PROGRAM TO HANDLE BASIC EVENTS

PROGRAM TO HANDLE BASIC EVENTS

MESSAGE MAP,
SAVING THE VIEW’S STATE,
INITIALIZING A VIEW CLASS DATA MEMBERS.

Message map:

 When the user presses the left mouse button in a view window, windows sends a message “WM_LBUTTONDOWN” to that window.

 If the program needs to take an action in response to WM_LBUTTONDOWN, view class must have a member function that looks like:

Void CMyView::OnLButtonDown(UINT nFlags, CPoint point)
{
// event processing code here
}

 Class header file must also have the prototype:

afx_msg void OnLButtonDown (UINT nFlags, CPoint point);

the afx_msg is a “no-op” that alerts that this is a prototype for a message map function.

Saving the View’s state:

 The view’s OnDraw function draws an image based on the view’s current state and user actions can alter that state.

 Two view class members used often are m_rectEllipse and m_nColor.

 m_rectEllipse is an object of class CRect, which holds the current bounding rectangle of an ellipse.

 m_nColor is an integer that holds the current ellipse color value.

 By convention, MFC library nonstatic class data member names begin with m_.

Initializing a view class data member:

 The most efficient place to initialize a class data member is in the constructor:

CMyView::CMyView:m_rectEllipse(0,0,200,200)
{
………..
}

Steps to create the application:

1. Run AppWizard to create second application.
2. Add the m_rectEllipse and m_nColor data members to secondView.
 In the workspace window set to ClassView, right click the secondView class, select Add Member variable and then insert the data members:
private:
CRect m_rectEllipse;
int m_nColor;
3. Use ClassWizard to add a secondView class message handles.
 Choose ClassWizard by right-click on the source window.
 Select
Class Name: secondView
Object ID: SecondView
Messages: WM_LBUTTONDOWN (double-click)

the OnLButtonDown function name should appear in the Member functions list box.

4. Edit the OnLButtonDown code in secondView.cpp.

 Click the edit code button in the ClassWizard and the code here:

void secondView::OnLButtonDown (UINT nFlags, CPoint point)
{
if (m_rectEllipse.PtInRect(point))
{
if (m_nColor == GRAY_BRUSH)
{
m_nColor = WHITE_BRUSH;
}
else
{
m_nColor = GRAY_BRUSH;
}
InvalidateRect (m_rectEllipse);
}
}







5. Edit the constructor and the OnDraw function in secondView class.

secondView :: secondView() : m_rectEllipse(0,0,200,200)
{
m_nColor = GRAY_BRUSH;
}

void secondView :: OnDraw (CDC* pDC)
{
pDC -> SelectStockObject (m_nColor);
pDC -> Ellipse (m_rectEllipse);
}

6. Build and run the program.