swift - KVO Swift : newValue is always nil

Question

有以下例子:

import UIKit

@objc class ViewController: UIViewController {

    @objc dynamic var buggyApple: NSObject?

    var obs: NSKeyValueObservation?

    override func viewDidLoad() {
        super.viewDidLoad()
        obs = observe(\.buggyApple, changeHandler: { (obj, change) in
            print(change)
        })
        buggyApple = NSObject()
        buggyApple = NSObject()
    }
}

我在调试控制台中得到相应的输出:

NSKeyValueObservedChange<Optional<NSObject>>(kind: __C.NSKeyValueChange, newValue: Optional(nil), oldValue: Optional(nil), indexes: nil, isPrior: false)
NSKeyValueObservedChange<Optional<NSObject>>(kind: __C.NSKeyValueChange, newValue: Optional(nil), oldValue: Optional(nil), indexes: nil, isPrior: false)

在检查 ViewController 时LLDB 中回调内断点上的对象，我得到以下信息:

(lldb) po obj.buggyApple
▿ Optional<NSObject>
  - some : <NSObject: 0x600000908fa0>

所以，事实上，新NSObject已成功发送，然而，change仍然是引用nil .我的代码是否有问题以及如何解决？

Answer 1

您需要提供new options 的选项observer 的输入参数在 change 中接收新值的方法.

obs = observe(\.buggyApple, options: [.new], changeHandler: { (obj, change) in
    print(change)
})