (flag)

SynchronizationContext.SetSynchronizationContext((SynchronizationContext) this._dispatcherSynchronizationContext);

while (frame.Continue && this.GetMessage(ref msg, IntPtr.Zero, 0, 0))

this.TranslateAndDispatchMessage(ref msg);

if (this._frameDepth != 1 || !this._hasShutdownStarted)

return;

this.ShutdownImpl();

}

finally

{

if (flag)

SynchronizationContext.SetSynchronizationContext(current);

}

finally

{

--this._frameDepth;

if (this._frameDepth == 0)

this._exitAllFrames = false;

}

Ok,从上述反编译的代码可以看到，WPF还是通过Dispatcher内部实现了传统的Win32消息循环，如GetMessage，TranslateMessage，DispatchMessage。

下面是win32消息泵的实现，大家可以对比下：

BOOL bRet;

while( (bRet = GetMessage( &msg, hWnd, 0, 0 )) != 0)
{
    if (bRet == -1)
    {
        // handle the error and possibly exit
    }
    else
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
}

巴特西

聊聊WPF中的Dispatcher

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