Passing struct property name as an argument, then accessing the property

Question

I want to write a partial compare function, defined for all Equatable structs, that would be able to compare two struct instances, but ignoring any properties specified using an argument. Example, where the unicorn operator (🦄 ) represents what I am missing:

struct Struct1: Equatable {
    x: Int
    y: Int
    z: Int
}

func compare<T: Equatable>(_ a: T, _ b: T, ignoring ignoredProperties: [Property]) {   // N.B. “Property” is a made-up type.
    var alteredA = a
    for property in ignoredProperties {
        alteredA🦄 property = b🦄 property
    }
    return alteredA == b
}

let s1 = Struct1(x:1, y:2, z:3)
let s2 = Struct1(x:1, y:2, z:4)  // s2 is the same as s1, except for the value of property “z”

compare(s1, s2, ignoring: [])                  // -> false
compare(s1, s2, ignoring: [Property(Struct1.z)])       // -> true

What can I use instead of 🦄 and Property ?

N.B. I don’t need the ignored properties to be specified at runtime – in this example, they could actually be known at compile time.

N.B. 2. If I need to change my struct into a class, so be it, but I don’t really known how to do what I need using a class either.

simonWasHere · Accepted Answer · 2017-02-16 10:39:04Z

3

You can pass property keypaths to a function and use them like this:

class Person: NSObject {
    dynamic var firstName: String = ""
    dynamic var lastName: String = ""

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }
}

func setProperty<T: NSObject>(object: T, path: String, value: Any) {
    object.setValue(value, forKey: path)
}

let firstNamePath = #keyPath(Person.firstName)
let chris = Person(firstName: "chris", lastName: "jones")
setProperty(object: chris, path: firstNamePath, value: "notChrisAnyMore")
print(chris.firstName)

edited Feb 16, 2017 at 10:39

answered Feb 16, 2017 at 10:30

simonWasHere

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2 Comments

Philippe-André Lorin Over a year ago

Good. Will be the accepted answer if nobody proposes a way of sticking with structs.

simonWasHere Over a year ago

I don't think it can be done with a struct, could be wrong :)

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