Windows 有很多的 “ 空闲时间 ”，在这期间所有的消息队列都是空的，Windows 就在等待键盘或者鼠标的输入。 那么能否在空闲期间从某种程度上获取控制并绘制随机矩形，而一旦有消息加载到程序的消息队列，就释放控制呢？这正是 PeekMessage 函数的 ” 用武之地 “

PeekMessage

The PeekMessage function dispatches incoming sent messages, checks the thread message queue for a posted message, and retrieves the message (if any exist).

BOOL PeekMessage(
  LPMSG lpMsg,         // message information
  HWND hWnd,           // handle to window
  UINT wMsgFilterMin,  // first message
  UINT wMsgFilterMax,  // last message
  UINT wRemoveMsg      // removal options
);

Parameters

lpMsg

[out] Pointer to an MSG structure that receives message information.

hWnd

[in] Handle to the window whose messages are to be examined.

wMsgFilterMin

[in] Specifies the value of the first message in the range of messages to be examined.

wMsgFilterMax

[in] Specifies the value of the last message in the range of messages to be examined.

wRemoveMsg

[in] Specifies how messages are handled. This parameter can be one of the following values.

Value	Meaning
PM_NOREMOVE	Messages are not removed from the queue after processing by PeekMessage.
PM_REMOVE	Messages are removed from the queue after processing by PeekMessage.

You can optionally combine the value PM_NOYIELD with either PM_NOREMOVE or PM_REMOVE. This flag prevents the system from releasing any thread that is waiting for the caller to go idle (see WaitForInputIdle).

By default, all message types are processed. To specify that only certain message should be processed, specify one of more of the following values.

Value	Meaning
PM_QS_INPUT	Windows 98, Windows 2000: Process mouse and keyboard messages.
PM_QS_PAINT	Windows 98, Windows 2000: Process paint messages.
PM_QS_POSTMESSAGE	Windows 98, Windows 2000: Process all posted messages, including timers and hotkeys.
PM_QS_SENDMESSAGE	Windows 98, Windows 2000: Process all sent messages.

Return Values

If a message is available, the return value is nonzero.

If no messages are available, the return value is zero. 

有了这个 PeekMessage 函数，我们只需简单修改一下正常的消息循环，如下:

while (TRUE)
{
if(PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
{
if(msg.message == WM_QUIT)
break;
TranslateMessage(&msg);
DispatchMessage(&msg);