Swift: Making the Associated Type Parameter Readable in Protocols

I’m still getting used to Associated Types in Swift. Even though they’ve been around for a while, I first starting to understand them from this talk by @alexisgallagher.

So I was happy yesterday when I used them to sort of solve a common problem in iOS development: View Controller Data Injection with Storyboards and Segues in Swift.

I updated my blog post from yesterday, but my initial Protocol looked something like this:

Notice the thing! Since each View Controller would be injected with something specific – it could be text-based or number-based or array-based, anything! I didn’t know how to name the parameter. So thing seemed like the most fitting parameter name.

The implementation was something like this then:

I didn’t exactly like the whole thing here – it’s not that great with readability. So this morning, I though of a crazy solution for this, tried it out, and it actually worked!

Instead of using thing, I leave the parameter name blank in the protocol!

Apparently, this now let’s me name the parameter anything I want in the implementation!

Now, the implementation is super clear! The more I use associated types, the more I like them!

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  • Bryan Hoke

    In inject(thing: T) — which is equivalent to inject(_ thing: T)thing is just a local parameter name that has no bearing on the call-site or function name, so you could already give thing a different name in implementations of inject without the need to replace it with a wildcard. However, I do agree that inject(_: T) reads better than inject(thing: T) in the protocol declaration.

  • Jessy Catterwaul

    Natasha, you don’t need to explicitly define a typealias.


    class MyStringDependencyViewController: UIViewController, Injectable {

    private var textDependency: String!

    func inject(text: String) {

    textDependency = text



    class MyIntDependencyViewController: UIViewController, Injectable {

    private var numberDependency: Int!

    func inject(number: Int) {

    numberDependency = number



    Also, I never use T. I either give the type a real name, or use the 🃏 emoji.


    protocol Injectable {

    associatedtype Injected

    func inject(_: Injected)


    • Good to know! I also fixed your code formatting with html tags (vs markdown).

    • Decided I like it without the explicit typealias better, so updated the blog post! Thanks.

  • Nick Kohrn

    So, does this work by calling the inject(text: String) method on an instance of the view controller?

    For instance, in prepareForSegue:, when you create an instance of a view controller to access a property to set, you then gain access to the property. But if the property is set a private, then you can’t access it. Is that what this is for? Call inject: on the destination view controller and passing the item to send so that that view controller can set its own private variable?

  • Heistings

    Hey, I’ve just found that, there is no need to use the _, it’s still possible to use a new parameter name when implementing a protocol:


    protocol TestProtocol {

    func method(a: String)


    struct TestStruct: TestProtocol {

    func method(b: String) {




    var b: TestProtocol = TestStruct()


  • Di

    Nice. I usually use something like this, inspired by NSCell 🙂