Tracking the Mouse Cursor
Windows-based applications often perform tasks that involve tracking the
position of the mouse cursor. Most drawing applications, for example, track it
during drawing operations, allowing the user to draw in a window's client area by
dragging the mouse. Word-processing applications also track the cursor, enabling
the user to select a word or block of text by clicking and dragging the mouse.
Tracking the cursor typically involves processing the
WM_LBUTTONDOWN,
WM_MOUSEMOVE, and
WM_LBUTTONUP messages. A window determines when to begin tracking the cursor by checking
the cursor position provided in the
lParam parameter of the WM_LBUTTONDOWN message. For example, a word-processing
application would begin tracking the cursor only if the WM_LBUTTONDOWN message
occurred while the cursor was on a line of text, but not if it was past the end of
the document.
A window tracks the position of the cursor by processing the stream of
WM_MOUSEMOVE messages posted to the window as the mouse moves. Processing the
WM_MOUSEMOVE message typically involves a repetitive painting or drawing operation in
the client area. For example, a drawing application might redraw a line
repeatedly as the mouse moves. A window uses the WM_LBUTTONUP message as a signal to
stop tracking the cursor.
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