ObservableBox.swift

class MutableBox<T> {
    var value: T

    init(value: T) {
        self.value = value
    }
}

extension MutableBox: CustomDebugStringConvertible {
    var debugDescription: String {
        return String(reflecting: self.value)
    }
}

class ObservableBox<T>: MutableBox<T> {
    var valueChanged: (T) -> Void = { _ in }

    override var value: T {
        didSet {
            self.valueChanged(oldValue)
        }
    }
}

let boxedArray = ObservableBox(value: [Int]())
boxedArray.valueChanged = { oldValue in
    print("notification. oldValue: \(oldValue)")
}
print(boxedArray)
boxedArray.value.append(1)
print(boxedArray)
boxedArray.value.removeAll()
print(boxedArray)

// Output:

// []
// notification. oldValue: []
// [1]
// notification. oldValue: [1]
// []