KVO对Swift的计算属性 [英] KVO on Swift's computed properties

问题描述

只是想知道在计算属性中是否可以在Swift 2.2中使用KVO！？

just wondering if this is possible in Swift 2.2, KVO on a computed property!?

ie：

   var width = 0
   var height = 0

   private var area : Double {
          get {
                 return with * height
          }
   }

   self.addOberser(self, forKeyPath: "area", ......

客户端代码是修改with或height触发器observeValueForKeyPath吗？

Would a client code modifying the with or height trigger observeValueForKeyPath?

在参与市长之前检查一下类重构.KVO的语法很烦人，因为如果有人手头的答案（我假设答案是否定的），它甚至不值得去游乐场。

Just checking before engaging on a mayor class refactor. KVO's syntax being as annoying as it's is not worth even a playground if someone has an answer at hand (I am assuming answer is NO)

问候！
~d

regards! ~d

推荐答案

该代码无法运作有两个原因：

That code won't work for two reasons:

1. 您必须将动态属性添加到区域属性，如采用Cocoa设计模式在中使用Swift与Cocoa和Objective-C 。

1. You must add the dynamic attribute to the area property, as described in the section "Key-Value Observing" under "Adopting Cocoa Design Patterns" in Using Swift with Cocoa and Objective-C.

您必须声明 area 取决于 width 和 height ，如键值观察编程中的https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/KeyValueObserving/Articles/KVODependentKeys.html\"rel =noreferrer>注册从属关键字指南 。 （这适用于Objective-C和Swift。）为此，您还必须将 dynamic 添加到 width 和高度。

You must declare that area depends on width and height as described in "Registering Dependent Keys" in the Key-Value Observing Programming Guide. (This applies to Objective-C and Swift.) And for this to work, you also have to add dynamic to width and height.

（你可以改为调用 willChangeValueForKey 和 didChangeValueForKey 每当 width 或 height 更改时，它就是通常更容易实现 keyPathsForValuesAffectingArea 。）

(You could instead call willChangeValueForKey and didChangeValueForKey whenever width or height changes, but it's usually easier to just implement keyPathsForValuesAffectingArea.)

因此：

class MyObject: NSObject {

    dynamic var width: Double = 0
    dynamic var height: Double = 0

    dynamic private var area: Double {
        return width * height
    }

    class func keyPathsForValuesAffectingArea() -> Set<String> {
        return [ "width", "height" ]
    }

    func register() {
        self.addObserver(self, forKeyPath: "area", options: [ .Old, .New ], context: nil)
    }

    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        print("observed \(keyPath) \(change)")
    }

}

let object = MyObject()
object.register()
object.width = 20
object.height = 5

输出：

observed Optional("area") Optional(["old": 0, "new": 0, "kind": 1])
observed Optional("area") Optional(["old": 0, "new": 100, "kind": 1])

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