NSOutlineViewDelegate

NSOutlineViewDelegate is a protocol in Apple’s AppKit that defines optional methods a delegate can implement to customize the presentation and behavior of an NSOutlineView. The delegate works alongside the NSOutlineViewDataSource, which supplies the hierarchical data; the data source determines structure and content, while the delegate handles appearance, interaction, and presentation details.

A primary role of the delegate is to supply and configure views for items in a view-based outline. The method outlineView:viewForTableColumn:item: lets you return and configure the view used to display a given item. The delegate can also customize the appearance of the outline cell itself with methods such as outlineView:willDisplayOutlineCell:forTableColumn:item:.

The delegate also influences user interaction and behavior. It can participate in decisions about item expansion and selection, providing methods that enable or restrict expanding or collapsing items and selecting items. It can also influence layout by adjusting row heights via outlineView:heightOfRowByItem: and by supplying a custom row view through rowViewForItem:.

Additional capabilities supported by the delegate include editing and interaction enhancements, drag-and-drop handling, tooltips, and other UI refinements, depending on the features supported by the outline view and the app’s needs. Implementers typically provide a subset of these methods to tailor the outline’s look and feel, performance, and interactivity.

In practice, developers implement NSOutlineViewDelegate to customize how hierarchical data is presented and interacted with, while NSOutlineViewDataSource supplies the underlying data structure. The combination enables rich, flexible user interfaces for hierarchical content in macOS applications.