Array of Protocols and Subscript

Question

craigaps OP

Created Apr ’22

Replies 3

Boosts 0

Views 1.3k

Participants 2

Hi

I have an array of protocols and would like to retrieve items from the array based on an enum. Something like this:

enum ServiceType: String {
   case car
   case boat
}

protocol ServiceDescriptor {
   var contact: String { get }
   var type: ServiceType { get }
}

struct CarService: ServiceDescriptor {
   let contact: String
   let type: ServiceType
}

struct BoatService: ServiceDescriptor {
   let contact: String
   let type: ServiceType
}

class Repair {
    init(allowedRepairs: [ServiceDescriptor]) {
        self.allowedRepairs = allowedRepairs
    }

    let allowedRepairs: [ServiceDescriptor]

    subscript(type: ServiceType) -> ServiceDescriptor? {
       get {
           return allowedRepairs.first(where: { $0.type == type })
       }
    }
}


let carService = CarService(contact: "Fred", type: .car)
let boatService = BoatService(contact: "Jane", type: .boat)

let repair = Repair(allowedRepairs: [carService, boatService])

print(repair.allowedRepairs[type: ServiceType.boat]). // error

I end up with errors in Playgrounds as:

no exact matches in call to subscript

found candidate with type '(ServiceType) -> Array.SubSequence' (aka '(ServiceType) -> ArraySlice< ServiceDescriptor >')

The following works:

print(repair.allowedRepairs[0])

I'm obviously coming about this the wrong way, I wanted to avoid a dictionary i.e let allowedRepairs: [ServiceType: ServiceDescriptor] because that would lend itself to assigning the wrong ServiceDescriptor to a ServiceType and it kinda duplicates the data.

Is there a another way to have a custom argument in subscript instead of int?

Many thanks

Boost

Answer 1

Claude31 OP

Apr ’22

In an array, the index must be Int.

So as you guessed, you should use dictionary.

You could do something like this (not tested):

enum ServiceType: String {
   case car
   case boat
}

protocol ServiceDescriptor {
   var contact: String { get }
   var type: ServiceType { get }
}

struct CarService: ServiceDescriptor {
   let contact: String
   let type: ServiceType
}

struct BoatService: ServiceDescriptor {
   let contact: String
   let type: ServiceType
}


typealias ServiceDict = [ServiceType: ServiceDescriptor]
class Repair {
    init(allowedRepairs: ServiceDict) {
        self.allowedRepairs = allowedRepairs
    }

    let allowedRepairs: ServiceDict

}


let carService = CarService(contact: "Fred", type: .car)
let boatService = BoatService(contact: "Jane", type: .boat)

let repair = Repair(allowedRepairs: [.car: carService, .boat:boatService])

print(repair.allowedRepairs[ServiceType.boat]) // No error

0

Answer 2

craigaps OP

Apr ’22

Hi Claude31

I thought this might have to be the solution and of course it works. I did try making enum ServiceType: Int and doing repairs.allowRepairs[ServiceType.boat] which also worked so long as the dictionary value matched the index of the enum.

So another issue will be JSON codable of Dictionary[ServiceType: ServiceDescriptor], all sorts of error here.

Wouldn't happen to have some ideas around codable of a Dictionary[Enum: Protocols]?

0

Answer 3

Claude31 OP

Apr ’22

If you use Int, then of course you can use it to index an array (just take care to start from 0).

To make a protocol Codable, just declare it as Codable ; idem for enum.

If you get an error and need to define encode and decode, get details here with an example:

https://developer.apple.com/forums/thread/79327

0