Swift Type Erasure example

type_erasure.swift

//: Playground - noun: a place where people can play


protocol ObjectMapper {
    
    associatedtype SourceType
    
    associatedtype ResultType
    
    func map(_ object: SourceType) -> ResultType
    
}


struct AnyObjectMapper<SourceType, ResultType>: ObjectMapper {
    
    private let _box: _AnyMapperBox<SourceType, ResultType>
    
    init<MapperType: ObjectMapper>(_ mapper: MapperType) where MapperType.SourceType == SourceType, MapperType.ResultType == ResultType {
        _box = _MapperBox(mapper)
    }
    
    func map(_ object: SourceType) -> ResultType {
        return _box.map(object)
    }
    
}


class _AnyMapperBox<SourceType, ResultType>: ObjectMapper {
    
    func map(_ object: SourceType) -> ResultType {
        fatalError("This method is abstract")
    }
    
}


class _MapperBox<Base: ObjectMapper>: _AnyMapperBox<Base.SourceType, Base.ResultType> {
    
    private let _base: Base
    
    init(_ base: Base) {
        _base = base
    }
    
    override func map(_ object: Base.SourceType) -> Base.ResultType {
        return _base.map(object)
    }
    
}


class Article { }

typealias ArticleDict = [AnyHashable: Any]

class ArticleMapper: ObjectMapper {
    func map(_ object: ArticleDict) -> Article {
        // Mapping...
        return Article()
    }
}

class ArticleService {
    var mapper: AnyObjectMapper<ArticleDict, Article>!
}


let service = ArticleService()
service.mapper = AnyObjectMapper(ArticleMapper())