The Container widget, introduced in Motif 2.0, is an approximation to an MVC (Model/View/Controller) component for the toolkit.
What the widget does is lay out application objects in a variety of styles, and control the selection of these objects. The application objects themselves are pictorially represented by the IconGadget component; the Container supplies the layout and selection mechanisms for its IconGadget children.
The Container supports three layout styles: a Tree (or Outliner) style, a Grid style, and a free-format Spatial style. The style can be changed dynamically to provide an alternative view of the application objects.
Each IconGadget can be associated with arbitrary data, using constraint resources defined by the Container. The extra data is known as "detail", and the Container knows how to lay out its children so that the detail is visible in a tabular arrangement, aligned with the IconGadget to which it pertains. Think of the detail as application attributes which can be displayed on request, in order to provide more information about the application objects on display to the user.
For more details of the Container widget, see XmContainer(3X) in the OSF/Motif Programmer's Reference.
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