swift - 在 Swift 中使用 DI 时避免胖初始化器

Question

我试图理解 Swift 中的 DI。

我知道有 Swinject 等框架可以帮助 DI 但是我很想自己理解这一点，而不是使用太多框架的魔力。

以下面的代码为例。

我的 ProfileService 的初始化程序只会随着这个伪服务的扩展而继续增长并变得越来越胖，并且由于项目包含多个类，所以相同的模式会重复很多次。

如何避免这种情况？我希望找到一种方法来支持轻松维护，同时仍然获得直接和简单代码注入(inject)的所有好处。

我在想也许可以使用 Protocol 和 Struct 来包含依赖项并将其注入(inject)，但是无法理解如何最好地实现它。

import UIKit

class UserService {
    func currentUser() -> String {
        return "some username"
    }
}

class AvatarService {
    func currentUserAvatarUrl() -> String {
        return "https://foo.bar/image.png"
    }
}

class MessageService {
    func currentInbox() -> [String:String] {
        return [
            "9279n1n2283":"something something",
            "m2j292i2m2n":"something something something"
        ]
    }
}

class ProfileService {
    private let userService: UserService
    private let avatarService: AvatarService
    private let messageService: MessageService

    init(userService: UserService, avatarService: AvatarService, messageService: MessageService) {
        self.userService = userService
        self.avatarService = avatarService
        self.messageService = messageService
    }

    func getLoggedInUser() -> String {
        return userService.currentUser()
    }

    func getUserAvatar() -> String {
        return avatarService.currentUserAvatarUrl()
    }

    func getInboxMessages() -> [String:String] {
        return messageService.currentInbox()
    }
}

let userService = UserService()
let avatarService = AvatarService()
let messageService = MessageService()

let profileService = ProfileService(userService: userService, avatarService: avatarService, messageService: messageService)

profileService.getLoggedInUser()
profileService.getUserAvatar()
profileService.getInboxMessages()

Answer 1

我的建议是根本不要直接对这些类进行依赖注入(inject)。我会做这样的事情:

import Foundation

// MARK: Protocols

protocol UserService {
    var username: String { get }
}

protocol AvatarService {
    var userAvatarURL: URL { get }
}

protocol MessageService {
    var inbox: [String: String] { get }
}

protocol GlobalServiceContext {
    var userService: UserService { get }
    var avatarService: AvatarService { get }
    var messageService: MessageService { get }
}

// MARK: Mock Implementations

class MockUserService: UserService {
    let username = "test_user"
}

class MockAvatarService: AvatarService {
    let userAvatarURL = URL(string: "https://en.wikipedia.org/static/images/project-logos/enwiki.png")!
}

class MockMessageService: MessageService {
    let inbox = [
        "test subject 1": "test message 1",
        "test subject 2": "test message 2",
        "test subject 3": "test message 3",
    ]
}

class MockServiceContext: GlobalServiceContext {
    let userService: UserService = MockUserService()
    let avatarService: AvatarService = MockAvatarService()
    let messageService: MessageService = MockMessageService()
}

// MARK: Mock Implementations

class ProdUserService: UserService {
    // TODO: Substitute real implementation here
    let username = "prod_user"
}

class ProdAvatarService: AvatarService {
    // TODO: Substitute real implementation here
    let userAvatarURL = URL(string: "https://en.wikipedia.org/static/images/project-logos/enwiki.png")!
}

class ProdMessageService: MessageService {
    let inbox = [ // TODO: Substitute real implementation here
        "Prod subject 1": "Prod message 1",
        "Prod subject 2": "Prod message 2",
        "Prod subject 3": "Prod message 3",
    ]
}

class ProdServiceContext: GlobalServiceContext {
    let userService: UserService = ProdUserService()
    let avatarService: AvatarService = ProdAvatarService()
    let messageService: MessageService  = ProdMessageService()
}

// MARK: Usage

let ServiceContext: GlobalServiceContext = MockServiceContext()

class ProfileService {
    var username: String { return ServiceContext.userService.username }
    var userAvatarURL: URL { return ServiceContext.avatarService.userAvatarURL }
    var inbox: [String:String] { return ServiceContext.messageService.inbox }
}

let profileService = ProfileService()
print(profileService.username)
print(profileService.userAvatarURL)
print(profileService.inbox)

它包含您所有的全局状态(您的 API、服务、数据库等):

• 成为一个单一的对象
• 全局可用
• 其值可以替代模拟实现