无法推断通用参数“ S”

Question

So I'm trying to make a small dependency injection container using generics and protocol conformance. However i'm having some trouble compiling it.

I wonder if there is a swift expert that can tell me what the problem is.

import Foundation

protocol Service: NSObjectProtocol {
    init()
}

protocol ServicesType {
    func add<S: Service>(service: S.Type)
    func get<S: Service>() throws -> S
}

class Services: ServicesType {

    var services: [Service.Type] = []

    func add<S: Service>(service: S.Type) {
        if services.contains(where: { service -> Bool in return service == service }) { return }
        services.append(service)
    }

    func get<S: Service>() throws -> S {
        guard let first = services.first(where: { service -> Bool in return service == service }) else { throw ServicesTypeError.notFound }
        return first.init() as! S
    }
}

enum ServicesTypeError: Error {
    case notFound
}

Generic parameter 'S' could not be inferred is the error message when i try to compile the line below

let creditCardService: CreditCardServiceType = try self.services.get()

protocol CreditCardServiceType : Service {
    var cards: [CreditCard] { get }
}
class CreditCardService: CreditCardServiceType {
    internal required init() {}
    var cards: [CreditCard] = []
}

Answer 1

It took a little modification to get this working in a Playground, but this seems to work.

If I'm right (in bold),

let creditCardService: **CreditCardServiceType** = try self.services.get()

Needs to be:

let creditCardService: **CreditCardService** = try self.services.get()

Full code:

import Foundation

protocol Service: NSObjectProtocol {
    init()
}

protocol ServicesType {
    func add<S: Service>(service: S.Type)
    func get<S: Service>() throws -> S
}

class Services: ServicesType {

    var services: [Service.Type] = []

    func add<S: Service>(service: S.Type) {
        if services.contains(where: { service -> Bool in return service == service }) { return }
        services.append(service)
    }

    func get<S: Service>() throws -> S {
        guard let first = services.first(where: { service -> Bool in return service == service }) else { throw ServicesTypeError.notFound }
        return first.init() as! S
    }
}

enum ServicesTypeError: Error {
    case notFound
}

class CreditCard {

}

protocol CreditCardServiceType : Service {
    var cards: [CreditCard] { get }
}

class CreditCardService: NSObject, CreditCardServiceType {

    required override init() {
        super.init()
    }
    var cards: [CreditCard] = []
}

let services = Services()
services.add(service: CreditCardService.self)

let creditCardService: CreditCardService = try services.get()

Answer 2

Ok what i ended up with is the following. Not as type safe as i wanted but good enough.

import Foundation

protocol Service: NSObjectProtocol {
    init()
}

protocol ServicesType {

    func add<S: Service>(service: S.Type) throws
    func get<S>() throws -> S

}

class Services: ServicesType {

    var services: [Service.Type] = []

    func add<S: Service>(service: S.Type) throws {
        if services.contains(where: { existing -> Bool in return existing == service }) { throw ServicesTypeError.allReadyAdded}
        services.append(service)
    }

    func get<S>() throws -> S {
        guard let first = services.first(where: { existing -> Bool in return existing is S }) else { throw ServicesTypeError.notFound }
        return first.init() as! S
    }

}

enum ServicesTypeError: Error {
    case allReadyAdded
    case notFound
}

usage:

try self.services.add(service: ProfileService.self)
let profileService: ProfileServiceType = try self.services.get()