In Swift how can I filter an array of objects conforming to a protocol by their class?

Question

I have a protocol, 'VariousThings', and two classes which conform to it, 'ThingType1' and 'ThingType2'. I've put some objects of these two types of classes into an array containing 'VariousThings'. I now want to just take all the objects out of that array that are of class type 'ThingType2' for example. How can I do this?

Here's what I have so far:

protocol VariousThings: class {

}

class ThingType1: VariousThings {

}

class ThingType2: VariousThings {

}


let array: [VariousThings] = [ThingType1(), ThingType2()]


func itemsMatchingType(type: VariousThings.Type) -> [VariousThings] {
    return array.filter { variousThing in
        return (variousThing.self === type)
    }
}


let justThingTypes1: [VariousThings] = itemsMatchingType(ThingType1)

Answer 1

I would use compactMap instead of filter here in order to give you better type safety. You can use a conditional downcast to filter out the elements you want and generics in order to preserve type information. This takes advantage of the fact that compactMap can filter out nil results from the transform function.

let array: [VariousThings] = [ThingType1(), ThingType2()]    

func itemsMatchingType<T : VariousThings>(_ type: T.Type) -> [T] {
  return array.compactMap { $0 as? T }
}

let justThingTypes1 = itemsMatchingType(ThingType1.self) // of type [ThingType1]

Now the array you get out of your itemsMatchingType function is [ThingType1] if you pass in ThingType1, rather than simply [VariousThings]. That way you don't have to deal with ugly forced downcasts later down the line.

Answer 2

You could use a generic

func itemsMatchingType<T : VariousThings>(type: T.Type) -> [VariousThings] {
  return array.filter { $0 is T }
}

Answer 3

You can use the filter for this:

let justThingsTypes1 = array.filter { $0 is ThingType1 }