SWIFTY SWIFT in OBJECTIVE-C

Often, Swift and Objective code must coexist but some Swift types are not compatible with Objective-C. However, we shouldn't avoid using them as they provide power and safety.

What is this repository for?

• Working examples and documentation of using Swift-only types such as Swift enumerations and structures with Objective-C - and how to mitigate the impedences that exist.

• Based in large part on a 2015 lecture given by Andy Matuschak https://www.youtube.com/watch?v=q_Y070VAP0c

There is now an official proposal to help with this.####

https://github.com/apple/swift-evolution/blob/master/proposals/0058-objectivecbridgeable.md http://www.russbishop.net/bridges-of-siracusa-county

What does it do?

Objective-C excludes some Swift types, such as those listed here:

• Generics
• Tuples
• Enumerations defined in Swift without Int raw value type
• Structures defined in Swift
• Top-level functions defined in Swift
• Global variables defined in Swift
• Typealiases defined in Swift
• Swift-style variadics
• Nested types
• Curried functions

This app has examples of working with Swift enumerations and stuctures in Objective-C. While these are not directly compatible, there are techniques to alleviate this to varying degrees. It demonstrates four mechanisms.

They are:

###Passing a Swift-only type in an Objective-C compatible Swift class### For example, passing a UIView subclass written in Swift containing a Swift-only type to an Objective-C UIViewController. You'll have to do the layout yourself, but it's the simplest technique to understand.

###Creating a Swift-only type within an Objective-C class from anywhere via a shim defined in a Swift extension of an Objective-C class

For example, using a Swift-only type

struct User {
	let name: String
	let profileImageURL: NSURL
}

and an Objective-C compatible container.

class ProfileView: UIView {
	var user:User?
	
	init(user: User) {
    	self.user = user
    }
}

Create a shim in a Swift extension.

extension ObjectiveCClass

	func showProfileViewWithUser(user: User) {
    	self.showProfileForUserWithName(user.name, profileImageURL: user.profileImageURL)
	}

In ObjectiveCClass.h:

-(void)showProfileForUserWithName:(NSString *)name profileImageURL: (NSURL *)url;

And calling it from a Swift class

let url = NSURLComponents(string:"https://github.com/danwallacenz")?.URL
let user = User(name: "Daniel Wallace", profileImageURL: url!)

objectiveCClass.showProfileViewWithUser(user)

or from an Objective-C class.

[objectiveCClass showProfileViewWithName:aName profileImageURL: aURL];

Creating a Swift object & setting its Swift-only type from Objective-C

You can use this technique in setters as well.

// preferably in an extension		
extension ProfileView {

	@objc convenience init(name:String, profileImageURL:NSURL ){
    	
    	let user = User(name: name, profileImageURL: profileImageURL)
    	self.init(user: user)
	}

and calling it from Objective-C by:

self.profileView = [[ProfileView alloc] initWithName: aName profileImageURL: aURL];  

###Passing a Swift-only type between the two languages using the Box Pattern
You can't use these from within an Objective-C class, but you can pass it to on the a Swift class.

class Box<T> {
    let value: T    
    init(_ value: T){ self.value = value }
}

In ObjectiveCClass.h: (typing by comment :)

	// Strong typing using comments
	@property (readwrite, strong) id /* Box<User> */ _user;

In a Swift extension create a computed property:

extension ObjectiveCClass {
	
	// Can store the User struct and pass it on to something else. Not usable in Obj-C code though.
	var user: User {
    	get {
        	return (_user as! Box<User>).value
   		}
    	set{
       		 _user = Box(newValue)
    	}
	}
}

From a Swift client, set the Boxed property on the receiver like this:

let user = User(name: "Addy Osmani", profileImageURL: aURL)
 objectiveCClass.user = user

Send it on to a Swift class which looks like:

class MySwiftClass { 
	...
	private var user:User?
	...
	
func setUser(user: AnyObject?){
    if let user = user as? Box<User> {
        self.user = user.value
    }
}

from Objective-C:

[mySwiftClass setUser:self._user];

Inspecting the Boxed User class in Objective-C you'll see: self._user = interop.Box<interop.User>

Using this, you can expose the 'partial' values of the struct as Objective-C properties:

extension ObjectiveCClass {
	
	// Set the user.name property of the User from Swift and Objective-C.
	// Set user.profileImageURL in a similar way.
	
	@objc var userName: String {
		get { return user.name }
		set {
			user = User(
				name: newValue,
				profileImageURL: user.profileImageURL
			)
		}
	}

Convoluted I know. Still...we want to use these powerful and safe Swift-only types.

There are examples of using Swift enums as well as Swift structs in the source.

Who do I talk to?

• https://developer.apple.com/library/ios/documentation/Swift/Conceptual/BuildingCocoaApps/
• https://itunes.apple.com/us/book-series/swift-programming-series/id888896989?mt=11
• https://developer.apple.com/videos/play/wwdc2015/401/
• Avoiding the overuse of @objc in Swift http://www.jessesquires.com/avoiding-objc-in-swift/
• Andy Matuschak haha https://andymatuschak.org