An object that manages an ordered collection of data items and presents them using customizable layouts.
- iOS 6.0+
- tvOS 9.0+
When adding 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, which is an object that conforms to the
UICollection Protocol and is provided by your app. Data in the collection view is organized into individual items, which can then be grouped 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
UICollection class that your data source configures and provides.
In addition to its cells, a collection view can present data using other types of views too. These supplementary views can be things like section headers and footers that are separate from the individual cells but still convey some sort of 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 it in your user interface, you use the methods of
UICollection object to ensure that the visual presentation of items matches the order in your data source object. Thus, whenever you add, delete, or rearrange data in your collection, you use the methods of this class 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
Collection Views and Layout Objects
A very important object associated with a collection view is the layout object, which is a subclass of the
UICollection class. The layout object is responsible for defining the organization and location of all cells and supplementary views inside the collection view. Although it defines their locations, the layout object does not actually apply that information to the corresponding views. Because the creation of cells and supplementary views involves coordination between the collection view and your data source object, the collection view actually applies layout information to the views. Thus, in a sense, the layout object is like another data source, only providing visual information instead of item data.
You normally specify a layout object when creating a collection view but you can also change the layout of a collection view dynamically. The layout object is stored in the
collection property. Setting this property directly updates the layout immediately, without animating the changes. If you want to animate the changes, you must call the
set method instead.
If you want to create an interactive transition—one that is driven by a gesture recognizer or touch events—use the
start method to change the layout object. That method installs an intermediate layout object whose purpose is to work with your gesture recognizer or event-handling code to track the transition progress. When your event-handling code determines that the transition is finished, it calls the
cancel method to remove the intermediate layout object and install the intended target layout object.
Creating Cells and Supplementary Views
The collection view’s data source object provides both the content for items and the views used to present that content. When the collection view first loads its content, it asks its data source to provide a view for each visible item. To simplify the creation process for your code, the collection view requires that you always dequeue views, rather than create them explicitly in your code. There are two methods for dequeueing views. The one you use depends on which type of view has been requested:
dequeueto get a cell for an item in the collection view.
Reusable Cell(with Reuse Identifier: for:)
dequeuemethod to get a supplementary view requested by the layout object.
Reusable Supplementary View(of Kind: with Reuse Identifier: for:)
Before you call either of these methods, you must tell the collection view how to create the corresponding view if one does not already exist. For this, you must register either a class or a nib file with the collection view. For example, when registering cells, you use the
register(_: method. As part of the registration process, you specify the reuse identifier that identifies the purpose of the view. This is the same string you use when dequeueing the view later.
After dequeueing the appropriate view in your delegate method, configure its content and return it to the collection view for use. After getting the layout information from the layout object, the collection view applies it to the view and displays it.
For more information about implementing the data source methods to create and configure views, see
Reordering Items Interactively
Collection views allow you to move items around based on user interactions. Normally, the order of items in a collection view is defined by your data source. If you support the ability for users to reorder items, you can configure a gesture recognizer to track the user’s interactions with a collection view item and update that item’s position.
To begin the interactive repositioning of an item, call the
begin method of the collection view. While your gesture recognizer is tracking touch events, call the
update method to report changes in the touch location. When you are done tracking the gesture, call the
cancel method to conclude the interactions and update the collection view.
During user interactions, the collection view invalidates its layout dynamically to reflect the current position of the item. If you do nothing, the default layout behavior repositions the items for you, but you can customize the layout animations if you want. When interactions finish, updates its data source object with the new location of the item.
UICollection class provides a default gesture recognizer that you can use to rearrange items in its managed collection view. To install this gesture recognizer, set the
installs property of the collection view controller to
Interface Builder Attributes
Table 1 lists the attributes that you configure for collection views in Interface Builder.
The number of prototype cells. This property controls the specified number of prototype cells for you to configure in your storyboard. Collection views must always have at least one cell and may have multiple cells for displaying different types of content or for displaying the same content in different ways.
The layout object to use. Use this control to select between the
When the flow layout is selected, you can also configure the scrolling direction for the collection view’s content and whether the flow layout has header and footer views. Enabling header and footer views adds reusable views to your storyboard that you can configure with your header and footer content. You can also create those views programmatically.
When a custom layout is selected, you must specify the
When the Flow layout is selected, the Size inspector for the collection view contains additional attributes for configuring flow layout metrics. Use those attributes to configure the size of your cells, the size of headers and footers, the minimum spacing between cells, and any margins around each section of cells. For more information about the meaning of the flow layout metrics, see
A collection view has no direct content of its own to internationalize. Instead, you internationalize the cells and reusable views of the collection view. For more information about internationalization, see Internationalization and Localization Guide.
A collection view has no content of its own to make accessible. If your cells and reusable views contain standard UIKit controls such as
UIText, you can make those controls accessible. When a collection view changes its onscreen layout, it posts the
For general information about making your interface accessible, see Accessibility Programming Guide for iOS.