Advanced MFC Programming

Chapter 6. Dialog Box
Dialog Box Unit
When creating a dialog template, we can read its dimension in the status bar of Developer Studio
(Figure 6-1). However this size is measured in dialog box unit rather than screen pixels. This means if we
create a dialog template with a size of 100×100 (measured in dialog box unit), its actual size will not be 100
pixel × 100 pixel. For any dialog template, its horizontal base unit is equal to the average width of the
characters that is used by the template, and its vertical base unit is equal to the height of the font. The
dialog box is measured by the base units: each horizontal base unit is equal to 4 horizontal dialog units and
each vertical base unit is equal to 8 vertical dialog units. This is a very complex calculation, fortunately in
class CDialog there is a member function CDialog::MapDialogRect(…) that can be used to implement the
dimension conversion so we do not need to calculate the details.
If we want the initial size of a dialog box to be exactly the same with its template size, we need to call
function CDialog::MapDialogRect(…) to convert its template size to screen pixels then call CWnd::
MoveWindow(…) to resize the dialog box before it is displayed.
Tracking Size and Maximized Size
There are two types of tracking sizes: minimum tracking size and maximum tracking size, which
correspond to limit sizes that can be set to a window by dragging one of its resizable border. The
maximized size of a window is the size when it is in the maximized state (when a window is maximized, it
doesn’t have to take up the whole screen). There is no “minimized size” here because when a window is
minimized, it will become an icon.
The size of
dialog template
is shown on the
status bar
Figure 6-1. Dialog box dimension
These sizes can all be customized. To provide user defined sizes, we can override function CWnd::
OnGetMinMaxInfo(…), which will be called when any of the above sizes is needed by the system. We can
provide our own sizes within the overridden function.
Function CWnd::OnGetMinMaxInfo(…) has the following format:
afx_msg void CWnd::OnGetMinMaxInfo(MINMAXINFO *lpMMI);
It is the handler of WM_GETMINMAXINFO message.
Whenever the system needs to know the tracking sizes or maximized size of a window, it sends a
WM_GETMINMAXINFO message to it. In MFC, this message is handled by function CWnd::
OnGetMinMaxInfo(…). The input parameter of this function is a MINMAXINFO type pointer, if we want to
customize the default implementation, we can change the members of MINMAXINFO. Structure MINMAXINFO is
defined as follows:
typedef struct tagMINMAXINFO {
POINT ptReserved;
POINT ptMaxSize;
POINT ptMaxPosition;
POINT ptMinTrackSize;
POINT ptMaxTrackSize;
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