Cocoa Touch Layer

The Cocoa Touch layer contains key frameworks for building iOS apps. These frameworks define the appearance of your app. They also provide the basic app infrastructure and support for key technologies such as multitasking, touch-based input, push notifications, and many high-level system services. When designing your apps, you should investigate the technologies in this layer first to see if they meet your needs.

High-Level Features

The following sections describe some of the key technologies available in the Cocoa Touch layer.

AirDrop

AirDrop lets users share photos, documents, URLs, and other kinds of data with nearby devices. Support for sending files to other iOS devices using AirDrop is built into the existing UIActivityViewController class. This class displays different options for sharing the content that you specify. If you are not yet using this class, you should consider adding it to your interface.

To receive files sent using AirDrop, your app must do the following:

  • Declare support for the appropriate document types in Xcode. (Xcode adds the appropriate keys to your app’s Info.plist file.) The system uses this information to determine whether your app can open a given file.

  • Implement the application:openURL:sourceApplication:annotation: method in your app delegate. (The system calls this method when a new file is received.)

Files sent to your app are placed in the Documents/Inbox directory of your app’s home directory. If you plan to modify the file, you must move it out of this directory before doing so. (The system allows your app to read and delete files in this directory only.) Files stored in this directory are encrypted using data protection, so you must be prepared for the file to be inaccessible if the device is currently locked.

For more information about using an activity view controller to share data, see UIActivityViewController Class Reference.

Text Kit

Text Kit is a full-featured, high-level set of classes for handling text and fine typography. Using Text Kit, you can lay out styled text into paragraphs, columns, and pages; you can flow text around arbitrary regions such as graphics; and you can use it to manage multiple fonts. If you were considering using Core Text to implement text rendering, you should consider Text Kit instead. Text Kit is integrated with all UIKit text-based controls to enable apps to create, edit, display, and store text more easily—and with less code than was previously possible in iOS.

Text Kit comprises new UIKit classes, along with extensions to existing classes, including the following:

For more information about Text Kit, see Text Programming Guide for iOS.

UIKit Dynamics

Apps can now specify dynamic behaviors for UIView objects and for other objects that conform to the UIDynamicItem protocol. (Objects that conform to this protocol are called dynamic items.) Dynamic behaviors offer a way to improve the user experience of your app by incorporating real-world behavior and characteristics into your app’s user interface. UIKit dynamics supports the following types of behaviors:

  • A UIAttachmentBehavior object specifies a connection between two dynamic items or between an item and a point. When one item (or point) moves, the attached item also moves. The connection is not completely static, though. An attachment behavior has damping and oscillation properties that determine how the behavior changes over time.

  • A UICollisionBehavior object lets dynamic items participate in collisions with each other and with the behavior’s specified boundaries. The behavior also lets those items respond appropriately to collisions.

  • A UIGravityBehavior object specifies a gravity vector for its dynamic items. Dynamic items accelerate in the vector’s direction until they collide with other appropriately configured items or with a boundary.

  • A UIPushBehavior object specifies a continuous or instantaneous force vector for its dynamic items.

  • A UISnapBehavior object specifies a snap point for a dynamic item. The item snaps to the point with a configured effect. For example, a dynamic item can snap to the point as if it were attached to a spring.

Dynamic behaviors become active when you add them to an animator object, which is an instance of the UIDynamicAnimator class. The animator provides the context in which dynamic behaviors execute. A given dynamic item can have multiple behaviors, but all of those behaviors must be animated by the same animator object.

For information about the behaviors you can apply, see UIKit Framework Reference.

Multitasking

Battery life is an important consideration for users of iOS devices and the multitasking model in iOS is designed to maximize battery life while giving apps the time they need to do critical work. When the user presses the Home button, the foreground app shifts to a background execution context. If the app has no more work to do, it is suspended from active execution and put into a "freeze-dried” state, where it remains in memory but does not execute any code. Apps that do need specific types of work can ask the system for background execution time. For example:

  • An app can request a finite amount of time to complete some important task.

  • An app that supports specific services (such as audio playback) can request time to provide those services.

  • An app can use local notifications to generate user alerts at designated times, whether or not the app is running.

  • An app can download content periodically from the network.

  • An app can download content in response to a push notification.

For information on how to support the iOS multitasking model, see iOS App Programming Guide.

Auto Layout

Auto layout helps you build dynamic interfaces with very little code. Using Auto Layout, you define rules for how to lay out the elements in your user interface. These rules express a larger class of relationships and are more intuitive to use than the springs and struts model that was used previously. For example, you can specify that a button always be 20 points from the left edge of its parent view.

The entities used in Auto Layout are Objective-C objects called constraints. Constraints provide several benefits:

  • They support localization through the swapping of strings alone, instead of requiring you to update your layouts.

  • They support mirroring of user interface elements for right-to-left languages, such as Hebrew and Arabic.

  • They promote a better separation of responsibilities between objects in the view and controller layers.

    A view object usually has values for its standard size, its positioning within its superview, and its positioning relative to its sibling views. A view controller can override these values if something nonstandard is required.

For more information about using Auto Layout, see Auto Layout Guide.

Storyboards

Storyboards are the recommended way to design your app’s user interface. Storyboards let you design your entire user interface in one place so that you can see all of your views and view controllers and understand how they work together. An important part of storyboards is the ability to define segues, which are transitions from one view controller to another. These transitions allow you to capture the flow of your user interface in addition to the content. You can define these transitions visually, in Xcode, or initiate them programmatically.

You can use a single storyboard file to store all of your app’s view controllers and views, or you can use multiple view storyboards to organize portions of your interface. At build time, Xcode takes the contents of the storyboard file and divides it into discrete pieces that can be loaded individually for better performance. Your app never needs to manipulate these pieces directly. The UIKit framework provides convenience classes for accessing the contents of a storyboard from your code.

For more information about using storyboards to design your interface, see Xcode Overview. For information about how to access storyboards from your code, see the UIStoryboard Class Reference.

UI State Preservation

State preservation provides a seamless experience for users by having your app appear to be always running, even when it was not. If the system encounters memory pressure, it may be forced to terminate one or more background apps quietly. When an app moves from the foreground to the background, it can preserve the state of its views and view controllers. During its next launch cycle, it can use that preserved state information to restore its views and view controllers to their previous configurations and make it appear as if the app had never quit.

For more information about how to add state preservation support to your app, see iOS App Programming Guide.

Apple Push Notification Service

Apple Push Notification service provides a way to alert users about new information, even when your app is not actively running. Using this service, you can push text notifications, add a badge to your app icon, or trigger audible alerts on user devices at any time. These messages let users know that they should open your app to receive the related information. In iOS 7, you can even push silent notifications to let your app know that new content is available to download.

From a design standpoint, there are two parts to making push notifications work for iOS apps. First, the app must request the delivery of notifications and process the notification data once it is delivered. Second, you need to provide a server-side process to generate the notifications in the first place. This process lives on your own local server and works with Apple Push Notification Service to trigger the notifications.

For more information about how to configure your app to use remote notifications, see Local and Push Notification Programming Guide.

Local Notifications

Local notifications complement the existing push notification mechanism by giving apps a way to generate the notifications locally instead of relying on an external server. Apps running in the background can use local notifications as a way to get a user’s attention when important events happen. For example, a navigation app running in the background can use local notifications to alert the user when it is time to make a turn. Apps can also schedule the delivery of local notifications for a future date and time and have those notifications delivered even if the app is not running.

An advantage of local notifications is that they are independent of your app. After a notification is scheduled, the system manages the delivery of it. Your app does not even have to be running when the notification is delivered.

For more information about using local notifications, see Local and Push Notification Programming Guide.

Gesture Recognizers

Gesture recognizers detect common types of gestures, such as swipes and pinches in your app’s views. Because they use the same heuristics as the system for detecting gestures, gesture recognizers offer a consistent behavior for your apps. To use one, you attach the gesture recognizer to your view and give it an action method to perform when the gesture occurs. The gesture recognizer does the difficult work of tracking the raw touch events and determining when they constitute the intended gesture.

All gesture recognizers are based on the UIGestureRecognizer class, which defines the basic behavior. UIKit supplies standard gesture recognizer subclasses to detect taps, pinches, pans, swipes, rotations. You can also tailor the behavior of most gesture recognizers to your app’s needs. For example, you can tell a tap gesture recognizer to detect a specific number of taps before calling your action method.

For more information about the available gesture recognizers, see Event Handling Guide for iOS.

Standard System View Controllers

Many system frameworks define view controllers for standard system interfaces. Whenever possible, use the provided view controllers rather than create your own. You are encouraged to use these view controllers in your apps to present a consistent user experience. Whenever you need to perform one of the following tasks, you should use a view controller from the corresponding framework:

  • Display or edit contact information. Use the view controllers in the Address Book UI framework.

  • Create or edit calendar events. Use the view controllers in the Event Kit UI framework.

  • Compose an email or SMS message. Use the view controllers in the Message UI framework.

  • Open or preview the contents of a file. Use the UIDocumentInteractionController class in the UIKit framework.

  • Take a picture or choose a photo from the user’s photo library. Use the UIImagePickerController class in the UIKit framework.

  • Shoot a video clip. Use the UIImagePickerController class in the UIKit framework.

For information on how to present and dismiss view controllers, see View Controller Programming Guide for iOS. For information about the interface presented by a specific view controller, see View Controller Catalog for iOS.

Cocoa Touch Frameworks

The following sections describe the frameworks of the Cocoa Touch layer and the services they offer.

Address Book UI Framework

The Address Book UI framework (AddressBookUI.framework) is an Objective-C programming interface that you use to display standard system interfaces for creating new contacts and for editing and selecting existing contacts. This framework simplifies the work needed to display contact information in your app and also makes sure that your app uses the same interfaces as other apps, thus ensuring consistency across the platform.

For more information about the classes of the Address Book UI framework and how to use them, see Address Book Programming Guide for iOS and Address Book UI Framework Reference for iOS.

Event Kit UI Framework

The Event Kit UI framework (EventKitUI.framework) provides view controllers for presenting the standard system interfaces for viewing and editing calendar-related events. This framework builds upon the event-related data in the Event Kit framework, which is described in “Event Kit Framework.”

For more information about the classes and methods of this framework, see Event Kit UI Framework Reference.

Game Kit Framework

The Game Kit framework (GameKit.framework) implements support for Game Center, which lets users share their game-related information online. Game Center provides support for the following features:

  • Aliases, to allow users to create their own online persona. Users sign in to Game Center and interact with other players anonymously through their alias. Players can set status messages as well as mark specific people as their friends.

  • Leaderboards, to allow your app to post user scores to Game Center and retrieve them later. You might use this feature to show the best scores among all users of your app.

  • Matchmaking, to allow you to create multiplayer games by connecting players who are logged into Game Center. Players do not have to be local to each other to join a multiplayer game.

  • Achievements, to allow you to record the progress a player has made in your game.

  • Challenges, to allow a player to challenge a friend to beat an achievement or score. (iOS 6 and later)

  • Turn-based gaming, to create persistent matches whose state is stored in iCloud.

For more information about how to use the Game Kit framework, see Game Center Programming Guide and Game Kit Framework Reference.

iAd Framework

The iAd framework (iAd.framework) lets you deliver banner-based advertisements from your app. Advertisements are incorporated into standard views that you integrate into your user interface and present when you want. The views themselves work with Apple’s iAd Service to automatically handle all the work associated with loading and presenting rich media ads and responding to taps in those ads.

For more information about using iAd in your apps, see iAd Programming Guide and iAd Framework Reference.

Map Kit Framework

The Map Kit framework (MapKit.framework) provides a scrollable map that you can incorporate into your app’s user interface. Beyond just displaying a map, you can use the framework interfaces to customize the map’s content and appearance. You can flag points of interest using annotations, and you can use custom overlays to intersperse your own content with the map content. For example, you might use an overlay to draw a bus route, or use annotations to highlight nearby shops and restaurants.

In addition to displaying maps, the Map Kit framework integrates with the Maps app and Apple’s map servers to facilitate directions. From the Maps app, users can delegate the providing of directions to any app that supports directions. Apps that provide specialized types of directions, such as subway routes, can register to provide those directions when asked. Apps can also request walking and driving directions from Apple servers and merge that route information with their custom directions to provide a complete point-to-point experience for the user.

For more information about using classes of the Map Kit framework, see Location and Maps Programming Guide and Map Kit Framework Reference.

Message UI Framework

The Message UI framework (MessageUI.framework) provides support for composing email or SMS messages from your app. The composition support consists of a view controller interface that you present in your app. You can populate the fields of this view controller to set the recipients, subject, body content, and any attachments you want to include with the message. After presenting the view controller, the user then has the option of editing the message before sending it.

For more information about the classes of the Message UI framework, see Message UI Framework Reference. For information about using the classes of this framework, see System Messaging Programming Topics for iOS.

Twitter Framework

The Twitter framework (Twitter.framework) has been replaced by the Social framework, which supports a UI for generating tweets and support for creating URLs to access the Twitter service. For more information, see “Social Framework.”

UIKit Framework

The UIKit framework (UIKit.framework) provides crucial infrastructure for implementing graphical, event-driven apps in iOS, including the following:

  • Basic app management and infrastructure, including the app’s main run loop

  • User interface management, including support for storyboards and nib files

  • A view controller model to encapsulate the contents of your user interface

  • Objects representing the standard system views and controls

  • Support for handling touch- and motion-based events

  • Support for a document model that includes iCloud integration; see Document-Based App Programming Guide for iOS

  • Graphics and windowing support, including support for external displays; see View Programming Guide for iOS

  • Multitasking support; see “Multitasking”

  • Printing support; see Drawing and Printing Guide for iOS

  • Support for customizing the appearance of standard UIKit controls

  • Support for text and web content

  • Cut, copy, and paste support

  • Support for animating user-interface content

  • Integration with other apps on the system through URL schemes and framework interfaces

  • Accessibility support for disabled users

  • Support for the Apple Push Notification service; see “Apple Push Notification Service”

  • Local notification scheduling and delivery; see “Local Notifications”

  • PDF creation

  • Support for using custom input views that behave like the system keyboard

  • Support for creating custom text views that interact with the system keyboard

  • Support for sharing content through email, Twitter, Facebook, and other services

In addition to providing the fundamental code for building your app, UIKit also incorporates support for some device-specific features, such as the following:

  • The built-in camera (where present)

  • The user’s photo library

  • Device name and model information

  • Battery state information

  • Proximity sensor information

  • Remote control information from attached headsets

For information about the classes of the UIKit framework, see UIKit Framework Reference.