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.