Cannot use mutating getter on immutable value: 'self' is immutable error

Question

I'm trying to reuse an older piece of Swift code, but getting an error 'Cannot use mutating getter on immutable value: 'self' is immutable error'. Xcode wanted to add 'mutating' before the func, and offered to do so through a 'fix'. So the error is gone there but still remains at the 'Text' statements.

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

Because when you call x inside the struct, it's not clear whether the contentView itself is mutable or not. A value type gets mutable only when it is defined as a var.

So it would help if you wrapped it with an immutable value before using it inside a builder function inside the struct.

like this:

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

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

---

💡Note: Lazy property's value will be associated on the first call and this mutates the object. So they are considered as mutating

Answer 2

A getter cannot mutate

This is mainly 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.

Lazy is mutating:

struct ContentView: View {
    lazy var x : Double = 3.0

    var body: some View {
       Text("\(x)") // ERROR
    }
}

Will result in the follow ERROR:

Cannot use mutating getter on immutable value: 'self' is immutable

SwiftUI - special case

Because the body variable is a computed property, you can't mutate/set variables. There's a way around that though.

Mark the variable with a @State property wrapper.

Example. The following code won't compile:

struct ContentView: View {
    var currentDate = Date()
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

    var body: some View {
        Text("\(currentDate)")
            .onReceive(timer) { input in
                currentDate = input // ERROR: Cannot assign to property: 'self' is immutable
        }
    }
}

Yet the following will, just because it has @State

struct ContentView: View {
    @State var currentDate = Date()
    let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()

    var body: some View {
        Text("\(currentDate)")
            .onReceive(timer) { input in
                currentDate = input
        }
    }
}

For more on that see here