Message Sending and Receiving
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Consider the sending part of the process and how the framework responds.
Most messages result from user interaction with the program. Commands are generated by mouse clicks in menu items or toolbar buttons or by accelerator keystrokes. The user also generates Windows messages by, for example, moving or resizing a window. Other Windows messages are sent when events such as program startup or termination occur, as windows get or lose the focus, and so on. Control-notification messages are generated by mouse clicks or other user interactions with a control, such as a button or list-box control in a dialog box.
The Run member function of class CWinApp retrieves messages and dispatches them to the appropriate window. Most command messages are sent to the main frame window of the application. The WindowProc predefined by the class library gets the messages and routes them differently, depending on the category of message received.
Now consider the receiving part of the process.
The initial receiver of a message must be a window object. Windows messages are usually handled directly by that window object. Command messages, usually originating in the application's main frame window, get routed to the command-target chain described in Command Routing.
Each object capable of receiving messages or commands has its own message map that pairs a message or command with the name of its handler.
When a command-target object receives a message or command, it searches its message map for a match. If it finds a handler for the message, it calls the handler. For more information about how message maps are searched, see How the Framework Searches Message Maps. Refer again to the figure Commands in the Framework.