Simple Outline for Cache layer in Swift

gistfile1.swift

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

import UIKit

protocol DataSource {
    func fetchValue() -> String?
}

protocol DataSourceCacheStrategy {
    func retrieve<T>(from: [DataSource], using: DataSource -> T?) -> T?
}

struct DataSourceCache {
    let strategy: DataSourceCacheStrategy
    let localDataSource: DataSource
    let remoteDataSource: DataSource
    
    func fetch<T>(applyFunction: DataSource -> T?) -> T? {
        let sources = [localDataSource, remoteDataSource]
        return strategy.retrieve(sources, using: applyFunction)
    }
    
}
struct BasicDataSourceCacheStrategy: DataSourceCacheStrategy {
    
    func retrieve<T>(from: [DataSource], using: DataSource -> T?) -> T? {
        for dataSource in from {
            if let result = using(dataSource) {
                return result
            }
        }
        return nil
    }
}

struct MoreFancyDataSourceCacheStrategy: DataSourceCacheStrategy {

    func retrieve<T>(from: [DataSource], using: DataSource -> T?) -> T? {
        let cacheValidation: T? -> Bool = { $0 != nil }
        
        for dataSource in from {
            let result = using(dataSource)
            if cacheValidation(result) {
                return result
            }
        }
        return nil
    }
}

struct LocalDataSource: DataSource {
    func fetchValue() -> String? {
        return nil // "from local source"
    }
}
struct RemoteDataSource: DataSource {
    func fetchValue() -> String? {
        return "from remote source"
    }
}

let strategy = BasicDataSourceCacheStrategy()
let cache = DataSourceCache(strategy: strategy, localDataSource: LocalDataSource(), remoteDataSource: RemoteDataSource())

let result = cache.fetch { $0.fetchValue() }
println("Result \(result)")