Drawing in a View

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Nearly all drawing in your application occurs in the view's OnDraw member function, which you must override in your view class. (The exception is mouse drawing, discussed in Interpreting User Input Through a View.) Your OnDraw override:

1. Gets data by calling the document member functions you provide.
   
   
2. Displays the data by calling member functions of a device-context object that the framework passes to OnDraw.

When a document's data changes in some way, the view must be redrawn to reflect the changes. Typically, this happens when the user makes a change through a view on the document. In this case, the view calls the document's UpdateAllViews member function to notify all views on the same document to update themselves. UpdateAllViews calls each view's OnUpdate member function. The default implementation of OnUpdate invalidates the view's entire client area. You can override it to invalidate only those regions of the client area that map to the modified portions of the document.

The UpdateAllViews member function of class CDocument and the OnUpdate member function of class CView let you pass information describing what parts of the document were modified. This "hint" mechanism lets you limit the area that the view must redraw. OnUpdate takes two "hint" arguments. The first, lHint, of type LPARAM, lets you pass any data you like, while the second, pHint, of type CObject*, lets you pass a pointer to any object derived from CObject.

When a view becomes invalid, Windows sends it a WM_PAINT message. The view's OnPaint handler function responds to the message by creating a device-context object of class CPaintDC and calls your view's OnDraw member function. You do not normally have to write an overriding OnPaint handler function.

A device context is a Windows data structure that contains information about the drawing attributes of a device such as a display or a printer. All drawing calls are made through a device-context object. For drawing on the screen, OnDraw is passed a CPaintDC object. For drawing on a printer, it is passed a CDC object set up for the current printer.

Your code for drawing in the view first retrieves a pointer to the document, then makes drawing calls through the device context. The following simple OnDraw example illustrates the process:

void CMyView::OnDraw( CDC* pDC )
{
    CMyDoc* pDoc = GetDocument();
    CString s = pDoc->GetData();   // Returns a CString
    CRect rect;

    GetClientRect( &rect );
    pDC->SetTextAlign( TA_BASELINE | TA_CENTER );
    pDC->TextOut( rect.right / 2, rect.bottom / 2, 
                  s, s.GetLength() );
}

In this example, you would define the GetData function as a member of your derived document class.

The example prints whatever string it gets from the document, centered in the view. If the OnDraw call is for screen drawing, the CDC object passed in pDC is a CPaintDC whose constructor has already called BeginPaint. Calls to drawing functions are made through the device-context pointer. For information about device contexts and drawing calls, see class CDC in the Class Library Reference and Working with Window Objects.

For more examples of how to write OnDraw, see the MFC Samples.