An XI interface is a window containing any number of user interface objects. Most XI objects are operable by the user and will invoke some action in the application. (Some XI objects are display-only.) Together the objects inside the interface behave in a coherent fashion to achieve the objectives of the program. You can think of these objects as analogous to controls in the XVT parlance or widgets in the vocabulary of some native tool kits.

For an overview of the kinds of objects an XI interface can hold, all types of XI objects are found in this image.

As you can see, an XI interface can hold any number of XI objects such as forms, lists, containers and groups. These objects might in turn hold other XI interface objects such as edit fields, columns and buttons. (Edit fields are called edit controls in MS-Windows.) In this sense, the XI object structure is hierarchical with the interface at the top of the tree, composite objects such as forms, lists, containers and groups on the next level, and edit fields, columns and buttons on the lowest level.

In the following diagram, you will see an example tree showing how XI interface objects are related to one another. Imagine that this tree corresponds to an interface that has a form with one edit field, one line of descriptive text, a list with one column and one row, a group (of either edit fields or columns), a rectangle, a line, and a container with one button. In the real world this interface would not be a very practical, but it is useful for illustrating the parent-child relationships between XI objects. When we describe XI objects in more detail in the next section, keep this diagram in mind. Note that buttons can be direct children of an interface if you do not wish to put them in a container. 

XI Objects

 When you look at the diagram "XI Object Hierarchy", you can see the relationships XI objects have to one another. Every XI object must be created within an interface object, and therefore each XI object must have an interface as one of its ancestors. This makes sense from the user’s point of view because whenever you create a control, you must have a window to put it in.

You can also see that lists, forms, groups, containers, buttons, rectangles, lines, and static text are children of the interface while edit fields, buttons and columns are grandchildren. The XI objects in the object hierarchy that are children of the interface object are summarized following this paragraph. Since list, form and container objects have children of their own, we call them composite objects. Columns are children of lists, edit fields are children of forms, and buttons are children of containers: 

• Lists:
  XI’s spreadsheet-style lists can be scrolled by the user and the user can edit each cell in the spreadsheet if your application permits it. In an XI list, columns are the only child objects of the list. Cells and rows are “pseudo-objects” because they are not actually instantiated when the interface is created. Nevertheless, you can treat them like real objects and “use” them to get and set text, get and set attributes, set focus, and so on.
• Forms:
  Forms contain any number of edit fields that can be edited. Edit fields are child objects of forms. (Edit fields are like edit controls in MS-Windows).
• Containers:
  Containers serve to arrange buttons. You can either stack buttons on top of one another, or arrange them end to end. Buttons can also be arranged in a grid. Buttons are child objects of containers. In addition, if a container contains radio buttons, the container serves to group the radio buttons together so that only one will be checked at a time. Buttons can also be children of the interface.
• Groups:
  You can have groups of edit fields or columns. In XI, groups are there to make it easier to validate data entry for associated edit fields or columns. Groups have no appearance in the window. To group controls visually, use XI rectangle controls.
• Static Text:
  Static text is used to label objects on the interface.
• Rectangles:
  Rectangles are used to visually group objects on the interface.
• Lines:
  Lines are used to visually separate objects on the interface.