Basic Setup

@H_403_12@ Swift is designed to provide seamless compatibility with Cocoa and Objective-C. You can use Objective-C APIs (ranging from system frameworks to your own custom code) in Swift,and you can use Swift APIs in Objective-C. This compatibility makes Swift an easy,convenient,and powerful tool to integrate into your Cocoa app development workflow.

@H_403_12@ This guide covers three important aspects of this compatibility that you can use to your advantage when developing Cocoa apps:

• Interoperabilitylets you interface between Swift and Objective-C code,allowing you to use Swift classes in Objective-C and to take advantage of familiar Cocoa classes,patterns,and practices when writing Swift code.

• Mix and matchallows you to create mixed-language apps containing both Swift and Objective-C files that can communicate with each other.

• Migrationfrom existing Objective-C code to Swift is made easy with interoperability and mix and match,making it possible to replace parts of your Objective-C apps with the latest Swift features.

@H_403_12@ Before you get started learning about these features,you need a basic understanding of how to set up a Swift environment in which you can access Cocoa system frameworks.

Setting Up Your Swift Environment

@H_403_12@ To start experimenting with accessing Cocoa frameworks in Swift,create a Swift-based app from one of the Xcode templates.

@H_403_12@ To create a Swift project in Xcode

1. Choose File > New > Project > (iOSorOS X) > Application >your template of choice.

2. Click the Language pop-up menu and choose Swift.

@H_403_12@ A Swift project’s structure is nearly identical to an Objective-C project,with one important distinction: Swift has no header files. There is no explicit delineation between the implementation and the interface,so all the information about a particular class resides in a single.swiftfile.

@H_403_12@ From here,you can start experimenting by writing Swift code in the app delegate,or you can create a new Swift class file by choosing File > New > File > (iOSorOS X) > Source > Swift.

Understanding the Swift Import Process

@H_403_12@ After you have your Xcode project set up,you can import any framework from the Cocoa platform to start working with Objective-C from Swift.

@H_403_12@ Any Objective-C framework (or C library) that’s accessible as amodulecan be imported directly into Swift. This includes all of the Objective-C system frameworks—such as Foundation,UIKit,and SpriteKit—as well as common C libraries supplied with the system. For example,to import Foundation,simply add this import statement to the top of the Swift file you’re working in:

@H_403_12@ This import makes all of the Foundation APIs—includingNSDate,NSURL,monospace; word-wrap:break-word; background:transparent">NSMutableData,and all of their methods,properties,and categories—directly available in Swift.

@H_403_12@ The import process is straightforward. Objective-C frameworks vend APIs in header files. In Swift,those header files are compiled down to Objective-C modules,which are then imported into Swift as Swift APIs. The importing determines how functions,classes,methods,and types declared in Objective-C code appear in Swift. For functions and methods,this process affects the types of their arguments and return values. For types,the process of importing can do the following things:

• Remap certain Objective-C types to their equivalents in Swift,likeidtoAnyObject

• Remap certain Objective-C core types to their alternatives in Swift,monospace; word-wrap:break-word; background:transparent">NSStringtoString

• Remap certain Objective-C concepts to matching concepts in Swift,like pointers to optionals

@H_403_12@ InInteroperability,you’ll learn more about these mappings and about how to leverage them in your Swift code.

@H_403_12@ The model for importing Swift into Objective-C is similar to the one used for importing Objective-C into Swift. Swift vends its APIs—such as from a framework—as Swift modules. Alongside these Swift modules are generated Objective-C headers. These headers vend the APIs that can be mapped back to Objective-C. Some Swift APIs do not map back to Objective-C because they leverage language features that are not available in Objective-C. For more information on using Swift in Objective-C,seeSwift and Objective-C in the Same Project.