swift2 Swift中的服务定位器模式

我对Swift中灵活的通用服务定位器设计模式实现感兴趣。
一种简单的方法可能如下：

// Services declaration

protocol S1 {
    func f1() -> String
}

protocol S2 {
    func f2() -> String
}

// Service Locator declaration
// Type-safe and completely rigid.

protocol ServiceLocator {
    var s1: S1? { get }
    var s2: S2? { get }
}

final class NaiveServiceLocator: ServiceLocator {
    var s1: S1?
    var s2: S2?
}

// Services imlementation

class S1Impl: S1 {
    func f1() -> String {
        return "S1 OK"
    }
}

class S2Impl: S2 {
    func f2() -> String {
        return "S2 OK"
    }
}

// Service Locator initialization

let sl: ServiceLocator = {
    let sl = NaiveServiceLocator()
    sl.s1 = S1Impl()
    sl.s2 = S2Impl()
    return sl
}()

// Test run

print(sl.s1?.f1() ?? "S1 NOT FOUND") // S1 OK
print(sl.s2?.f2() ?? "S2 NOT FOUND") // S2 OK

但是如果服务定位器能够处理任何类型的服务而不改变它的代码，那就更好了。在Swift中如何实现这一点？

注意：服务定位器是一个非常有争议的设计模式（有时甚至称为反模式），但是请在这里避免讨论这个主题。

实际上，我们可以利用Swift的类型推理能力来获得一个灵活的通用 * 和类型安全 * 的服务定位器。

protocol ServiceLocator {
    func getService<T>() -> T?
}

final class BasicServiceLocator: ServiceLocator {

    // Service registry
    private lazy var reg: Dictionary<String, Any> = [:]

    private func typeName(some: Any) -> String {
        return (some is Any.Type) ? "\(some)" : "\(some.dynamicType)"
    }

    func addService<T>(service: T) {
        let key = typeName(T)
        reg[key] = service
        //print("Service added: \(key) / \(typeName(service))")
    }

    func getService<T>() -> T? {
        let key = typeName(T)
        return reg[key] as? T
    }

}

然后可按如下方式使用：

// Services declaration

protocol S1 {
    func f1() -> String
}

protocol S2 {
    func f2() -> String
}

// Services imlementation

class S1Impl: S1 {
    func f1() -> String {
        return "S1 OK"
    }
}

class S2Impl: S2 {
    func f2() -> String {
        return "S2 OK"
    }
}

// Service Locator initialization

let sl: ServiceLocator = {
    let sl = BasicServiceLocator()
    sl.addService(S1Impl() as S1)
    sl.addService(S2Impl() as S2)
    return sl
}()

// Test run

let s1: S1? = sl.getService()
let s2: S2? = sl.getService()

print(s1?.f1() ?? "S1 NOT FOUND") // S1 OK
print(s2?.f2() ?? "S2 NOT FOUND") // S2 OK

这已经是一个可用的实现了，但是它对于允许延迟服务初始化也是有用的。

protocol ServiceLocator {
    func getService<T>() -> T?
}

final class LazyServiceLocator: ServiceLocator {

    /// Registry record
    enum RegistryRec {

        case Instance(Any)
        case Recipe(() -> Any)

        func unwrap() -> Any {
            switch self {
                case .Instance(let instance):
                    return instance
                case .Recipe(let recipe):
                    return recipe()
            }
        }

    }

    /// Service registry
    private lazy var reg: Dictionary<String, RegistryRec> = [:]

    private func typeName(some: Any) -> String {
        return (some is Any.Type) ? "\(some)" : "\(some.dynamicType)"
    }

    func addService<T>(recipe: () -> T) {
        let key = typeName(T)
        reg[key] = .Recipe(recipe)
    }

    func addService<T>(instance: T) {
        let key = typeName(T)
        reg[key] = .Instance(instance)
        //print("Service added: \(key) / \(typeName(instance))")
    }

    func getService<T>() -> T? {
        let key = typeName(T)
        var instance: T? = nil
        if let registryRec = reg[key] {
            instance = registryRec.unwrap() as? T
            // Replace the recipe with the produced instance if this is the case
            switch registryRec {
                case .Recipe:
                    if let instance = instance {
                        addService(instance)
                    }
                default:
                    break
            }
        }
        return instance
    }

}

它可以按以下方式使用：

// Services declaration

protocol S1 {
    func f1() -> String
}

protocol S2 {
    func f2() -> String
}

// Services imlementation

class S1Impl: S1 {
    let s2: S2
    init(s2: S2) {
        self.s2 = s2
    }
    func f1() -> String {
        return "S1 OK"
    }
}

class S2Impl: S2 {
    func f2() -> String {
        return "S2 OK"
    }
}

// Service Locator initialization

let sl: ServiceLocator = {
    let sl = LazyServiceLocator()
    sl.addService { S1Impl(s2: sl.getService()!) as S1 }
    sl.addService { S2Impl() as S2 }
    return sl
}()

// Test run

let s1: S1? = sl.getService()
let s2: S2? = sl.getService()
//let s2_: S2? = sl.getService()

print(s1?.f1() ?? "S1 NOT FOUND") // S1 OK
print(s2?.f2() ?? "S2 NOT FOUND") // S2 OK

我认为结合使用服务定位器和依赖注入可以避免前一种模式的一些缺点。

swift2 Swift中的服务定位器模式

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