An @dfn{outline view} is a specialised form of table view designed for displaying hierachical data in a tree like format. It looks alot like a Windows' TreeView Control, but operates differently, provides much more powerful functionality and it is less tedious to programme.
The node's in the outline view can be collapsed and expanded and display a list of sub-nodes. This makes the outline view hierachical.
It uses the @code{NSOutlineView} class, which inherits from @code{NSTableView}. This means that most of the behaviour that applies to tableviews also applies to outline views, such as changing columns/rows and their display, etc. It extends the tableview with a hierachial layout for data, in which nodes can be expanded and contracted.
Like the table view, the outline view control uses a data source object to get it's data, as well as a delegate to modify it's behaviour. These are objects implementing the informal protocols @code{NSOutlineViewDataSource} and @code{NSOutlineViewDelegate}. Although a delegate object is optional, outline views require a data source object.
See the @cite{GNUstep GUI Reference} for more information about outline views (including class documentation).
@section Using a Data Source
The data source for an outline view implements the @code{NSOutlineViewDataSource} informal protocol. Some of it's methods are compulsory; some are not.
Note that a parameter in many of the delegate's methods is an untyped object @var{item}. This object is supplied by you, and the outline view passes it back to your delegate as a representation of a node or leaf row.
The outline view requires you implement the following methods:
