如何复制结构并同时修改其属性之一? [英] How to copy a struct and modify one of its properties at the same time?

问题描述

如果我想将视图控制器的状态表示为单个结构，然后实施撤消机制，我将如何更改该结构上的一个属性，同时获取前一个状态的副本?

If I want to represent my view controller's state as a single struct and then implement an undo mechanism, how would I change, say, one property on the struct and, at the same time, get a copy of the the previous state?

struct A {
   let a: Int
   let b: Int

   init(a: Int = 2, b: Int = 3) {
      self.a = a
      self.b = b
   }
}

let state = A()

现在我想要一个state的副本，但b =4.如何在不构造新对象且不必为每个属性指定值的情况下做到这一点?

Now I want a copy of state but with b = 4. How can I do this without constructing a new object and having to specify a value for every property?

推荐答案

注意，当您为常量 a和b使用占位符值时，您将无法构造A和其他任何值，但此占位符除外.改写初始化器.您也可以编写自定义方法来更改struct中的任何值:

Note, that while you use placeholder values for constants a and b you are not able to construct instance of A with any other values but this placeholders. Write initializer instead. You may write custom method that change any value in struct also:

struct A {
    let a: Int
    let b: Int

    init(a: Int = 2, b: Int = 3) {
        self.a = a
        self.b = b
    }

    func changeValues(a: Int? = nil, b: Int? = nil) -> A {
        return A(a: a ?? self.a, b: b ?? self.b)
    }
}

let state = A()
let state2 = state.changeValues(b: 4)

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