Swift is designed to provide seamless compatibility with Cocoa and Objective-C. You can use Objective-C APIs in Swift, and you can use Swift APIs in Objective-C. This makes Swift an easy, convenient, and powerful tool to integrate into your development workflow.
This guide covers three important aspects of Swift and Objective-C 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.
Setting Up Your Swift Environment
To start experimenting with Cocoa app development using Swift, create a new Swift project from one of the provided Xcode templates.
To create a Swift project in Xcode
Choose File > New > Project > (iOS, watchOS, tvOS, or OS X) > Application > your template of choice.
Click the Language pop-up menu and choose Swift.
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—all of the information about a class, function, or constant resides in a single
.swift file. This is discussed in more detail in Swift and Objective-C in the Same Project.
From here, you can start experimenting by writing Swift code in the app delegate or a new Swift file you create by choosing File > New > File > (iOS, watchOS, tvOS, or OS X) > Source > Swift.
Understanding the Swift Import Process
After you have your Xcode project set up, you can import any framework from Cocoa or Cocoa Touch to start working with Objective-C from Swift.
Any Objective-C framework or C library that supports modules can 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 use Foundation APIs from a Swift file, add the following import statement to the top of the file:
With this import statement, that Swift file can now access all of Foundation’s classes, protocols, methods, properties, and constants.
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 process 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 have the following effects:
Remap certain Objective-C types to their equivalents in Swift, like
Remap certain Objective-C core types to their alternatives in Swift, like
Remap certain Objective-C concepts to matching concepts in Swift, like pointers to optionals
For more information on using Objective-C in Swift, see Interacting with Objective-C APIs.
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, see Swift and Objective-C in the Same Project.