An object that manages an ordered collection of data items and presents them using customizable layouts.
@MainActor class UICollectionView : [UIScrollView](<https://developer.apple.com/documentation/uikit/uiscrollview>)
When you add a collection view to your user interface, your app's main job is to manage the data associated with that collection view. The collection view gets its data from the data source object, stored in the collection view’s dataSource property.
For your data source, you can use a UICollectionViewDiffableDataSource object, which provides the behavior you need to simply and efficiently manage updates to your collection view's data and user interface. Alternatively, you can create a custom data source object by adopting the UICollectionViewDataSource protocol.
Data in the collection view is organized into individual items, which you can group into sections for presentation. An item is the smallest unit of data you want to present. For example, in a photos app, an item might be a single image. The collection view presents items onscreen using a cell, which is an instance of the UICollectionViewCell class that your data source configures and provides.
Figure 1 A collection view using the flow layout
In addition to its cells, a collection view can present data using other types of views. These supplementary views can be, for example, section headers and footers that are separate from the individual cells but still convey information. Support for supplementary views is optional and defined by the collection view’s layout object, which is also responsible for defining the placement of those views.
Besides embedding a UICollectionView in your user interface, you use the methods of the collection view to ensure that the visual presentation of items matches the order in your data source object. A UICollectionViewDiffableDataSource object manages this process automatically. If you're using a custom data source, then whenever you add, delete, or rearrange data in your collection, you use the methods of UICollectionView to insert, delete, and rearrange the corresponding cells.
You also use the collection view object to manage the selected items, although for this behavior the collection view works with its associated delegate object.