NSCollectionView

NSCollectionView is a macOS user interface component in AppKit that presents a scrollable collection of views, typically arranged in a grid or list. It is the macOS counterpart to UICollectionView and is used to display homogeneous or heterogeneous items in a flexible, reusable way.

Core concepts revolve around a layout object that defines how items are sized and positioned. The collection view uses an NSCollectionViewLayout to arrange its content. The built-in option most developers start with is NSCollectionViewFlowLayout, which supports grid and list patterns; more complex or custom layouts can be implemented to achieve specific arrangements. Each item in the collection is represented by an NSCollectionViewItem, which owns the view used to render content. Supplementary views, such as headers and footers, and decoration views provide additional chrome and visual structure around items.

Data source and delegate roles mirror other collection-based components. The NSCollectionViewDataSource protocol supplies the number of sections and items and creates the NSCollectionViewItem for a given index path. The NSCollectionViewDelegate handles user interactions, selection, and actions, while NSCollectionViewDelegateFlowLayout can provide sizing information when using the flow layout.

Registration and reuse are central to performance. You register item and supplementary view classes or nibs with identifiers, then the collection view dequeues configured items as needed via the data source method that creates items for represented objects at index paths.

In practice, NSCollectionView supports selection, drag and drop, and contextual menus, and is a versatile tool for presenting grids, lists, or custom content layouts in AppKit-based interfaces.