Often the widget invokes a callback from an action routine when the widget receives an event of a given type. Widgets that invoke callbacks have resources whose value is a list of callback procedures. A group of elements all of the same type. A resource class represents a group of resources with different names. A widget class represents the procedures and data structures shared by all widgets of that class.
Clients create their own windows and know how to resize themselves.
A selection often used to cut or copy data from one client and paste it into another client or another window of the same client. For example, windows are clipped by their parents. An association between pixel values and colors. Each color is represented by a triple of red, green, and blue values that result in a particular color on a particular screen. Each window has an associated colormap that determines what color is used to display each pixel.