Event handling in MFC (Microsoft Foundation Class) is a fundamental aspect of developing interactive applications. In MFC, events such as button clicks, key presses, and window resizing are managed using the message map mechanism.
What is a Message Map in MFC?
A message map is a mechanism used to link Windows messages (events) to functions that handle those messages in an MFC class.
Message Handling in MFC
The **message map** is declared in the class using `BEGIN_MESSAGE_MAP` and `END_MESSAGE_MAP`. The macro `ON_COMMAND` is used to map a command (e.g., button click) to the appropriate handler function.
Example: Handling a Button Click
Suppose you have a button with an ID of `IDC_MYBUTTON`:
“`cpp
BEGIN_MESSAGE_MAP(CMyDialog, CDialog)
ON_COMMAND(IDC_MYBUTTON, &CMyDialog::OnMyButtonClick)
END_MESSAGE_MAP()
void CMyDialog::OnMyButtonClick()
{
AfxMessageBox(_T(“Button clicked!”));
}
“`
Handling Window Messages
For window messages like `WM_PAINT`, use `ON_WM_PAINT` in the message map:
“`cpp
BEGIN_MESSAGE_MAP(CMyView, CView)
ON_WM_PAINT()
END_MESSAGE_MAP()
void CMyView::OnPaint()
{
CPaintDC dc(this); // Device context for painting
dc.TextOut(10, 10, _T(“Handling WM_PAINT”));
}
“`
Handling Keyboard Events
To handle keyboard events such as `WM_KEYDOWN`:
“`cpp
BEGIN_MESSAGE_MAP(CMyDialog, CDialog)
ON_WM_KEYDOWN()
END_MESSAGE_MAP()
void CMyDialog::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
if (nChar == VK_ESCAPE) {
AfxMessageBox(_T(“Escape key pressed”));
}
}
“`
MFC’s message map system provides a structured way to handle various events, making event-driven programming easier.