How can I observe a specific element with Swift collection types using property observers?

Question

Inspired when answering the question of How do I know if a value of an element inside an array was changed?, The answer was using a Property Observer for checking i

Answer 1

You can use the willSet observer to calculate changes before they are applied. Like so:

struct YourStruct {
  var strings : [ String ] = [ "Hello", "World", "!" ] {
    willSet {
      // in here you have `newValue` containing the new array which
      // will be set. Do any comparison operations you want, like:
      let oldStrings = Set(strings)
      let newStrings = Set(newValue)
      print("removed: \(oldStrings.substract(newStrings))")
      print("added:   \(newStrings.substract(oldStrings))")
      // (Just for demonstration purposes, if they are Sets, they
      //  should be Sets in the first place, obviously.)
    }
  }
}

Answer 2

Thanks for @hnh, based on his answer, I ended up with:

class MyNumber: NSObject {

    // NOTE that it works in both "willSet" and "didSet"

    /// Array ///
    var arrayNumbers: [String] = ["one", "two", "three"] {
        willSet {
            let oldStrings = Set(arrayNumbers)
            let newStrings = Set(newValue)

            print("removed from array: \(oldStrings.subtracting(newStrings))")
            print("added to array:   \(newStrings.subtracting(oldStrings))")

            print("----------")
        }
    }

    /// Set ///
    var setNumbers: Set = ["one", "two", "three"] {
        didSet(newSet) {
            print("removed from set: \(newSet.subtracting(setNumbers))")
            print("added to set:   \(setNumbers.subtracting(newSet))")

            print("----------")
        }
    }

    var dictionaryNumbers = ["1": "one", "2": "two", "3": "three"] {
        didSet(modified) {
            let oldKeys = Set(dictionaryNumbers.keys)
            let newKeys = Set(modified.keys)

            let oldValues = Set(dictionaryNumbers.values)
            let newValues = Set(modified.values)

            print("removed from dictionary (keys): \(newKeys.subtracting(oldKeys)) (values): \(newValues.subtracting(oldValues))")
            print("added to dictionary (keys):   \(oldKeys.subtracting(newKeys)) (values):    \(oldValues.subtracting(newValues))")
            print("----------")

//            print("removed (values): \(newValues.subtracting(oldValues))")
//            print("added (values):   \(oldValues.subtracting(newValues))")

        }
    }
}

Execution:

let myNumber = MyNumber()

/// Array ///

// adding:
myNumber.arrayNumbers.append("four")
/* Logging:
 removed: [] means that nothing has been removed form the array
 added:   ["four"]
 ----------
 */

// updating:
myNumber.arrayNumbers[0] = "One"
/* Logging:
 removed: ["one"]
 added:   ["One"]
 ----------
 */

// deleting:
myNumber.arrayNumbers.removeLast()
/* Logging:
 removed: ["four"]
 added:   [] means that nothing has been added to the array
 ----------
 */


/// Set ///

// adding:
myNumber.setNumbers.insert("four")
/* Logging:
 removed from set: [] means that nothing has been removed form the set
 added to set:   ["four"]
 ----------
 */

// deleting:
myNumber.setNumbers.removeFirst()
/* Logging:
 removed from set: ["three"] // sets are unsorted...
 added to set:   [] means that nothing has been added to the set
 ----------
 */


/// Dictionary ///

// adding:
myNumber.dictionaryNumbers["4"] = "four"
/* Logging:
 removed from dictionary (keys): [] (values): []
 added to dictionary (keys):   ["4"] (values):    ["four"]
 ----------
 */

// updating:
myNumber.dictionaryNumbers["1"] = "One"
/* Logging:
 removed from dictionary (keys): [] (values): ["one"]
 added to dictionary (keys):   [] (values):    ["One"]
 ----------
 */

// deleting:
myNumber.dictionaryNumbers.removeValue(forKey: "2")
/* Logging:
 removed from dictionary (keys): ["2"] (values): ["two"]
 added to dictionary (keys):   [] (values):    []
 ----------
 */

This shows how to deal with array, set and dictionary.