无法在不可变值上使用可变 getter：“self”是不可变错误

Question

我正在尝试重用一段较旧的 Swift 代码，但收到错误“无法在不可变值上使用 mutating getter: 'self' 是不可变错误”。Xcode 想要在 func 之前添加“mutating”，并提供通过“修复”来做到这一点。所以错误消失了，但仍然保留在“文本”语句中。

import SwiftUI

struct ContentView: View {

     typealias PointTuple = (day: Double, mW: Double)
    let points: [PointTuple] = [(0.0, 31.98), (1.0, 31.89), (2.0, 31.77), (4.0, 31.58), (6.0, 31.46)]

    lazy var meanDays = points.reduce(0) { $0 + $1.0 } / Double(points.count)
    lazy var meanMW   = points.reduce(0) { $0 + $1.1 } / Double(points.count)

    lazy var a = points.reduce(0) { $0 + ($1.day - meanDays) * ($1.mW - meanMW) }
    lazy var b = points.reduce(0) { $0 + pow($1.day - meanDays, 2) }

    lazy var m = a / b
    lazy var c = meanMW - m * meanDays        
    lazy var x : Double = bG(day: 3.0)
    lazy var y : Double = bG(day: 5.0)
    lazy var z : Double = bG(day: 7.0)

    mutating func bG(day: Double) -> Double {
        return m * day + c
    }

    var body: some View {
        VStack {
            Text("\(x)")
            Text("\(y)")
            Text("\(z)")
        }
    }
}

#if DEBUG
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

Answer 1

因为当您x在 struct 内部调用时，尚不清楚它contentView本身是否可变。实际上，值类型 只有在定义为var.

因此，在结构体内部的构建器函数中使用它之前，您应该使用不可变值包装它。

像这样：

func xValue() -> Double {
    var mutatableSelf = self
    return mutatableSelf.x
}

var body: some View {
    VStack {
        Text("\(xValue())")
    }
}

Answer 2

This has nothing to do with SwiftUI. It's about a design Swift is enforcing with its getters. The principle is:

The getters should not mutate the object. Because developers may not be expecting that. They should only expect a change when you're using the setter or calling a mutating function. A getter is neither of them.

The following example works as expected:

struct Device {
    var isOn = true
}

let x = Device()
let y = Device()

y.isOn // Doing such will not cause the object to mutate.

Yet the following example, the getter will have a side-effect. The Swift architecture just doesn't allow it.

struct Device2 {

    var x = 3
    var isOn: Bool {
        x = 5
        return true
    }
}

let a = Device2()
let b = Device2()

a.isOn // Doing such will mutate the object. a.x will be '5'. While `b.x` will be '3'. Swift doesn't want to allow this.