Cannot use mutating getter on immutable value

Question

I tried to use ForEach() to generate labels in a list

List {
    ForEach(objects) { obj in
        Label(obj.id, systemImage: obj.icon)
    }
}

The icon of each label is decided by its id so I use lazy var to define the struct

struct Object: Identifiable {
    var id:Int
    lazy var icon:String={
        // ...
    }
}

And create an array for each one

var objects = [
    Object(id:1),
    Object(id:2),
    // ...
]

But it throw an error like Cannot use mutating getter on immutable value: 'obj' is a 'let' constant

Looks like a duplicate of https://stackoverflow.com/q/60767041/1187415 to me. — Martin R, Jan 10 '23 at 07:46

Quang Hà · Accepted Answer · 2023-01-11T00:07:46.003

Your objects in ForEach(objects) { obj in ... } will be copied. Copied value-type object (struct) is immutable, mean its properties could not be changed.

There is no need to use lazy var icon:String = { } because the icon value will be changed each time you call it (computation property), and it's not suitable for above immutable object logic.

Try to provide exact value before using in ForEach

struct MyObject: Identifiable {
    let id: Int
    let icon: String
}

var objects = [
    MyObject(id: 1, icon: "moonphase.waxing.crescent"),
    MyObject(id: 2, icon: "moonphase.waning.crescent"),
    // ...
]

Cannot use mutating getter on immutable value

1 Answers1