swift - 无法在不可变值: 'self' is immutable error上使用变异 setter/getter

Question

我正在尝试重用旧版的Swift代码，但收到错误消息“不能对不可变值使用变异getter:'self'是不可变错误”。 Xcode希望在功能之前添加“变异”，并提出通过“修复”来实现。因此错误在那里消失了，但仍然停留在“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

因为当您在结构内部调用x时，尚不清楚contentView本身是否可变。实际上，仅当将定义为var时，值类型才可变为。

因此，在将其用于struct内部的builder函数之前，应将其包装为不可变值。

像这样:

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

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