Swift:初始化具有大量属性的模型类的正确方法

Question

如何使用大量属性初始化类/结构？

这个问题可能在没有 Swift 上下文的情况下被问到，但 Swift 给它带来了 flavor ，所以我在标题和标签中添加了 Swift 标签。

假设您有一个包含 20 个属性的 User 类。他们中的大多数不应为零或为空。让我们假设这些属性不是相互依赖的。假设根据类的逻辑，它的 33% 应该是常量 (let)。假设其中至少 65% 没有有意义的默认值。您将如何设计此类并初始化它的实例？

到目前为止，我的想法很少，但似乎没有一个让我完全满意:

• 将所有属性线性放入类中并创建巨大的 init 方法:

  class User {
      // there is 20 properties like that
      let id : String
      let username : String
      let email : String
      ...
      var lastLoginDate : Date
      var lastPlayDate : Date
  
      // then HUUUUGE init
      init(id: String, 
           username: String,
           ...
           lastPlayDate: Date) {
      }
  }
  

• 尝试将属性分组为子类型并分别处理较小的 init

  class User {
      struct ID {
          let id : String
          let username : String
          let email : String
      }
      struct Activity {
          var lastLoginDate : Date
          var lastPlayDate : Date 
      }
      let id : ID
      ...
      var lastActivity : Activity
  
      // then not so huge init
      init(id: ID, 
           ...
           lastActivity: Activity) {
      }
  }
  

• 另一种解决方案是稍微打破要求:要么声明一些可选属性并在 init 之后设置值，要么声明虚拟默认值并在 init 之后设置正常值，这在概念上似乎是相同的

  class User {
      // there is 20 properties like that
      let id : String
      let username : String
      let email : String
      ...
      var lastLoginDate : Date?
      var lastPlayDate : Date?
  
      // then not so huge init
      init(id: String, 
           username: String,
           email: String) {
      }
  }
  
  // In other code 
  var user = User(id: "1", username: "user", email: "user@example.com"
  user.lastLoginDate = Date()
  

是否有一个很好的范例/模式来处理这种情况？

Answer 1

你可以尝试构建器模式。

例子

class DeathStarBuilder {

    var x: Double?
    var y: Double?
    var z: Double?

    typealias BuilderClosure = (DeathStarBuilder) -> ()

    init(buildClosure: BuilderClosure) {
        buildClosure(self)
    }
}

struct DeathStar : CustomStringConvertible {

    let x: Double
    let y: Double
    let z: Double

    init?(builder: DeathStarBuilder) {

        if let x = builder.x, let y = builder.y, let z = builder.z {
            self.x = x
            self.y = y
            self.z = z
        } else {
            return nil
        }
    }

    var description:String {
        return "Death Star at (x:\(x) y:\(y) z:\(z))"
    }
}

let empire = DeathStarBuilder { builder in
    builder.x = 0.1
    builder.y = 0.2
    builder.z = 0.3
}

let deathStar = DeathStar(builder:empire)

示例取自此处:https://github.com/ochococo/Design-Patterns-In-Swift

如果您正在寻找更像 Java 的解决方案，您可以尝试类似的方法。

替代示例

final class NutritionFacts {
    private let servingSize: Int
    private let servings: Int
    private let calories: Int
    private let fat: Int
    private let sodium: Int
    private let carbs: Int
 
    init(builder: Builder) {
        servingSize = builder.servingSize
        servings = builder.servings
        calories = builder.calories
        fat = builder.fat
        sodium = builder.sodium
        carbs = builder.carbs
    }
 
    class Builder {
        let servingSize: Int
        let servings: Int
 
        private(set) var calories = 0
        private(set) var fat = 0
        private(set) var carbs = 0
        private(set) var sodium = 0
 
        init(servingSize: Int, servings: Int) {
            self.servingSize = servingSize
            self.servings = servings
        }
 
        func calories(value: Int) -> Builder {
            calories = value
            return self
        }
 
        func fat(value: Int) -> Builder {
            fat = value
            return self
        }
 
        func carbs(value: Int) -> Builder {
            carbs = value
            return self
        }
 
        func sodium(value: Int) -> Builder {
            sodium = value
            return self
        }
 
        func build() -> NutritionFacts {
            return NutritionFacts(builder: self)
        }
    }
}
 
let facts = NutritionFacts.Builder(servingSize: 10, servings: 1)
    .calories(value: 20)
    .carbs(value: 2)
    .fat(value: 5)
    .build()

示例取自:http://ctarda.com/2017/09/elegant-swift-default-parameters-vs-the-builder-pattern