Handling Windows Messages in the View

To implement mouse-driven drawing in Scribble, you must add code that handles several Windows messages related to mouse activity. You will use WizardBar to help add this message-handling code.

Suggested Reading

WM_LBUTTONDOWN, Win32 SDK Reference
WM_LBUTTONUP, Win32 SDK Reference
WM_MOUSEMOVE, Win32 SDK Reference
GetCapture, Win32 SDK Reference
CWnd::OnLButtonDown, Microsoft Foundation Class Reference
CWnd::OnLButtonUp, Microsoft Foundation Class Reference
CWnd::OnMouseMove, Microsoft Foundation Class Reference
CView Overview, Microsoft Foundation Class Reference
CClientDC Overview, Microsoft Foundation Class Reference
Declaring Message Handler Functions, Visual C++ Programmer’s Guide
Overview: Text Editor, Visual C++ User’s Guide

In Scribble, Windows sends a user action message to a window, in this case the currently active view. The view uses its message map to determine whether it has a member function that can handle the message. If it does, the view calls the handler.

The following table correlates the user action to the message sent by Windows to Scribble’s view and the corresponding member function that handles the message:

User	Message sent from Windows to the window	Handler
Press left mouse button	WM_LBUTTONDOWN	`CScribbleView::OnLButtonDown`
Move mouse (whether or not mouse button is pressed)	WM_MOUSEMOVE	`CScribbleView::OnMouseMove`
Release left mouse button	WM_LBUTTONUP	`CScribbleView::OnLButtonUp`

The view handles mouse-drawing messages because it’s in the view that Scribble’s drawing takes place. The view represents that part of the document that can be seen at any one time.

The message handlers track mouse activity and draw in the view accordingly. They also call member functions of the document to update its data. As the user draws a stroke, the points that make up the stroke are stored in the document’s stroke list.