如何创建面向协议的通用服务？答案

【问题标题】：How to create protocol-oriented generic services?如何创建面向协议的通用服务？
【发布时间】：2016-04-25 12:05:43
【问题描述】：

我正在尝试创建一个为视图控制器提供数据的协议。我正在尝试采用协议方法并使事情变得灵活，因此视图控制器可以使用任何类型的数据进行一致性。

但是，我收到错误消息：Protocol 'Serviceable' can only be used as a generic contraint because it has Self or associated type requirements

这就是我想要做的：

protocol Serviceable {
    associatedtype DataType
    func get(handler: ([DataType] -> Void)?)
}

struct PostService: Serviceable {
    func get(handler: ([Postable] -> Void)? = nil) {
        print("Do something...")
    }
}

struct AuthorService: Serviceable {
    func get(handler: ([Authorable] -> Void)? = nil) {
        print("Do something...")
    }
}

protocol Postable {
    var title: String { get set }
    var content: String { get set }
}

protocol ServiceControllable: class {
    var service: Serviceable { get } // Error: Protocol 'Serviceable' can only be used as a generic contraint because it has Self or associated type requirements
}

extension ServiceControllable {
    func setupDataSource() {
        service.get { items in
            // Do something
        }
    }
}

class MyViewController: ServiceControllable {
    let service: Serviceable = PostService() // Error: Same as above

    override func viewDidLoad() {
        super.viewDidLoad()
        setupDataSource()
    }
}

如何进行设置，以便我的视图控制器可以实现 ServiceControllable 并可以访问用于填充表、集合等的通用 setupDataSource？

【问题讨论】：

标签： ios swift generics swift2 protocols

【解决方案1】：

你想要这样的东西。

import UIKit

protocol Serviceable {
    associatedtype DataType
    func get(handler: ([DataType] -> Void)?)
}

struct PostService: Serviceable {
    func get(handler: ([Postable] -> Void)? = nil) {
        print("Do something...")
    }
}

protocol Authorable {}

struct AuthorService: Serviceable {
    func get(handler: ([Authorable] -> Void)? = nil) {
        print("Do something...")
    }
}

protocol Postable {
    var title: String { get set }
    var content: String { get set }
}

protocol ServiceControllable: class {

    // THIS is the way to use generic-constraint-protocols in protocols.
    associatedtype _Serviceable: Serviceable

    var service: _Serviceable { get }

}

extension ServiceControllable {
    func setupDataSource() {
        service.get { items in
            // Do something
        }
    }
}

class MyViewController: UIViewController, ServiceControllable {

    let service = PostService()

    override func viewDidLoad() {
        super.viewDidLoad()
        setupDataSource()
    }
}

相关文档部分：Protocol Associated Type Declaratio.

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