- 1 year ago
C++ WM_LBUTTONDOWN not triggering when using WM_NCHITTEST
In the Windows message handling system, the WM_LBUTTONDOWN
message is sent to a window's message queue when the left mouse button is pressed. However, when handling the WM_NCHITTEST
message, which is used for determining the hit-test area of a non-client region, the WM_LBUTTONDOWN
message might not be triggered by default.
The WM_NCHITTEST
message is usually handled in the window's WndProc
function to determine how the window should respond to mouse events in the non-client area. By default, the WM_NCHITTEST
handler consumes the left mouse button event in the non-client area and doesn't propagate it as a WM_LBUTTONDOWN
message.
If you want to receive the WM_LBUTTONDOWN
message when the left mouse button is pressed in the non-client area, you can explicitly forward the message in the WM_NCHITTEST
handler. Here's an example:
cppLRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {
switch (message) {
case WM_NCHITTEST:
{
// Determine hit-test result
LRESULT hitTestResult = DefWindowProc(hWnd, message, wParam, lParam);
// Forward left mouse button down event
if (hitTestResult == HTCLIENT) {
if (wParam == HTNOWHERE || wParam == HTCAPTION) {
// Left button is pressed in non-client area
SendMessage(hWnd, WM_LBUTTONDOWN, wParam, lParam);
}
}
return hitTestResult;
}
case WM_LBUTTONDOWN:
{
// Left mouse button down event handling
// ...
return 0;
}
// Other message handling cases
// ...
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
In the WM_NCHITTEST
handler, the hit-test result is determined using DefWindowProc
and then checked if it is HTCLIENT
(indicating the left button is pressed in the client area). If the hit-test result is HTNOWHERE
or HTCAPTION
(indicating the left button is pressed in the non-client area), the WM_LBUTTONDOWN
message is explicitly sent to the window using SendMessage
.
By forwarding the WM_LBUTTONDOWN
message explicitly in the WM_NCHITTEST
handler, you can ensure that the message is triggered when the left mouse button is pressed in the non-client area of the window.