Advanced MFC Programming

Chapter 4. Button
Implementing Subclass
Class CBitmapButton gives us another member function that can be used to associate bitmaps with an
owner-draw button: CBitmapButton::LoadBitmaps(…). This function has two versions, the first version
allows us to load bitmaps with string IDs, the second version allows us to load bitmaps with integer IDs.
To use this function, we must first implement subclass for the owner-draw button. “Subclass” is a very
powerful technique in Windows programming. It allows us to write a procedure, attach it to a window,
and use it to intercept messages sent to this window then process it. By doing this, we are able to customize
the window’s behavior within the procedure.
Subclass is supported by class CWnd, so theoretically all windows (including client window, dialog box,
dialog common controls...) can be “subclassed”. There are two functions to implement subclass, one is
CWnd::SubclassWindow(…), which allows us to customize the normal behavior of a window. Here we will
use the other one: CWnd::SubclassDlgItem(…), which is specially designed to implement subclass for the
common controls contained in a dialog box.
In MFC, implementing subclass is very simple. We don’t need to write a special procedure to handle
the intercepted messages. All we need to do is designing a class as usual, adding message handlers for the
messages we want to process, and implementing the message handlers. Then we can declare a variable
using the newly designed class, and call function CWnd::SubclassDlgItem(…) to implement subclass.
Function CWnd::SubclassDlgItem(…) has two parameters:
BOOL CWnd::SubclassDlgItem(UINT nID, CWnd *pParent);
Parameter nID indicates which control we are dealing with, and pParent is the pointer to the control’s
parent window.
Class CBitmapButton uses subclass to change the default behavior of a button. If we use automatic
method to load the bitmaps, the subclass procedure is transparent to the programmer. However, if we want
to load the bitmaps by ourselves, we must implement subclass first.
Bitmap Button
Sample 4.3\Btn demonstrates how to associate bitmaps with an owner draw button by calling function
CBitmapButton::LoadBitmaps(…). It is based on sample 4.2\Btn. There is nothing new in this sample,
except that button IDC_PLAY is implemented differently.
In the previous samples, variable CBtnDlg::m_btnPlay is declared as a CBitmapButton type variable.
In the new sample, instead of using automatic method to load the bitmaps, we first implement the subclass
then load the bitmaps manually in function CBitmapButton::LoadBitmaps(…):
……
……
BOOL CBtnDlg::OnInitDialog()
{
CDialog::OnInitDialog();
}
m_btnPlay.SubclassDlgItem(IDC_PLAY, this);
m_btnPlay.LoadBitmaps
(
“PLAYU”,
“PLAYD”,
NULL,
NULL
);
m_btnPlay.SizeToContent();
return TRUE;
Here, function CBitmapButton::AutoLoad(…) is replaced by three new functions. The first function
added is CWnd::SubclassDlgItem(…). The second function is CBitmapButton::LoadBitmaps(…). This
function has four parameters, which are the bitmap IDs corresponding to button’s “Up”, “Down”,
“Focused” and “Disabled” states respectively. They could be either string IDs or integer IDs. The last
function is CBitmap::SizeToContent(), which allows us to set bitmap button’s size to the size of the
associated bitmaps. If we don’t call this function, the bitmaps may not fit well into the button.
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