8.4 The class hierarchy for DUIM-Gadgets

8.4.1 The <gadget> class and its subclasses

The base class for the majority of DUIM gadgets is the <gadget> class, which is itself a subclass of <object>. All other DUIM gadgets are subclasses of <gadget>, with the exception of <list-item>, <tree-node>, and <table-item>.

The immediate subclasses of <gadget> are shown in Table 8.1. Only <value-gadget> and <page> have any subclasses defined. See Section 8.4.2 on page 509 and Section 8.4.2 on page 509 for details of these subclasses.

The <gadget> class provides a number of subclasses that allow particular parts of a user interface to be created:

<menu>: Use this class to add a menu to the menu bar of any application frame. Menus themselves contain commands created using the menu-specific button and collection gadgets described in Section 8.4.4 and Section 8.4.5.
<tool-bar>: This class is used to add a tool bar to an application frame. A tool bar is a row of buttons that duplicates the functionality of the most commonly used menu commands, thereby providing the user with quick access to the most useful operations in the application.
<scroller>: This is a generic scrolling gadget that can be used in a number of situations.
<viewport>: A viewport can be used to create a generic pane for displaying specialized contents that you may have defined. Use this class when there is no other class provided for displaying the objects in question.
<splitter>: This class can be used to split the current view in half. This allows the user, for example, to create a second view of the same document.

The <gadget> class provides a number of subclasses that allow general spatial and grouping capability, in addition to the layout functionality described in Chapter 7, "DUIM-Layouts Library". These are as follows:

<label>: This class is used to assign label to many other types of gadget. Many gadgets can be assigned one or more labels, usually by means of a label: init-keyword. This class is used to assign any label.
<separator>: This allows a line to be drawn between any two gadgets or groups of gadgets, so as to provide a visible barrier between them.
<spacing>: This allows you to specify how much space should be placed between any two gadgets or groups of gadgets.
<border>: This allows a visible border to be placed around any number of gadgets.
<group-box>: This allows you to group together any number of related gadgets in a frame. Grouped elements are usually displayed with a border and label identifying the grouping.

Table 8.1 Overall class hierarchy for the DUIM-Gadgets library
<object>
	<gadget>
		<action-gadget>
		<value-gadget>	See Section 8.4.2
		<label>
		<menu>
		<tool-bar>
		<scroller>
		<separator>
		<viewport>
		<spacing>
		<border>
		<group-box>
		<splitter>
		<page>	See Section 8.4.3
	<list-item>
	<tree-node>
	<table-item>