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聊聊WPF中的Dispatcher
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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);
}
}
