Observe changes in any property of any member of an array - arrays

Question

I have an array which stores objects of a class:
class Apple {
var color = "Red"
}
let myApple = Apple()
var apples = [Apple]()
apples.append(myApple)
// Func should be called here
myApple.color = "Blue"
let otherApple = Apple()
// Func should not be called here,
// because otherApple is not a member of apples array
otherApple.color = "Green"
I want to run a function when any property of any member of "apples" array has changed. When calling this function, I need to pass the item of the array whose property is changed as a parameter.
I thought of using didSet on color property, but in that case function is called when otherApple's property is changed too. This is not what I want. I only want to run the function when a property of a member of the array has changed. If it is not a member, the function should not run.
Using didSet, running the function in any case, and checking membership in the beginning of the function might be an idea but I feel this is not a good way.
How can I properly achieve this with Swift?
Edit: Apple's guide for Using Key-Value Observing in Swift

Answer 1

You need to add the observer to all the Apple objects that you're adding in apples array.
First of all create a property named observers of type [NSKeyValueObservation] at class level, i.e.
var observers = [NSKeyValueObservation]()
Now, create a method that will append new Apple instances in apples array and add observer to it,
func addNewApple(_ apple: Apple) {
observers.append(apple.observe(\.color, options: [.new], changeHandler: { (apple, changes) in
if let newValue = changes.newValue {
print(newValue)
}
}))
apples.append(apple)
}
To observe a property of an object, that must be marked it #objc dynamic. So, the Apple definition goes like,
class Apple: NSObject {
#objc dynamic var color = "Red"
}
Now, you can use it as described below,
let myApple = Apple()
self.addNewApple(myApple)
myApple.color = "Blue"
Combining all the bits and pieces, the whole code can be written like,
class VC: UIViewController {
var apples = [Apple]()
var observers = [NSKeyValueObservation]()
override func viewDidLoad() {
super.viewDidLoad()
let myApple = Apple()
self.addNewApple(myApple)
myApple.color = "Blue"
let otherApple = Apple()
otherApple.color = "Green"
}
func addNewApple(_ apple: Apple) {
observers.append(apple.observe(\.color, options: [.new], changeHandler: { (apple, changes) in
if let newValue = changes.newValue {
print(newValue)
}
}))
apples.append(apple)
}
}