iOS 基础 - KVO
一些 iOS 基础知识，业务开发中经常用到，面试时也常会被问到，这里总结一下，此篇文章讲解 KVO。
Key-Path Expressions
对于实例的属性访问可以采用 [keyPath: .属性名] 这样的方式，后面讲 KVO 时，可以看到这种表达式的用处。
struct Person {
var name: String
}
struct Book {
var title: String
var authors: [Person]
var primaryAuthor: Person {
return authors.first!
}
}
let abelson = Person(name: "Harold Abelson")
let sussman = Person(name: "Gerald Jay Sussman")
let book = Book(title: "Structure and Interpretation of Computer Programs", authors: [abelson, sussman])
print(book[keyPath: \Book.title])
print(book[keyPath: \.primaryAuthor.name])KVO
KVO 主要应用场景是观察对象属性的变化，能被观察的对象属性需满足下面两个条件：
- 首先要利用 Objective-C Runtime 的特性，所以需要继承自 NSObject。
- 属性需要是 @objc dynamic。
class Child: NSObject {
let name: String
@objc dynamic var age: Int
init(name: String, age: Int) {
self.name = name
self.age = age
super.init()
}
func celebrateBirthday() {
age += 1
}
}通过在要观察的对象上调用 observe，通过前面讲的 Key-Path Expressions 来指定属性 .age，方法会返回一个 NSKeyValueObservation，当不再需要监听变化时，可以通过 invalidate 方法来结束监听：
let mia = Child(name: "Mia", age: 5)
let observation = mia.observe(\.age, options: [.initial, .old]) { (child, change) in
if let oldValue = change.oldValue {
print("\(child.name)’s age changed from \(oldValue) to \(child.age)")
} else {
print("\(child.name)’s age is now \(child.age)")
}
}
mia.celebrateBirthday()
observation.invalidate()示例 1：监听摄像头闪光灯模式的变化
private var flashModeObservation: NSKeyValueObservation?
flashModeObservation = videoDevice.observe(\.flashMode) { [weak self] device, _ in
self?.flashModeChanged(device)
}示例 2：监听 AVPlayerItem 的状态变化
private var statusObservation: NSKeyValueObservation?
// item is AVPlayerItem
statusObservation = item.observe(\.status) { [weak self] item, _ in
self?.itemStatusChanged(item)
}