draw line doesn't work, what might be the problem? - c++

Question

I want to draw the line by clicking the mouse on the first coordinate where the line should start and second coordinate where the line should end.
When I run my project nothing happens.
I cannot find out what my code is missing.
LONG WINAPI WndProc(HWND hWnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
BOOL fDraw = FALSE;
POINT ptPrevious = { 0 };
HPEN Pen = CreatePen(PS_SOLID, 3, RGB(0, 0, 255));
switch (Message) {
case WM_LBUTTONDOWN: {
fDraw = TRUE;
ptPrevious.x = LOWORD(lParam);
ptPrevious.y = HIWORD(lParam);
break;
}
case WM_LBUTTONUP: {
if (fDraw)
{
hdc = GetDC(hWnd);
MoveToEx(hdc, ptPrevious.x, ptPrevious.y, NULL);
LineTo(hdc, LOWORD(lParam), HIWORD(lParam));
ReleaseDC(hWnd, hdc);
}
fDraw = FALSE;
break;
}
case WM_MOUSEMOVE: {
if (fDraw)
{
hdc = GetDC(hWnd);
MoveToEx(hdc, ptPrevious.x, ptPrevious.y, NULL);
LineTo(hdc, ptPrevious.x = LOWORD(lParam),
ptPrevious.y = HIWORD(lParam));
ReleaseDC(hWnd, hdc);
}
break;
}
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, Message, wParam, lParam);
}
return 0;
}

Answer 1

Fundamentally, the problem is because none of your data is persistent. Since they are all local variables, their content disappears when WndProc returns. fDraw is set to FALSE on every message, and will never be true for the WM_LBUTTONDOWN or WM_MOUSEMOVE messages. Therefore nothing happens.
You'll want to create some sort of a class to hold fDraw, ptPrevious, and a structure to hold the coordinates of the line(s) to draw. Use InvalidateRect in the WM_MOUSEMOVE and WM_LBUTTONUP messages. Then only draw them in response to a WM_PAINT message (using the DC provided in the paint message).
Using the CWnd MFC class can greatly simplify those tasks.

Answer 2

Because declared variables are local variables, variables can be declared with Static.
static BOOL fDraw = FALSE;
static POINT ptPrevious = { 0 };
In addition, you forget to use the SelectObject function to select pen into DC.
You can also use WM_PAINT redrawing to improve your code:
Example:
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
HDC hdc;
PAINTSTRUCT ps;
static BOOL fDraw = FALSE;
static BOOL fDraw_begin = FALSE;
static POINT ptPrevious;
static RECT rcClient;
static POINT pt;
static HPEN Pen;
switch (message)
{
case WM_CREATE:
{
Pen = CreatePen(PS_SOLID, 3, RGB(255, 0, 0));
break;
}
case WM_LBUTTONDOWN:
{
ptPrevious.x = (LONG)LOWORD(lParam);
ptPrevious.y = (LONG)HIWORD(lParam);
return 0;
}
case WM_LBUTTONUP:
{
if (fDraw = TRUE)
{
fDraw = FALSE;
fDraw_begin = TRUE;
InvalidateRect(hwnd, &rcClient, TRUE);
UpdateWindow(hwnd);
}
return 0;
}
case WM_MOUSEMOVE:
{
if (wParam && MK_LBUTTON)
{
GetClientRect(hwnd, &rcClient);
hdc = GetDC(hwnd);
SetROP2(hdc, R2_NOTXORPEN);
if (!IsRectEmpty(&rcClient)) // Detecting whether the rectangular area is empty
{
MoveToEx(hdc, ptPrevious.x, ptPrevious.y, NULL);
LineTo(hdc, (LONG)LOWORD(lParam),
(LONG)HIWORD(lParam));
}
MoveToEx(hdc, ptPrevious.x, ptPrevious.y, NULL);
LineTo(hdc, (LONG)LOWORD(lParam),
(LONG)HIWORD(lParam));
pt.x = (LONG)LOWORD(lParam);
pt.y = (LONG)HIWORD(lParam);
fDraw = TRUE;
ReleaseDC(hwnd, hdc);
}
return 0;
}
case WM_PAINT:
{
hdc = BeginPaint(hwnd, &ps);
SelectObject(hdc, Pen);
if (fDraw_begin)
{
fDraw_begin = FALSE;
MoveToEx(hdc, ptPrevious.x, ptPrevious.y, NULL);
LineTo(hdc, pt.x,
pt.y);
}
EndPaint(hwnd, &ps);
ReleaseDC(hwnd, hdc);
return 0;
}
case WM_DESTROY:
{
DeleteObject(Pen);
PostQuitMessage(0);
return 0;
}
}
return DefWindowProc(hwnd, message, wParam, lParam);
}

Answer 3

Do not get the DC three times, get it once and release it on mouse up.
Also, your drawing will be eventually validated, so you need to log the mouse move and repeat this stuff in WM_PAINT handler.