How to detect a keystroke in the Window Procedure method? - c++

Question

I am a bit new to c++ and I am trying to create a gui application to tell me whether my caps lock is active or not. I have already set up the basic UI and it starts up according to plan (by showing me my lock state via colors) but I cannot manage to change the window color at runtime.
here's my code :
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
bool state = false;
switch (uMsg) {
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
// All painting occurs here, between BeginPaint and EndPaint.
if ((GetKeyState(VK_CAPITAL) & 0x0001) != 0) {
FillRect(hdc, &ps.rcPaint, CreateSolidBrush(RGB(0, 255, 0)));
}
else {
FillRect(hdc, &ps.rcPaint, CreateSolidBrush(RGB(255, 0, 0)));
}
EndPaint(hwnd, &ps);
}
case WM_KEYUP:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
// All painting occurs here, between BeginPaint and EndPaint.
if ((GetKeyState(VK_CAPITAL) & 0x0001) != 0) {
FillRect(hdc, &ps.rcPaint, CreateSolidBrush(RGB(0, 255, 0)));
}
else {
FillRect(hdc, &ps.rcPaint, CreateSolidBrush(RGB(255, 0, 0)));
}
EndPaint(hwnd, &ps);
}
return 0;
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
and thanks in advance.

Answer 1

Don't call (Begin/End)Paint() in your WM_KEYUP handler, and don't call GetKeyState() in your WM_PAINT handler. Have WM_KEY(DOWN|UP) save the desired color to a variable and then call InvalidateRect() whenver that variable changes value to trigger a repaint of your window. Let WM_PAINT draw the window using the current value of that variable as needed.
Also, your case blocks are missing break statements. And you are leaking the HBRUSH returned by CreateSolidBrush().
Try something more like this:
COLORREF color;
void UpdateColorForCapsLock()
{
if (GetKeyState(VK_CAPITAL) & 0x0001) {
color = RGB(0, 255, 0);
}
else {
color = RGB(255, 0, 0);
}
}
RESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
switch (uMsg) {
case WM_CREATE:
{
UpdateColorForCapsLock();
break;
}
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
HBRUSH hBrush = CreateSolidBrush(color);
FillRect(hdc, &ps.rcPaint, hBrush);
DeleteObject(hBrush);
EndPaint(hwnd, &ps);
return 0;
}
case WM_KEYDOWN:
case WM_KEYUP:
{
if (wParam == VK_CAPITAL)
{
UpdateColorForCapsLock();
InvalidateRect(hwnd, NULL, TRUE);
}
break;
}
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

Answer 2

Capture the WM_KEYDOWN message. If i's a Caps Lock key press (check the parameters), make your window repaint itself by calling InvalidateRect.