WPF Dispatcher 一次小重构

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几个月之前因为项目需要，需要实现一个类似于WPF Dispatcher类的类，来实现一些线程的调度。之前因为一直做Asp.Net，根本没有钻到这个层次去，做的过程中，诸多不顺，重构了四五次，终于实现，满足项目要求。 Dispatcher的源码对我来说，看的确实很累，各种累关联，不知所云。当时仅有的周永恒的博客看了六七遍也只是知道了大概的轮廓。今天我这里讲的，就是按照当时的项目需求的方向所理解和收获的一些知识，分享出来。太全面的东西，我讲出来只怕误人子弟了，大家还是去参照老周的博客吧。O(∩_∩)O~

一、Dispatcher几个重要的方法。

Dispatcher译名是调度者，他的工作就是不断的从线程的消息队列中取出一个消息，通过TranslateAndDispatchMessage派发出去，WndProcHook来处理捕获到的消息。

1.Run()

Run()是dispatcher的一个静态方法，这个方法在WPF应用程序运行起来之前，已经被调用了，可以通过调用堆栈来看到。

而Run()调用的是一个所谓的消息泵，---》PushFrame(new DispatcherFrame());---》PushFrameImpl(frame);

它的源码如下，关键的就是拿个while循环，通过GetMessage来不断的获取消息，然后派发出去。

[SecurityCritical, SecurityTreatAsSafe ]
private void PushFrameImpl(DispatcherFrame frame)
{
MSG msg = new MSG();
//.............................
while(frame.Continue)
{
if (!GetMessage(ref msg, IntPtr.Zero, , ))
break; TranslateAndDispatchMessage(ref msg);
} // If this was the last frame to exit after a quit, we
//................. 略了一些代码
}

而这里的GetMessage和TranslateAndDispactchMessage 本质上都是调用的Win32的API函数。和GetMessage对应的还有个PeekMessage,前者一直等待有消息到才返回，后者获取一次就返回。上文的意思就是遇上错误或者窗口退出消息就中断。在WPF中还有一个隐藏的窗口，是窗口类，但没有窗体，专门来捕获派发过来的消息（工作线程中委Invoke或者BeginInvoke的方法也是包装成了消息）。在Dispatcher的构造函数中：

MessageOnlyHwndWrapper window = new MessageOnlyHwndWrapper();
_window = new SecurityCriticalData<MessageOnlyHwndWrapper>( window );
_hook = new HwndWrapperHook(WndProcHook);
_window.Value.AddHook(_hook);

而在WndProcHook 方法中，主要的就是处理消息。而其中又调用了ProcessQueue() 这个方法。

2. ProcessQueue();

在ProcessQueue中，主要是处理BeginInvoke和Invoke 进入队列的任务（或者说操作）。关键代码如下,从队列中 dequeue()出来后，invoke执行了。而这里的DispatcherOperation就包含了我们之前Invoke或者BeginInvoke进来的操作，到这里就是正真被执行了。各个版本的源码会有些出入，下载安装的.net4.0源码和反编译出来的源码是有些出入的。

private void ProcessQueue()
{
DispatcherPriority maxPriority = DispatcherPriority.Invalid; // NOTE: should be Priority.Invalid
DispatcherOperation op = null;
DispatcherHooks hooks = null;
//..............
{
op = _queue.Dequeue();
hooks = _hooks;
}
// .........
op.Invoke(); //..........
}

3.BeginInvokeImpl()

上面好像有点倒叙，讲了出队列，没有说入队列。我们在工作线程调用的方法，如下

private void ButtonOnclick(object sender, EventArgs e)
{
Button.Dispatcher.BeginInvoke(DispatcherPriority.Normal, new Action(() =>
{
Button.Content = _random.NextDouble();//显示一个随机值。
}));
}

BeginInvoke和Invoke，Dispatcher都重载了多个版本，Invoke的最终调用的是InvokeImpl，BeginInvoke最终调用的是BeginInvokeImpl，而InvokeImpl内部还是调用的BeginInvokeImpl。在这个方法中关键代码如下：

[FriendAccessAllowed] //used by DispatcherExtensionMethods in System.Windows.Presentation.dll
internal DispatcherOperation BeginInvokeImpl(DispatcherPriority priority, Delegate method, object args, int numArgs)
{
ValidatePriority(priority, "priority");
if(method == null)
{
throw new ArgumentNullException("method");
} DispatcherOperation operation = null;
DispatcherHooks hooks = null;
bool succeeded = false; // Could be a non-dispatcher thread, lock to read
lock(_instanceLock)
{
if (!_hasShutdownFinished && !Environment.HasShutdownStarted)
{
operation = new DispatcherOperation(this, method, priority, args, numArgs); // Add the operation to the work queue
operation._item = _queue.Enqueue(priority, operation); // Make sure we will wake up to process this operation.
succeeded = RequestProcessing(); if (succeeded)
{
hooks = _hooks;
}
else
{
// Dequeue and abort the item. We can safely return this to the user.
_queue.RemoveItem(operation._item);
operation._status = DispatcherOperationStatus.Aborted;
}
}
else
{
// Rather than returning null we'll create an aborted operation and return it to the user
operation = new DispatcherOperation(this, method, priority);
}
} // .................return operation;
}

通过Enqueue 进入了消息队列。而这里的_queue的定义是：

private PriorityQueue<DispatcherOperation> _queue;

就是一个带有优先级的操作队列。

二、实现一个小的Dispatcher

找到了上面几个重要点，就可以构建自己的调度者了。（不得不说一下，刚打死了一直小老鼠。这么恶劣的公司宿舍，点燃一支烟，压压惊，继续写代码，不容易啊）

1.自己的RUN()

[SecurityCritical, UIPermission(SecurityAction.LinkDemand, Unrestricted = true)]
public void Run()
{
var waitHandles = new WaitHandle[];// 这里是项目需要的两个handle， _stopEvent = new AutoResetEvent(false); 专门处理程序窗口停止。
waitHandles[] = _stopEvent;
waitHandles[] = _triggerEvent; var msg = new MSG();
while (!_isShutdown)
{
var index = MsgWaitForMultipleObjects(waitHandles, false, Infinite, 0x04BF);//等待事件函数，他能等待到是否触发了waitHandles中的handle 其他的消息交给peekmessage处理
switch (index)
{
case :
_isShutdown = true;
_stopEvent.Reset();
break;
case :
_triggerEvent.Reset();
while (TaskQueueCount > )
{
ProcessQueue();//处理invoke和beginInvoke。
}
break;
default:
while (PeekMessage(ref msg))
{
TranslateAndDispatchMessage(ref msg);
}
break;
}
}
}

MsgWaitForMultipleObjects:

private int MsgWaitForMultipleObjects(WaitHandle[] wHandles, bool fWaitAll, int dwMilliseconds, int dwWakeMask)
{
var i = wHandles.Length;
var intPtrs = new IntPtr[i];
for (int j = ; j < i; j++)
{
intPtrs[j] = wHandles[j].SafeWaitHandle.DangerousGetHandle();//这个转换的方法，找了两天。不容易啊。handle转化为inPtrs
}
return UnsafeNativeMethods.MsgWaitForMultipleObjects(i, intPtrs, fWaitAll, dwMilliseconds, dwWakeMask);//这个方法可以去查MSDN，也是个WIN32函数，下面的UnsafeNativeMethods中有写。
}

ProcessQueue

public void ProcessQueue()
{
DispatcherOperation operation = null;
var invalid = _queue.MaxPriority;
if (((invalid != DispatcherPriority.Invalid) && (invalid != DispatcherPriority.Inactive)) || _queue.Count == )
{
operation = _queue.Dequeue();
}
if (operation != null)
{
operation.Invoke();
operation.InvokeCompletions();
}
}

TranslateAndDispatchMessage 和 PeekMessage

[SecurityCritical]
private bool PeekMessage(ref MSG msg)
{
var nullHandleRef = new HandleRef(null, IntPtr.Zero);
return UnsafeNativeMethods.PeekMessage(ref msg, nullHandleRef, , , );
}
[SecurityCritical]
private void TranslateAndDispatchMessage(ref MSG msg)
{
bool handled = ComponentDispatcher.RaiseThreadMessage(ref msg); if (!handled)
{
UnsafeNativeMethods.TranslateMessage(ref msg);
UnsafeNativeMethods.DispatchMessage(ref msg);
}
}

UnsafeNativeMethods精简了很多。

public class UnsafeNativeMethods
{
[SuppressUnmanagedCodeSecurity, SecurityCritical, DllImport("user32.dll", EntryPoint = "GetMessageW", CharSet = CharSet.Unicode, SetLastError = true, ExactSpelling = true)]
private static extern int IntGetMessageW([In, Out] ref MSG msg, HandleRef hWnd, int uMsgFilterMin, int uMsgFilterMax); /// <summary>
/// TranslateMessage
/// </summary>
/// <param name="msg"></param>
/// <returns></returns>
[SecurityCritical, SuppressUnmanagedCodeSecurity, DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
internal static extern bool TranslateMessage([In, Out] ref MSG msg); /// <summary>
/// DispatchMessage
/// </summary>
/// <param name="msg"></param>
/// <returns></returns>
[SecurityCritical, SuppressUnmanagedCodeSecurity, DllImport("user32.dll", CharSet = CharSet.Auto)]
internal static extern IntPtr DispatchMessage([In] ref MSG msg); [SuppressUnmanagedCodeSecurity, SecurityCritical, DllImport("user32.dll", EntryPoint = "MsgWaitForMultipleObjectsEx", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
private static extern int IntMsgWaitForMultipleObjectsEx(int nCount, IntPtr[] pHandles, int dwMilliseconds, int dwWakeMask, int dwFlags); /// <summary>
/// 等待消息和事件
/// </summary>
/// <param name="nCount"></param>
/// <param name="pHandles"></param>
/// <param name="fWaitAll"></param>
/// <param name="dwMilliseconds"></param>
/// <param name="dwWakeMask"></param>
/// <returns></returns>
[SuppressUnmanagedCodeSecurity, SecurityCritical, DllImport("user32.dll", EntryPoint = "MsgWaitForMultipleObjects", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
internal static extern int MsgWaitForMultipleObjects(int nCount, IntPtr[] pHandles, bool fWaitAll, int dwMilliseconds, int dwWakeMask); /// <summary>
/// PeekMessage
/// </summary>
/// <param name="msg"></param>
/// <param name="hwnd"></param>
/// <param name="msgMin"></param>
/// <param name="msgMax"></param>
/// <param name="remove"></param>
/// <returns></returns>
[SecurityCritical, SuppressUnmanagedCodeSecurity, DllImport("user32.dll", CharSet = CharSet.Auto)]
internal static extern bool PeekMessage([In, Out] ref MSG msg, HandleRef hwnd, int msgMin, int msgMax, int remove); [ComImport, SecurityCritical, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("8f1b8ad8-0b6b-4874-90c5-bd76011e8f7c"), SuppressUnmanagedCodeSecurity]
internal interface ITfMessagePump
{
[SecurityCritical]
void PeekMessageA(ref MSG msg, IntPtr hwnd, int msgFilterMin, int msgFilterMax, int removeMsg, out int result);
[SecurityCritical]
void GetMessageA(ref MSG msg, IntPtr hwnd, int msgFilterMin, int msgFilterMax, out int result);
[SecurityCritical]
void PeekMessageW(ref MSG msg, IntPtr hwnd, int msgFilterMin, int msgFilterMax, int removeMsg, out int result);
[SecurityCritical]
void GetMessageW(ref MSG msg, IntPtr hwnd, int msgFilterMin, int msgFilterMax, out int result);
} /// <summary>
/// 获取消息
/// </summary>
/// <param name="msg"></param>
/// <param name="hWnd"></param>
/// <param name="uMsgFilterMin"></param>
/// <param name="uMsgFilterMax"></param>
/// <returns></returns>
/// <exception cref="Win32Exception"></exception>
[SecurityCritical]
public static bool GetMessageW([In, Out] ref MSG msg, HandleRef hWnd, int uMsgFilterMin, int uMsgFilterMax)
{
var index = IntGetMessageW(ref msg, hWnd, uMsgFilterMin, uMsgFilterMax);
switch (index)
{
case -:
throw new Win32Exception(); case :
return false;
}
return true;
} [SecurityCritical]
internal static int MsgWaitForMultipleObjectsEx(int nCount, IntPtr[] pHandles, int dwMilliseconds, int dwWakeMask, int dwFlags)
{
int num = IntMsgWaitForMultipleObjectsEx(nCount, pHandles, dwMilliseconds, dwWakeMask, dwFlags);
if (num == -)
{
throw new Win32Exception();
}
return num;
}
}

其他的类，差不多都是拿来主义。还有一些设计公司内部的东西，不便贴出来，但核心的东西都说了。

三、调用

1.我们可以把操作压入自己的dispatcher中。

Dispather.BeginInvoke(new Action<object>(TriggerAction), DispatcherPriority.Normal, );

2.让timer跑起来

public MainWindow()
{
InitializeComponent();
_thread = new Thread(Run);
_thread.Start();
var id = Thread.CurrentThread.ManagedThreadId;
} [SecurityCritical, UIPermission(SecurityAction.LinkDemand, Unrestricted = true)]
private void Run()
{
var mytimer = new System.Windows.Forms.Timer();//windows form中的timer 必须依赖一个窗体线程，如果没有这个下面的_dispatcher.Run() tick中的方法是不会有效果的。

mytimer.Tick += ButtonOnclick;
mytimer.Enabled = true;
mytimer.Interval = ;
//var id = Thread.CurrentThread.ManagedThreadId;
_dispatcher.Run();//这是必须的这里纯粹只是一个例子在这里没有使用意义。
}

结语：学习源码确实有很多收获，但是确实很累，要是没有别人指点，我几乎是不可能做出来的，特别是对核心代码的提炼，很难分清，重要的一点就是要明白需求。之前也没有接触过底层的函数，可能做C++的人接触的比较多，像windows的消息机制，这里理解起来就比我要快的多了。在这里分享出来，与君共勉！如果对你有帮助，就支持一个吧。

写完11点多了，洗洗睡了。园友们晚安！