Drawing Lines with the Mouse - Win32 Programmer's Reference

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Drawing Lines with the Mouse

The example in this section demonstrates how to track the mouse cursor. It contains portions of a window procedure that enables the user to draw lines in a window's client area by dragging the mouse.

When the window procedure receives a WM_LBUTTONDOWN message, it captures the mouse and saves the coordinates of the cursor, using the coordinates as the starting point of the line. It also uses the ClipCursor function to confine the cursor to the client area during the line drawing operation.

During the first WM_MOUSEMOVE message, the window procedure draws a line from the starting point to the current position of the cursor. During subsequent WM_MOUSEMOVE messages, the window procedure erases the previous line by drawing over it with an inverted pen color. Then it draws a new line from the starting point to the new position of the cursor.

The WM_LBUTTONUP message signals the end of the drawing operation. The window procedure releases the mouse capture and frees the mouse from the client area.

LRESULT APIENTRY MainWndProc(hwndMain, uMsg, wParam, lParam)

HWND hwndMain;

UINT uMsg;

WPARAM wParam;

LPARAM lParam;

{

HDC hdc; /* handle of device context */

RECT rcClient; /* client area rectangle */

POINT ptClientUL; /* client upper left corner */

POINT ptClientLR; /* client lower right corner */

static POINTS ptsBegin; /* beginning point */

static POINTS ptsEnd; /* new endpoint */

static POINTS ptsPrevEnd; /* previous endpoint */

static BOOL fPrevLine = FALSE; /* previous line flag */

switch (uMsg) {

case WM_LBUTTONDOWN:

/* Capture mouse input. */

SetCapture(hwndMain);

* Retrieve the screen coordinates of the client area,

* and convert them into client coordinates.

GetClientRect(hwndMain, &rcClient);

ptClientUL.x = rcClient.left;

ptClientUL.y = rcClient.top;

* Add one to the right and bottom sides, because the

* coordinates retrieved by GetClientRect do not

* include the far left and lowermost pixels.

ptClientLR.x = rcClient.right + 1;

ptClientLR.y = rcClient.bottom + 1;

ClientToScreen(hwndMain, &ptClientUL);

ClientToScreen(hwndMain, &ptClientLR);

* Copy the client coordinates of the client area

* to the rcClient structure. Confine the mouse cursor

* to the client area by passing the rcClient structure

* to the ClipCursor function.

SetRect(&rcClient, ptClientUL.x, ptClientUL.y,

ptClientLR.x, ptClientLR.y);

ClipCursor(&rcClient);

* Convert the cursor coordinates into a POINTS

* structure, which defines the beginning point of the

* line drawn during a WM_MOUSEMOVE message.

ptsBegin = MAKEPOINTS(lParam);

return 0;

case WM_MOUSEMOVE:

* When moving the mouse, the user must hold down

* the left mouse button to draw lines.

if (wParam & MK_LBUTTON) {

* Retrieve a device context (DC) for the client

* area.

hdc = GetDC(hwndMain);

* The following function ensures that pixels of

* the previously drawn line are set to white and

* those of the new line are set to black.

SetROP2(hdc, R2_NOTXORPEN);

* If a line was drawn during an earlier

* WM_MOUSEMOVE message, draw over it. This erases

* the line by setting the color of its pixels to

* white.

if (fPrevLine) {

MoveToEx(hdc, ptsBegin.x, ptsBegin.y,

(LPPOINT) NULL);

LineTo(hdc, ptsPrevEnd.x, ptsPrevEnd.y);

}

* Convert the current cursor coordinates to a

* POINTS structure, and then draw a new line.

ptsEnd = MAKEPOINTS(lParam);

MoveToEx(hdc, ptsBegin.x, ptsBegin.y,

(LPPOINT) NULL);

LineTo(hdc, ptsEnd.x, ptsEnd.y);

* Set the previous line flag, save the ending

* point of the new line, and then release the DC.

fPrevLine = TRUE;

ptsPrevEnd = ptsEnd;

ReleaseDC(hwndMain, hdc);

}

break;

case WM_LBUTTONUP:

* The user has finished drawing the line. Reset the

* previous line flag, release the mouse cursor, and

* release the mouse capture.

fPrevLine = FALSE;

ClipCursor(NULL);

ReleaseCapture();

return 0;

case WM_DESTROY:

PostQuitMessage(0);

break;

. /* Process other messages. */

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