Designing a Window Procedure
The following example shows the structure of a typical window procedure. The
window procedure uses the message argument in a
switch statement with individual messages handled by separate
case statements. Notice that each case returns a specific value for each message.
For messages that it does not process, the window procedure calls the
DefWindowProc function.
LRESULT CALLBACK MainWndProc(
HWND hwnd, // handle of window
UINT uMsg, // message identifier
WPARAM wParam, // first message parameter
LPARAM lParam) // second message parameter
{
switch (uMsg)
{
case WM_CREATE:
// Initialize the window.
return 0;
case WM_PAINT:
// Paint the window's client area.
return 0;
case WM_SIZE:
// Set the size and position of the window.
return 0;
case WM_DESTROY:
// Clean up window-specific data objects.
return 0;
//
// Process other messages.
//
default:
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
return 0;
}
The
WM_NCCREATE messageis sent just after your window is created, but if an application
responds to this message by returning FALSE,
CreateWindowEx function fails. The
WM_CREATE message is sent after your window is already created.
The
WM_DESTROY message is sent when your window is about to be destroyed. The
DestroyWindow function takes care of destroying any child windows of the window being
destroyed. The
WM_NCDESTROY message is sent just before a window is destroyed.
At the very least, a window procedure should process the
WM_PAINT message to draw itself. Typically, it should handle mouse and keyboard
messages as well. Consult the descriptions of individual messages to determine
whether your window procedure should handle them.
Your application can call the
DefWindowProc function as part of the processing of a message. In such a case, the
application can modify the message parameters before passing the message to
DefWindowProc, or it can continue with the default processing after performing its own
operations.
A dialog box procedure receives a
WM_INITDIALOG message instead of a
WM_CREATE message and does not pass unprocessed messages to the
DefDlgProc function. Otherwise, a dialog box procedure is exactly the same as a window
procedure.
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