Apple Inc. - Using Swift with Cocoa and Objective-C

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.

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

• Interoperability lets 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 match allows you to create mixed-language apps containing both Swift and Objective-C files that can communicate with each other.

• Migration from 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.

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.

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

To create a Swift project in Xcode

1. Choose File > New > Project > (iOS or OS X) > Application > your template of choice.

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

A Swift projects 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 .swift file.

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 > (iOS or OS X) > Source > Swift.

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.

Any Objective-C framework (or C library) thats accessible as a module can be imported directly into Swift. This includes all of the Objective-C system frameworkssuch as Foundation, UIKit, and SpriteKitas 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 youre working in:

Swift

• importFoundation

Interoperability is the ability to interface between Swift and Objective-C in either direction, letting you access and use pieces of code written in one language in a file of the other language. As you begin to integrate Swift into your app development workflow, its a good idea to understand how you can leverage interoperability to redefine, improve, and enhance the way you write Cocoa apps.

One important aspect of interoperability is that it lets you work with Objective-C APIs when writing Swift code. After you import an Objective-C framework, you can instantiate classes from it and interact with them using native Swift syntax.

To instantiate an Objective-C class in Swift, you call one of its initializers with Swift syntax. When Objective-C init methods come over to Swift, they take on native Swift initializer syntax. The init prefix gets sliced off and becomes a keyword to indicate that the method is an initializer. For init methods that begin with initWith, the With also gets sliced off. The first letter of the selector piece that had init or initWith split off from it becomes lowercase, and that selector piece is treated as the name of the first argument. The rest of the selector pieces also correspond to argument names. Each selector piece goes inside the parentheses and is required at the call site.

For example, where in Objective-C you would do this:

Objective-C

• UITableView * myTableView = [[ UITableView alloc ] initWithFrame : CGRectZero style : UITableViewStyleGrouped ];