Drawing and Refreshing

• Direct drawing (the result of a call to the Draw member function) is the general mechanism for rendering the view. Objects outside of the View System also can draw your view.
• Refreshing (the result of a call to the Refresh member function) is a device-dependent mechanism intended for rendering the view on a specific display device (the device associated with the view hierarchy's view root). The View System uses this mechanism to redraw those parts of the display that no longer accurately reflect the state of the view hierarchy.

Similarly, Refresh calls RefreshContents, then calls every child's Refresh, as shown in Figure 29. RefreshContents calls DrawContents by default, so the default behaviors of Draw and Refresh are the same.

Only the View System calls your view's Refresh member function, and only to render the view on the device that displays the view's hierarchy. You can override RefreshContents to provide optimizations for the refresh operation. For example, you can use cached, off-screen bitmaps to increase rendering speed in your RefreshContents implementation.

In contrast, any object can call your view's Draw member function, at any time, even before your view is attached to a display device. Even while your view is attached to a display device, anyone can call Draw to render your view on a different display device, such as a printer, with completely different graphic properties. Therefore, you must never use device-dependent operations of any kind in your DrawContents implementation.