Advanced MFC Programming

Chapter 12. Screen Capturing & Printing
Picking Up a Window
Often there are many windows contained in the desktop window. Each application may have one
mainframe window and several child windows. To let the user pick up a window with mouse clicking, we
must find a way to detect if there is a window under the current mouse cursor.
We can call function CWnd::WindowFromPoint(…) to retrieve the window under current mouse cursor.
If there is such a window, its handle will be returned by this function. Otherwise the function will return
NULL.
We also need to monitor mouse movement and respond to its activities when the user is selecting a
window. We all know that this can be implemented by handling mouse related messages: WM_LBUTTONDOWN,
WM_MOUSEMOVE, and WM_LBUTTONUP. Also we must be able to receive these messages even if the mouse
cursor is outside our application window. To implement this, we need to set window capture. By doing so,
the user can hold the left button and move it anywhere to pick up window. As long as the left button is held
down, we can receive WM_MOUSEMOVE messages (The capture will be released by the system if the left button
is released).
Another issue is how to indicate the selected window. To make the application easier to use, we need
to put some indication on the window that is being selected. From the previous section we know that the
DC of the desktop window can be obtained and used to draw anything anywhere on the screen. With the
desktop window DC, we can reverse the selected window when the mouse cursor is over it, and resume it
as the mouse cursor leaves the window.
The drawing mode can be set by calling function CDC::SetROP2(…) using R2_NOT flag. After this if we
draw a rectangle (By calling function CDC::Rectangle(…)) using the window’s position and size, the whole
window will be reversed. Calling this function twice using the same parameter will resume the original
window.
One thing we must pay attention to is that the application itself also has a mainframe window, and
therefore will be selected for capturing if the mouse cursor is over it. This is not a desirable feature. To
solve this problem, after function CWnd::WindowFromPoint(…) is called, we must check if the window
returned by this function belongs to the application itself.
Because the windows can overlap one another, when the mouse cursor is over one window, we should
not reverse the overlapped part (Figure 11-2). To solve this problem, we need to use region. We can create
a region that contains only the non-overlapped part of a window, which can be selected by the desktop
window DC. By doing this, the overlapped portion of the window will not be reversed when the function
CDC::Rectangle(…) is called.
Window A
Window B
If mouse cursor is over window A,
we do not want to reverse the part
that is overlapped by window B
Figure 11-2. Reverse a window to indicate that it can be selected for capturing
After the user has selected a specific window and executed Capture | Go! command, we can find out
the size and position of this window, start timer, call function CDC::BitBlt(…) to make a snapshot of the
window and store the data to the memory bitmap.
Dialog Box IDD_DIALOG_SELECT
Sample 12.2\GDI is based on sample 12.1\GDI. The new sample allows the user to select a specified
window for making snapshot. First a dialog box is added to the application, it will be used to let the user
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