In front, we analyzed the basic theory and basic functions of the message, followed, we will further discuss the implementation of messaging in the MFC.
The method of processing of MFC messages is initially read in the MFC in the MFC, and the influence of the rooted C in the mind, we may naturally think that one of the three major characteristics of C : virtual mechanisms to achieve message delivery, But after analysis, we see that things don't think about what we think, in the MFC message is processed by a so-called message mapping mechanism. why? Detailed reason is given in the "Visual C Technical Insider" (4th edition) of Pan Aimin teacher (4th edition). I briefly said. About 110 messages in the CWND class, there are other MFC classes, and there is too much news, and every derived class used in the program in C has a vTable, each virtual function is There is a 4-byte size entry address in vtable, so that each specific type of window or control is required, the application requires a 440KB size table to support the virtual message control function. If the above window or control can be barely implemented, what is the message for the menu command message and button? How do we deal with different menus and buttons? This message mapping system in the MFC library avoids the use of large VTABLE, and can process a variety of applications for command messages that can be processed while processing conventional Windows messages. To put it bluntly, the message mechanism in the MFC is a huge message and the corresponding table of its handle function, and then add some program code inside the application framework for this table. This can avoid programming in SDK The cumbersome CASE statement used.
The base class of the MFC message map CCMDTARGET must be derived from the CCMDTARGET class if you want your control to map mapping. The CCMDTARGET class is the basis of the MFC handling command message. MFC designs many member functions and some member data for this class. Basically to resolve message mapping issues, all response messages or events are derived from it, for example: application classes, framework, document classes, view classes And a wide variety of controls, etc., there are still many. However, 2 functions in this class are very important to message mapping, one is a static member function Dispatchcmdmsg, and the other is the virtual function oncmdmsg. DispatchCmdmsg is specifically used inside the MFC to distribute Windows messages. ONCMDMSG is used to transmit and send messages to update the status of the user interface object. CCMDTARGET's default implementation of OnCMDMSG: Search the message processing function of the specified command message in the message mapping array of the class and base class in the current command target (THIS). Here, use the virtual function getMessageMap to get the message map of the command target class mapping portal array _MessageEntries, and then match the command message ID in the array, the control notification code also the same message mapping entry. Where GetMessageMap is a virtual function, so you can confirm the exact class of the current command target. If you find a matching message mapping entry, use DispachCMDMSG to call this processing function; if you do not find it, use the _getBaseMessageMap to get the base class mapping array, find until you find or search for all the base classes (to ccmdtarget) If finally is not found, return FASLE. Each command target class derived from the ccmdtarget can overcmDMSG, use it to determine if a command can handle, if not, by calling the next command target's oncMDMSG, the command is sent to the next command target processing. Typically, when derived class overcmdmsg, the overcard mdmsg of the base class is to be called. In the MFC framework, some MFC command target classes overcmdmsg, such as the Frame window class overwrite the function, implements the MFC standard command message transmission path. If necessary, the application can override onCMDMSG, change the sending regulations in one or more classes, and implement a different transmission path to send a specified sector with the standard framework. For example, in the following cases, this can be processed: pass the command to a non-MFC default object in a class to interrupt the transmission order; in a new non-default object or in a command target that may be transmitted. The content of the message map is generated by classwizard, we can see that the message mapping is basically divided into 2 Most: There is a macro Declare_Message_Map () in the header file (.h), he is placed at the end of the class. It is a public attribute; in response, the implementation section (.cpp) adds a message mapping table, the content is as follows: begin_message_map (current class, current class base class) File: // {AFX_MSG_MAP (CMAINFRAME The entrance page of the message file: //}} AFX_MSG_MAP END_MESSAGE_MAP () But only the two are still not enough to complete a message, if a message work, must have the following three parts: 1. In the definition of the class Add corresponding function declarations; 2. Add corresponding message mapping entry items in the message mapping table of the class; 3. Add corresponding functional bodies in the implementation of the class;
