Swift map doesn't work when changing array property

Question

I have the following struct, with some properties:

struct Partner {
    let id: Int
    let nome: String
    let icone: String
    var isSelected : Bool
}

So I initialize a simple array and put some data in there:

var parceiros : [Partner] = [
    Partner(id: 1, nome: "Personal Profile", icone: "btPersonal",isSelected : true),
    Partner(id: 2, nome: "Professional Profile", icone: "btProfessional", isSelected: false)
]

But when I want to change the "isSelected" property with the high-order function Map, in the swift 4, the array don't update at all. Its weird because the var "_parceiro" have the right value in the return loop. But after the function the array returns to the original value.

private func select(partner: Partner){
    let _ = parceiros.map { (parceiro) -> Partner in
        var _parceiro = parceiro
        _parceiro.isSelected = parceiro.id == partner.id ? true : false
        return _parceiro
    }
}

Answer 1

You are confusing reference and value types. While you working with Swift arrays of structs (struct is value-type), they creating a copy of anything you are putting in it. When you retrieving anything from the struct, it will make another copy of it. Basically map creation a new array of new structs taken from old array. You had to assign that array back:

private func select(partner: Partner){
    parceiros = parceiros.map { (parceiro) -> Partner in
        var _parceiro = parceiro
        _parceiro.isSelected = parceiro.id == partner.id ? true : false
        return _parceiro
    }
}

Or you can use reference type: class. It means that instead of keeping copies of your structs, array will keep references to actual instance of the objects.

class Partner {
    let id: Int
    let nome: String
    let icone: String
    var isSelected : Bool
}

And the change a particular object inside it. You don't need to map then though. If you want to apply something for each member of array use forEach , if you want to apply something to part of array - use filter first:

private func select(partner: Partner){
        parceiros.forEach {  $0.isSelected = (parceiro.id == partner.id) }
}

Answer 2

map is not a mutating function. It can be used to iterate over a collection and apply the same transformation function to all elements of the collection, storing the transformed values in a new collection and returning that new collection.

You either need to use a simple loop and manually modify the selected value in the original array or simply use the return value of map .

private func select(partner: Partner) -> [Partner] {
    return parceiros.map { (parceiro) -> Partner in
        var _parceiro = parceiro
        _parceiro.isSelected = parceiro.id == partner.id
        return _parceiro
    }
}

parceiros = select(parceiros[0])

If you want to go for the map approach, you can also simplify the closure to a single line like below:

private func select(partner: Partner, from partners: [Partner]) -> [Partner] {
    return partners.map { return Partner(id: $0.id, nome: $0.nome, icone: $0.icone, isSelected: $0.id == partner.id)}
}

let selectedPartners = select(partner: parceiros[1], from: parceiros)

The approach using a regular loop:

private func select(partner: Partner){
    for i in parceiros.indices {
        parceiros[i].isSelected = parceiros[i].id == partner.id
    }
}