Repository Pattern in Swift using Type Erasure

repository-pattern.swift

//  Repository.swift
import Foundation

protocol Repository {
    associatedtype Model
    func get(id: String, completion: @escaping (Result<Model, Error>) -> Void)
    func save(_ item: Model, completion: @escaping (Result<Model, Error>) -> Void)
    func remove(_ item: Model, completion: @escaping (Result<Model, Error>) -> Void)
}

enum RepositoryError: Error {
    case keyNotFound
    case couldNotSave
    case couldNotRemove
}

// Type Erasure
struct AnyRepository<Model>: Repository {
    typealias Completion = (Result<Model, Error>) -> Void

    private let box: RepositoryBoxBase<Model>

    init<RepositoryType: Repository>(_ repository: RepositoryType) where Model == RepositoryType.Model {

        // If this has already been boxed, avoid boxing again
        if let erased = repository as? AnyRepository<Model> {
            box = erased.box
        } else {
            box = RepositoryBox(base: repository)
        }
    }

    func get(id: String, completion: @escaping Completion) {
        box.get(id: id, completion: completion)
    }

    func save(_ item: Model, completion: @escaping Completion) {
        box.save(item, completion: completion)
    }

    func remove(_ item: Model, completion: @escaping Completion) {
        box.remove(item, completion: completion)
    }
}

fileprivate final class RepositoryBox<RepositoryType: Repository>: RepositoryBoxBase<RepositoryType.Model> {
    typealias Completion = (Result<Model, Error>) -> Void

    fileprivate let base: RepositoryType

    fileprivate init(base: RepositoryType) {
        self.base = base
        super.init()
    }

    override func get(id: String, completion: @escaping Completion) {
        base.get(id: id, completion: completion)
    }

    override func save(_ item: Model, completion: @escaping Completion) {
        base.save(item, completion: completion)
    }

    override func remove(_ item: Model, completion: @escaping Completion) {
        base.remove(item, completion: completion)
    }
}

// Abstract Base Class
private class RepositoryBoxBase<Model>: Repository {
    typealias Completion = (Result<Model, Error>) -> Void

    func get(id: String, completion: @escaping Completion) {}

    func save(_ item: Model, completion: @escaping Completion) {}

    func remove(_ item: Model, completion: @escaping Completion) {}
}

extension Repository {
    public func eraseToAnyRepository() -> AnyRepository<Model> {
        .init(self)
    }
}