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,

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);
}

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.

`OnPaint` is a method of `CPluginWindow`. `OnDraw` is a method in the class `CView` and is more general. `CPluginWindow` inherits `CView`. — user51187286016, Apr 02 '22 at 15:25
@user51187286016 `OnPaint` is a message-handling routine in the [`CWnd` class](https://learn.microsoft.com/en-us/cpp/mfc/reference/cwnd-class?view=msvc-170#onpaint), which is the base class from which (almost) all other MFC windows and view types are derived. — Adrian Mole, Apr 02 '22 at 15:39

score 4 · Answer 1 · answered Apr 02 '22 at 16:28

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.

MFC OnDraw vs. OnPaint

1 Answers1