NSOutlineView

NSOutlineView is a Cocoa AppKit control for displaying hierarchical data in macOS applications. It is a subclass of NSTableView and presents items in a grouped, expandable list. Each item can have children, which can be shown or hidden by expanding or collapsing the item. NSOutlineView is data-driven and relies on the NSOutlineViewDataSource protocol to obtain the structure of the data and on the NSOutlineViewDelegate protocol to respond to user actions and customize presentation.

Key data source methods provide the tree structure: number of children of a given item, the child item for a given index, and whether an item is expandable. The root item is typically nil. Delegate methods are used for handling events such as selection changes, item expansion and collapse, and for supplying the views used to render each item in a given column. In modern AppKit, outline views are view-based, with per-item views created by outlineView:viewForTableColumn:item: or row view via outlineView:rowViewForItem:.

NSOutlineView supports common table features such as multiple selection, editing, reordering by drag and drop, and context menus. It can display different cell views per column and per item, and can integrate with a data model that represents a tree structure. It is commonly used for navigational interfaces, project outlines, and file systems, and is the standard way to present hierarchical data in macOS applications that use AppKit.

Developers typically implement a custom data model for the tree, implement the required data source methods, and provide appropriate views for display via the delegate. Accessibility support and configuration options allow fine-grained control over appearance and behavior.