如何使用属性观察器观察具有 Swift 集合类型的特定元素？

Question

在回答如何知道数组中元素的值是否已更改的问题时受到启发？, 答案是使用Property Observer来检查数组是否已被修改。

但是，如何确定属性观察器中集合类型中更新的元素是什么？例如：

class MyClass {
    var strings: [String] = ["hello", "world", "!"] {
        didSet(modifiedStrings) {
            print("strings array has been modified!!:")
            print(modifiedStrings)
        }
    }
}

let myClass = MyClass()
myClass.strings.append("a string")
myClass.strings[0] = "Hello"
myClass.strings.removeLast()

请注意，didSet每个添加、更新或删除操作都调用了代码，但我如何才能确切地知道受影响的元素是什么？甚至有没有办法通过将strings数组作为Property Observer来实现这一点？

我在询问 Swift 中的所有集合类型，因为我认为所有集合类型都应该具有相同的行为，这与观察有关。

谢谢。

Answer 1

感谢@hnh，根据他的回答，我得到了：

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))")

        }
    }
}

执行：

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):    []
 ----------
 */

这显示了如何处理数组、集合和字典。