UICollectionViewFlowLayout

UICollectionView is a fundamental framework class in Apple's UIKit framework for iOS, iPadOS, and tvOS. It provides a flexible and powerful way to display collections of data in a visually appealing and organized manner. Unlike UITableView which is designed for linear lists of data, UICollectionView allows for more complex and customizable layouts, enabling the presentation of items in grids, staggered arrangements, or any other arbitrary arrangement defined by a layout object.

At its core, UICollectionView manages the display of a collection of data items. It works in conjunction with a data source object, which provides the actual data to be displayed, and a delegate object, which handles user interactions and data management. The visual appearance of each individual item in the collection is determined by a cell, which is a subclass of UICollectionViewCell. These cells are reusable to optimize performance, similar to how UITableViewCells are reused.

The key to UICollectionView's flexibility lies in its layout system. Developers can use pre-defined layout classes like UICollectionViewFlowLayout for standard grid-like layouts, or they can create custom UICollectionViewLayout subclasses to implement highly bespoke arrangements. Layout objects are responsible for determining the position, size, and appearance of each cell and supplementary view within the collection view. Supplementary views, such as headers and footers, can also be added to provide additional context or visual structure to the collection. The ability to define custom layouts makes UICollectionView suitable for a wide range of applications, from photo galleries and dashboards to complex data visualizations.