Array of generics with metatypes (using AlamofireObjectMapper)

Question

I suspect I may be making the same mistake as described by Rob in this post here in that I should be doing this whole thing another way, but with that in mind:

I'm trying to use AlamofireObjectMapper in a generic way. It has a protocol

public protocol Mappable

I then have various model classes that adopt it

class Dog: Mappable
class Cat: Mappable
class Bird: Mappable

I have this method

func loadEntityArray<T: Mappable>(type: T.Type)

and the reason this is generic is because it calls a function load() that needs a completion block that uses this generic param. The 'type' argument is never actually used, but you can't make a func generic without the generic type being in the func's parameter list.

func load(completion:(Response<T, NSError> -> Void))

loadEntityArray is called from another method

func letsgo() { loadEntityArray(Dog.self); loadEntityArray(Cat.self) }

So far so good, this all works. But I want to pass an array of which models to load to letsgo() and I can't work out how to do this. If I change letsgo() to

func letsgo<T:Mappable>(models: [T.Type]) {
  for mod in models {
    loadEntityArray(mod)
  }
}

and then call letsgo() with 1 param like

letsgo([Dog.self])

it works, but as soon as I have an array of 2 or more, I get a compiler error 'cannot convert value of type NSArray to expected argument type [_.Type]' I don't now how I would explicitly type this array either.

letsgo([Dog.self, Cat.self])

I've tried various permutations and nothing seems to work. Am I doing something impossible here? It seems to me the compiler has enough information at compile time for this to work, so I'm not sure if this is a syntax thing or I'm doing something wrong here with generics.

What is the purpose of `loadEntityArray()`? Why are you trying to use the type of class as a parameter? — , Apr 08 '16 at 14:20

score 1 · Answer 1 · answered Apr 08 '16 at 14:44

Looking at your function :

func letsgo<T:Mappable>(models: [T.Type])

Its model parameter should be an Array of all the same Type. So an array of only Dog Types for example. That's why

letsgo([Dog.self])

works but

letsgo([Dog.self, Cat.self])

won't as it has multiple Types.

The solution :

Use Mappable.Type directly :

func loadEntityArray(type: Mappable.Type) {}

func letsgo(models: [Mappable.Type]) {
    for mod in models {
        loadEntityArray(mod.self)
    }
}

And cast your array of Types as an array of Mappable Types :

letsgo([Dog.self, Cat.self] as [Mappable.Type])

Hope this achieves what you're looking for !

Unfortunately not.. The loadEntityArray function needs to be generic itself because it calls the load function which is generic over type T. — Brynjar, Apr 10 '16 at 07:55

score 0 · Answer 2 · answered Apr 20 '16 at 12:41

So in the end I came to the conclusion that this is not possible. I changed the code such that I pass in an enum to letsgo() and in a switch statement on that enum I call loadEntityArray(Dog.self) etc. etc. with an explicitly coded call, for each of my possible types. Then the compiler can see all the possible types and is happy.

Array of generics with metatypes (using AlamofireObjectMapper)

2 Answers2