MFC OnDraw vs. OnPaint

Question

I am wondering about the difference between OnDraw() and OnPaint() in MFC.

After searching the Internet for a while, I found a useful article. In summary,

1. WM_PAINT will trigger OnPaint(), which calls OnDraw() and passes a CDC*:

   void CView::OnPaint()
   {
       // standard paint routine
       CPaintDC dc(this);
       OnPrepareDC(&dc);
       OnDraw(&dc);
   }
   

2. Another article mentions that when printing a document, OnPrint() also calls OnDraw() by passing a printer DC. Therefore, by overriding OnDraw(), you get screen painting and printing both in one function, which is convenient.

I tried to put my statements for drawing in either OnDraw() and OnPaint(). Either can work well. OnDraw() is a little easier because it has already gotten a pointer pDC.

Answer 1

Device contexts are an ancient abstraction. They have been described as early as 1982 in the first edition of Computer Graphics: Principles and Practice (probably even earlier) and seem to confuse people to this day.

The primary purpose of a device context is to abstract peculiarities of render devices (such as displays, printers, in-memory bitmaps, etc.) and provide a coherent interface. Code that's rendering into a device context generally does not need to know, which device is ultimately consuming the render commands.

The documentation entry titled Drawing in a View goes on to explain how the system is intended to work: In short, all painting should be performed in an OnDraw override that receives a device context. The system-provided OnPaint implementation then constructs a CPaintDC and calls OnDraw.

Up to this point this seems to be just an overly complex way to render the contents of a window. Things start to make sense when you implement, say, printing support. Now all you have to do is set up a printing device context and call OnDraw. Nothing in your OnDraw implementation needs to change.