Changing your name with typealias

The typealias declaration allows you to create an alias for a type (and is therefore pretty accurately named!). You can specify a name that can be used in place of any given type of definition. If this type is quite complex, a typeAlias can be a useful way to simplify its use.

How to do it...

We will use a typealias to replace an array definition:

1. First, let's create something we can store in an array. In this instance, let's create a Pug struct:

struct Pug { 
    let name: String 
}

2. Now, we can create an array that will contain instances of a Pug struct:

let pugs = [Pug]() 

As you may or may not know, the collective noun for a group of pugs is called a grumble.

3. We can set up a typealias to define an array of pugs as a Grumble:

typealias Grumble = [Pug] 

4. With this defined, we can substitute Grumble wherever we would use [Pug] or Array<Pug>:

var grumble = Grumble()

5. However, this isn't some new type – it is just an array with all the same functionalities:

let marty = Pug(name: "Marty McPug") 
let wolfie = Pug(name: "Wolfgang Pug") 
let buddy = Pug(name: "Buddy") 
grumble.append(marty) 
grumble.append(wolfie) 
grumble.append(buddy)

There's more...

The preceding example allows us to use types in a more natural and expressive way. In addition, we can use a typealias to simplify a more complex type that may be used in multiple places.

To see how this might be useful, we can partially build an object to fetch program information:

enum Channel { 
    case BBC1 
    case BBC2 
    case BBCNews 
    //... 
} 
 
class ProgrammeFetcher { 
     
    func fetchCurrentProgrammeName(forChannel channel: Channel, 
       resultHandler: (String?, Error?) -> Void) { 
        // ...  
        // Do the work to get the current programme 
        // ... 
        let exampleProgramName = "Sherlock" 
        resultHandler(exampleProgramName, nil) 
    } 
     
    func fetchNextProgrammeName(forChannel channel: Channel, 
       resultHandler: (String?, Error?) -> Void) { 
        // ...  
        // Do the work to get the next programme 
        // ... 
        let exampleProgramName = "Luther" 
        resultHandler(exampleProgramName, nil) 
    } 
} 
 

In the ProgrammeFetcher object, we have two methods that take a channel and a result handler closure. The result handler closure has the following definition. We have to define this twice; once for each method:

(String?, Error?) -> Void 

Instead, we can define this closure definition with a typealias called FetchResultHandler and replace each method definition with a reference to this typealias:

class ProgrammeFetcher { 
     
    typealias FetchResultHandler = (String?, Error?) -> Void 
     
    func fetchCurrentProgrammeName(forChannel channel: Channel, 
                                   resultHandler: FetchResultHandler) { 
        // Get next programme 
        let programmeName = "Sherlock" 
        resultHandler(programmeName, nil) 
    } 
     
    func fetchNextProgrammeName(forChannel channel: Channel, 
                                resultHandler: FetchResultHandler) { 
        // Get next programme 
        let programmeName = "Luther" 
        resultHandler(programmeName, nil) 
    } 
} 
 

Not only does this save us from defining the closure type twice, but it is also a better description of the function that the closure performs.

Using typealias doesn't affect how we provide closure to the method:

let fetcher = ProgrammeFetcher() 
fetcher.fetchCurrentProgrammeName(forChannel: .BBC1, 
   resultHandler: { programmeName, error in  
    print(programmeName as Any) 
})

See also

Further information about typealias can be found in Apple's documentation on the Swift language at http://swiftbook.link/docs/declarations.