Visual C++ and MFC Programming 2nd Edition

Chapter 12: Dialog-Based Windows Visual C++ and MFC Fundamentals

virtual LRESULT WindowProcedure(UINT message, WPARAM wParam, LPARAM lParam);

To process the messages, and because there can be so many of them, the window

procedure typically uses a switch control to list all necessary messages and process each

one in turn (some of the messages are those we reviewed in Lesson 4). After processing a

message, its case must return a value indicating whether the message was successfully

processed or not and how the message was processed.

Regardless of the number of messages you process, there will still be messages that you

did not deal with. It could be because they were not sent even though they are part of the

Windows control(s) used in your application. If you did not process some messages, you

should/must let the operating system take over and process it. This is done because the

operating system is aware of all messages and it has a default behavior or processing for

each one of them. Therefore, you should/must return a value for this to happen. The value

returned is typically placed in the default section of the switch condition and must simply

be a DefWindowProc() function. For a Win32 application, its syntax is:

LRESULT DefWindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

For an MFC application, the syntax used for this function is:

virtual LRESULT DefWindowProc(UINT message, WPARAM wParam, LPARAM lParam);

This function is returned to Windows, saying "There are messages I couldn't process. Do

what you want with them". The operating system would simply apply a default

processing to them. The values returned by the DefWindowProc() function should be the

same passed to the procedure.

The most basic message you can process is to make sure a user can close a window after

using it. This can be done with a function called PostQuitMessage(). Its syntax is:

VOID PostQuitMessage(int nExitCode);

This function takes one argument which is the value of the LPARAM argument. To

close a window, you can pass the argument as WM_QUIT.

The name of the window procedure must be assigned to the lpfnWndProc member

variable of the WNDCLASS variable.

Because we are using MFC to visually build our applications, you usually will not need

to define a window procedure to process Windows messages, unless the control you are

using is lacking a message that you find relevant. The Windows controls we will use in

this book have messages and notifications that apply the most regular behaviors they

need to offer. If you do not process all messages of a control, which will happen most of

the time, their default behavior are part of the AfxWndProc procedure. Therefore, you

can simply assign it to the lpfnWndProc member variable of your WNDCLASS variable:

CMainFrame::CMainFrame()

{

// Declare a window class variable

WNDCLASS WndCls;

WndCls.style = CS_VREDRAW | CS_HREDRAW;

WndCls.lpfnWndProc = AfxWndProc;

WndCls.cbClsExtra = 0;

WndCls.cbWndExtra = 0;