ios - 如何观察值(value)的变化并表现出来？在 swift

Question

我有一个变量，它获取不同类中数组中对象的数量，我如何跟踪当前类中这个变量的变化？我做了很多不同的尝试，但都失败了。

var digitIndex: Int! {
        set {
            self.digitIndex = newValue
        }
        get {
            return firstClass.indexesOfSelectedNodes().count
        }
    }

    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if context == &digitIndex {
            if let newValue = change?[NSKeyValueChangeNewKey] {
                print("newValue")
                infoLabel.text = "\(newValue)"
            }
        }
    }

Answer 1

1) 在你的代码中

var digitIndex: Int! {
        set {
            self.digitIndex = newValue
        }
        get {
            return firstClass.indexesOfSelectedNodes().count
        }
    }

digitIndex 是计算属性!你不能在它自己的 setter 中设置 self.digitIndex!

这段代码，即使经过编译，也将永远运行:-)

var i: Int {
    get {
        return 10
    }
    set {
        i = newValue
    }
}

i = 100
print(i)

2) 如何使用 willSet 和 didSet(对于 STORED 属性)？

class C1 {}
class C2 {}

class C {
    var objects: [AnyObject] = [C1(),C1(),C2()] {
        willSet {
            print(objects, objects.count, newValue)
        }
        didSet {
            print(objects, objects.count)
        }
    }
    func add(object: AnyObject) {
        objects.append(object)
    }
}

let c = C()
c.add(C1())
/*
[C1, C1, C2] 3 [C1, C1, C2, C1]
[C1, C1, C2, C1] 4
*/

var i: Int = 0 {
willSet {
    print(newValue, "will replace", i)
}
didSet {
    print(oldValue, "was replaced by", i)
}
}

i = 100
/*
100 will replace 0
0 was replaced by 100
*/

您可以根据自己的优势组合计算和存储的属性

// 'private' storage
var _j:Int = 0
var j: Int {
get {
    return _j
}
set {
    print(newValue)
    if newValue < 300 {
    _j = newValue
    } else {
        print(newValue, "refused")
    }
}
}

print(j) // prints 0
j = 200  // prints 200
print(j) // prints 200
j = 500  // prints 500 refused
print(j) // prints 200