学步园

当没有任何 Windows 消息正在被处理时，框架调用 的成员函数 （如“MFC 库参考”中所述）。

重写 OnIdle 以执行后台任务。默认版本更新用户界面对象（如工具栏按钮）的状态，并对框架在操作期间创建的临时对象执行清理工作。下图阐释了在队列中没有任何消息时消息循环调用 OnIdle 的方式。

消息循环

代码：

int CWinThread::Run()

{

	.......

	for(;;)

	{

		while (bIdle && //bIdle控制上图的是否有其他空闲任务

			!::PeekMessage(&m_msgCur,NULL,NULL,PM_NOREMOVE))

		{

			if (!OnIdle(lIdleCount++))

			{

				bIdle = FALSE;

			}

		}

		......//msg loop

	}



}

如果自己的程序想处理idle time，只要改写CWinApp派生类的OnIdle函数即可：

vitual BOOL OnIdel(LONG lCount);（返回False表示不再需要空闲处理,但下一次有消息来，处理完消息后，bIdle又置为True，又可以进行空闲处理）

lCount表示从上次有消息进来，到现在，OnIdle已经被调用了多少次，如上面的循环，没调一次OnIdle，lCount累加一次，

知道有消息到来时，lCount被重置为0.（WM_SYTEMTIMER和WM_PAINT消息不会使lCount置为0）.

以下两个程序在不同的空闲时间做不同的处理：

BOOL CMyApp::OnIdle(LONG lCount)
{
    BOOL bMore = CWinApp::OnIdle(lCount);

    if (lCount == 0)
    {
    TRACE("App idle for short period of time/n");
    bMore = TRUE;
    }
    else if (lCount == 10)
    {
    TRACE("App idle for longer amount of time/n");
        bMore = TRUE;
    }
    else if (lCount == 100)
    {
        TRACE("App idle for even longer amount of time/n");
        bMore = TRUE;
    }
    else if (lCount == 1000)
    {
        TRACE("App idle for quite a long period of time/n");
     // bMore is not set to TRUE, no longer need idle
     // IMPORTANT: bMore is not set to FALSE since CWinApp::OnIdle may
     // have more idle tasks to complete.
    }

    return bMore;
     // return TRUE as long as there is any more idle tasks
}

// In this example, four idle loop tasks are given various 
// opportunities to run:
// Task1 is always given a chance to run during idle time, provided
//   that no message has queued up while the framework was processing
//   its own idle loop tasks (at lCount levels 0 and 1).
// Task2 is given a chance to run only if Task1 has already run,
//   provided that no message has queued up while Task1 was running.
// Task3 and Task4 are given a chance to run only if both Task1 and
//   Task2 have already run, and no message has queued up in the mean
//   time.  If Task3 gets its chance to run, then Task4 always gets
//   a chance to run immediately after Task3.

BOOL CMyApp::OnIdle(LONG lCount)
{
   // In this example, as in most applications, you should let the
   // base class CWinApp::OnIdle complete its processing before you
   // attempt any additional idle loop processing.
   if (CWinApp::OnIdle(lCount))
      return TRUE;   

   // The base class CWinApp::OnIdle reserves the lCount values 0 
   // and 1 for the framework's own idle processing.   If you wish to
   // share idle processing time at a peer level with the framework,
   // then replace the above if-statement with a straight call to
   // CWinApp::OnIdle; and then add a case statement for lCount value
   // 0 and/or 1. Study the base class implementation first to 
   // understand how your idle loop tasks will compete with the 
   // framework's idle loop processing.

   switch (lCount)
   {
      case 2:
         Task1();
         return TRUE; // next time give Task2 a chance
      case 3:
         Task2();
         return TRUE; // next time give Task3 and Task4 a chance
      case 4:
         Task3();
         Task4();
         return FALSE; // cycle through the idle loop tasks again
   }
   return FALSE;
}
因为只有在OnIdle返回之后应用程序才能处理用户输入，因此在OnIdle中不应进行较长的任务。