Set value of read-only stored property during initializing in Swift

Question

I want to implement my custom MKAnnotation. I took a look at MKAnnotation protocol(MKAnnotation.h). It's as follow:

//
//  MKAnnotation.h
//  MapKit
//
//  Copyright (c) 2009-2014, Apple Inc. All rights reserved.
//

protocol MKAnnotation : NSObjectProtocol {

    // Center latitude and longitude of the annotation view.
    // The implementation of this property must be KVO compliant.
    var coordinate: CLLocationCoordinate2D { get }

    // Title and subtitle for use by selection UI.
    @optional var title: String! { get }
    @optional var subtitle: String! { get }

    // Called as a result of dragging an annotation view.
    @optional func setCoordinate(newCoordinate: CLLocationCoordinate2D)
}

Please note the coordinate property (which is a read-only stored property). And here is how I've implemented this protocol:

class RWDefaultPin: NSObject, MKAnnotation {
    var title:String = ""
    var subtitle:String = ""
    var groupTag:String = ""
    var coordinate: CLLocationCoordinate2D { get {
        return self.coordinate // this is obviously wrong because property's trying to return itself
    } };


    init(coordinate:CLLocationCoordinate2D) {
        super.init()
        self.coordinate = coordinate
    }
}

But obviously compiler complaints on my init method where I'm trying to assign to my coordinate property Cannot assign to 'coordinate' in 'self' obviously because it's a read-only property.

Previously in Objective-C we could overcome this issue as properties were backed by ivars.

I wish there was access modifier in Swift so I could define a private property in my class and set its value on init, and returning its value on get action of coordinate, but there is no such thing!

I don't quiet know how to fix this issue in Swift, or maybe I need to make it wide open and change my coordinate to be readable/writable?

Answer 1

You should be able to just add a setter to it and store the information in an inner coordinate value. Since you have a getter it is still conforming to the protocol:

var innerCoordinate: CLLocationCoordinate2D

var coordinate: CLLocationCoordinate2D { 
    get {
        return self.innerCoordinate
    } 
    set {
        self.innerCoordinate = newValue
    }
};

init(coordinate:CLLocationCoordinate2D) {
    super.init()
    self.innerCoordinate = coordinate
}

This is actually how I implement readonly and private properties (with protocols and the factory pattern). I setup protocols with the public interface and classes with private variables and setters. It is actually super clean way to setup your code (and gets around the lack of protected/private properties in Swift).

---

Here is a abstracted example of what I am talking about (if you care):

// this is your MKAnnotation in this example
protocol SomeProtocol {
    var getterProperty: String { get }
    var setterProperty: String { set get }

    func publicFunction(someStirng: String) -> ();

}

// setup a function that returns a class conforming to your needed protocol
func SomeClassMaker() -> SomeProtocol {
    // your internal class that no one can access unless by calling the maker function
    class SomeClassInternal: NSObject, SomeProtocol {

        // private and no one can get to me!
        var innerSetterProperty = "default setter";

        var getterProperty = "default getter"

        var setterProperty: String {
            get {
                return self.innerSetterProperty;
            }
            set {
                "hit"
                self.innerSetterProperty = newValue
            }
        }

        func publicFunction(someString: String) -> ()  {
            // anyone get me
            self.getterProperty = someString;
        }

        func privateFunction() -> () {
            // no one can get me except internal functions
        }

    }

    return SomeClassInternal();
}


// create the class
var classInstance = SomeClassMaker();

// totally fine!
classInstance.setterProperty = "secret string"
// prints "secret string"
classInstance.setterProperty;

// error! no public setter for "getter"
classInstance.getterProperty = "another secret"

classInstance.publicFunction("try secret again")
// prints "try secret again"
let blahed = classInstance.getterProperty

// error!
classInstance.privateFunction()