During idle time, the UI thread is also responsible for processing all messages in the application's Windows message queue.
在空闲时候，该UI线程同样要对应用程序的窗体消息队列中的所有消息进行负责。
Posted when the user presses a hot key registered by the RegisterHotKey function. The message is placed at the top of the message queue associated with the thread that registered the hot key.
WM_HOTKEY消息在用户键入被RegisterHotKey函数注册的热键时发送。该消息将位于消息队列的最前端，并且与注册了这个热键的进程关联。
The system creates a thread's message queue when the thread makes its first call to one of the USER or GDI functions.
所有线程刚生成时都是没有消息队列的。当线程第一次调用特定的函数时，消息队列就被创建了。
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