Painting with a custom widget is, at its heart, no different than offscreen painting; all you need is a widget subclass and a painter pointing to the widget, and you're all set. Yet, how do you know when to paint?
Qt's QWidget class defines an interface used by the rendering system to pass events to your widget: Qt defines the QEvent class to encapsulate the data about an event, and the QWidget class defines an interface that Qt's rendering system uses to pass events to your widget for processing. Qt uses this event system not just to indicate things such as mouse movements and keyboard input, but also for requests to paint the screen as well.
Let's look at painting first. QWidget defines the paintEvent method, which Qt's rendering system invokes, passing a QPaintEvent pointer. The QPaintEvent pointer includes the region that needs...