MFC Windows 程序设计>捕获鼠标

// In CMainWindow's message map ON_WM_LBUTTONDOWN () ON_WM_LBUTTONUP () void CMainWindow::OnLButtonDown (UINT nFlags, CPoint point) { SetCapture (); } void CMainWindow::OnLButtonUp (UINT nFlags, CPoint point) { ::ReleaseCapture (); }  

In between, CMainWindow receives WM_MOUSEMOVE messages that report the cursor position even if the cursor leaves it. Client-area mouse messages continue to report cursor positions in client coordinates, but coordinates can now go negative and can also exceed the dimensions of the window's client area.

A related function, CWnd::GetCapture, returns a CWnd pointer to the window that owns the capture. In the Win32 environment, GetCapture returns NULL if the mouse is not captured or if it's captured by a window belonging to another thread. The most common use of GetCapture is for determining whether your own window has captured the mouse. The statement

if (GetCapture () == this)

is true if and only if the window identified by this currently has the mouse captured.

How does capturing the mouse solve the problem with the rubber-banded line? By capturing the mouse in response to a WM_LBUTTONDOWN message and releasing it when a WM_LBUTTONUP message arrives, you're guaranteed to get the WM_LBUTTONUP message when the mouse button is released. The sample program in the next section illustrates the practical effect of this technique.

void CMainWindow::OnLButtonDown (UINT nFlags, CPoint point) { // // Record the anchor point and set the tracking flag. // m_ptFrom = point; m_ptTo = point; m_bTracking = TRUE; // // If capture is enabled, capture the mouse. // if (m_bCaptureEnabled) SetCapture (); } void CMainWindow::OnMouseMove (UINT nFlags, CPoint point) { // // If the mouse is moved while we're "tracking" (that is, while a // line is being rubber-banded), erase the old rubber-band line and // draw a new one. // if (m_bTracking) { CClientDC dc (this); InvertLine (&dc, m_ptFrom, m_ptTo); InvertLine (&dc, m_ptFrom, point); m_ptTo = point; } } void CMainWindow::OnLButtonUp (UINT nFlags, CPoint point) { // // If the left mouse button is released while we're tracking, release // the mouse if it's currently captured, erase the last rubber-band // line, and draw a thick red line in its place. // if (m_bTracking) { m_bTracking = FALSE; if (GetCapture () == this) ::ReleaseCapture (); CClientDC dc (this); InvertLine (&dc, m_ptFrom, m_ptTo); CPen pen (PS_SOLID, 16, RGB (255, 0, 0)); dc.SelectObject (&pen); dc.MoveTo (m_ptFrom); dc.LineTo (point); } }