以下用于游乐场：
struct Node {
let title: String
let leaf: Bool
var children: [String: Node] = [:]
}
let t = Node(title:"hello",leaf:false,children:[:])
let k1 = \Node.leaf
let k2 = \Node.children
t[keyPath: k1] // returns "false" works
t[keyPath: k2] // returns "[:]" works
@objcMembers class MyController : NSObject {
dynamic var tr: Node
var kvo : NSKeyValueObservation!
init(t: Node) {
tr = t
super.init()
kvo = observe(\.tr) { object,change in
print("\(object) \(change)")
}
}
}
let x = MyController(t: t)
x.tr = Node(title:"f",children:[:])
x
这个错误：
fatal error: Could not extract a String from KeyPath
Swift.ReferenceWritableKeyPath<__lldb_expr_3.MyController,
__lldb_expr_3.Node>: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-900.0.45.6/src/swift/stdlib/public/SDK/Foundation/NSObject.swift,
line 85
另外,请看到此错误：
error: Execution was interrupted,reason: EXC_BAD_INSTRUCTION
(code=EXC_I386_INVOP,subcode=0x0). The process has been left at the
point where it was interrupted,use “thread return -x” to return to
the state before expression evaluation.
是否有其他人能够得到这样的工作,或者这是我需要报告的错误？
@objcMembers class MyController : NSObject {
dynamic var tr: Node
// ...
Node是一个结构,因此不能直接用Obj-C表示.但是,编译器仍允许您将tr标记为动态 – 这需要@objc.虽然@objcMembers推断@objc为类的成员,但它只对可直接表示在Obj-C中的成员这样做,而tr则不是.
所以真的,编译器不应该让你把tr标记为动态 – 我继续前进到filed a bug here,其中has now been fixed将为Swift 5做好准备.
tr需要成为@objc&动态为你使用KVO,因为KVO需要方法调配,Obj-C运行时提供,而Swift运行时则不需要.所以要在这里使用KVO,你需要使Node成为一个类,并从NSObject继承,以便将tr暴露给Obj-C：
class Node : NSObject {
let title: String
let leaf: Bool
var children: [String: Node] = [:]
init(title: String,leaf: Bool,children: [String: Node]) {
self.title = title
self.leaf = leaf
self.children = children
}
}
(如果你再次看一下WWDC视频,你会看到他们观察的属性实际上是一个继承自NSObject的类的类型)
但是,在您给出的示例中,您并不真正需要KVO – 您可以将Node保留为结构,而是使用属性观察器：
struct Node {
let title: String
let leaf: Bool
var children: [String: Node] = [:]
}
class MyController : NSObject {
var tr: Node {
didSet {
print("didChange: \(tr)")
}
}
init(t: Node) {
tr = t
}
}
let x = MyController(t: Node(title:"hello",children: [:]))
x.tr = Node(title:"f",leaf: false,children: [:])
// didChange: Node(title: "f",children: [:])
而且因为Node是一个值类型,didSet也会触发对其属性的任何更改：
x.tr.children["foo"] = Node(title: "bar",children: [ // "foo": kvc_in_playground.Node(title: "bar",children: [:]) // ])
