valueForKeyTest.swift

//: Playground - noun: a place where people can play

import Cocoa

// When you’re downloading objects from the web, it’s common to need to merge changes
// from the server to already-existing local objects. (If your data model allows for
// mutable objects, as with Core Data, that is.)
//
// The below is a Swift translation of how I’ve done this in Objective-C.
// Note that it works just fine in Swift — though it does require NSObject subclasses.
//
// The part I like is that updateLocalObjectWithServerObject is reusable
// (and I have reused my equivalent Objective-C code).
// It doesn’t care what the objects are, or whether they’re of the same type,
// and it doesn’t care what the types of the values are.
//
// An argument against is that it uses strings — see mergeablePropertyNames.
// Type one of those wrong, and you get a runtime exception.
// My counter to that is that you’ll find that bug very, very quickly.
// Even your testers won’t see it.
//
// My wish is that I could do this same thing — or something very much like it —
// in pure Swift (minus NSObject), and that it would work with structs as well as classes.

class LocalObject: NSObject {

    var dog = "Fido"
    var cat = "Hunter"
    var zebra = "Mr. Stripes"
    var numberOfAnimals = 17
    var fedTheTigers = false
    var attendingDoctors: [String]?
    let zoo = "On My Mac"

    static let mergeablePropertyNames = ["dog", "cat", "zebra", "numberOfAnimals", "fedTheTigers", "attendingDoctors"]
}

class ServerObject: NSObject {

    var dog = "Rex"
    var cat = "Hunter"
    var zebra = "Mr. Stripes"
    var numberOfAnimals = 33
    var fedTheTigers = true
    var attendingDoctors = ["Smith", "Tennant", "Capaldi"]
    let dateDownloaded = NSDate()
}

func valuesAreEqual(value1: AnyObject?, _ value2: AnyObject?) -> Bool {

    if let value1 = value1, value2 = value2 {
        return value1.isEqual(value2)
    }
    if value1 == nil && value2 == nil {
        return true
    }
    return false
}

func updateLocalObjectWithServerObject(localObject: AnyObject, serverObject: AnyObject, propertyNames: [String]) -> NSDictionary {

    // Returns a dictionary with the changes. The changes can then be used for notifications, updating the database, etc.

    let changeDictionary = NSMutableDictionary()

    propertyNames.forEach { (onePropertyName) in

        let localValue = localObject.valueForKey(onePropertyName)
        let serverValue = serverObject.valueForKey(onePropertyName)

        if valuesAreEqual(localValue, serverValue) {
            return
        }

        localObject.setValue(serverValue, forKey: onePropertyName)
        changeDictionary[onePropertyName] = serverValue
    }

    return changeDictionary.copy() as! NSDictionary
}

let x = LocalObject()
let y = ServerObject()

let changes = updateLocalObjectWithServerObject(x, serverObject: y, propertyNames: LocalObject.mergeablePropertyNames)

print(changes)

// Prints:
//{
//    attendingDoctors =     (
//        Smith,
//        Tennant,
//        Capaldi
//    );
//    dog = Rex;
//    fedTheTigers = 1;
//    numberOfAnimals = 33;
//}