Subclass UICollectionViewFlowLayout to get custom functionality such as animating cells that are added.

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A collection view is a way to present an ordered set of data items using a flexible and changeable layout. The most common use for collection views is to present items in a grid-like arrangement, but collection views in iOS are capable of more than just rows and columns. With collection views, the precise layout of visual elements is definable through subclassing and can be changed dynamically, so you can implement grids, stacks, circular layouts, dynamically changing layouts, or any type of arrangement you can imagine.

Collection views keep a strict separation between the data being presented and the visual elements used to present that data. In most cases, your app is solely responsible for managing the data. Your app also provides the view objects used to present that data. After that, the collection view takes your views and does all the work of positioning them onscreen. It does this work in conjunction with a layout object, which specifies the placement and visual attributes for your views and that can be subclassed to fit your app’s exact needs. Thus, you provide the data, the layout object provides the placement information, and the collection view merges the two pieces together to achieve the final appearance.

Collection View Basics

To present its content onscreen, a collection view cooperates with many different objects. Some objects are custom and must be provided by your app. For example, your app must provide a data source object that tells the collection view how many items there are to display. Other objects are provided by UIKit and are part of the basic collection view design.

Like tables, collection views are data-oriented objects whose implementation involves a collaboration with your app’s objects. Understanding what you have to do in your code requires a little background information about how a collection view does what it does.

A Collection View Is a Collaboration of Objects

The design of collection views separates the data being presented from the way that data is arranged and presented onscreen. Although your app is strictly responsible for managing the data to be presented, its visual presentation is managed by many different objects. Table 1-1 lists the collection view classes in UIKit and organizes them by the roles they play in implementing a collection view interface. Most of the classes are designed to be used as is without any need for subclassing, so you can usually implement a collection view with very little code. And when you want to go beyond the provided behavior, you can subclass and provide that behavior

When creating a collection view interface, you first add a UICollectionView object to your storyboard or nib file. Think of the collection view as the central hub, from which all other objects emanate. After adding that object, you can begin to configure any related objects, such as the data source or delegate. All configurations are centered around the collection view itself. For example, you never create a layout object without also creating a collection view object.

Reusable Views Improve Performance

Collection views employ a view recycling program to improve efficiency. As views move offscreen, they are removed from view and placed in a reuse queue instead of being deleted. As new content is scrolled onscreen, views are removed from the queue and repurposed with new content. To facilitate this recycling and reuse, all views displayed by the collection view must descend from the UICollectionReusableView class.

Collection views support three distinct types of reusable views, each of which has a specific intended usage:

Cells present the main content of your collection view. The job of a cell is to present the content for a single item from your data source object. Each cell must be an instance of the UICollectionViewCell class, which you may subclass as needed to present your content. Cell objects provide inherent support for managing their own selection and highlight state.

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