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

Solution 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

Solution 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.