面试题：Objective-C中KVO在复杂数据结构下的应用

Question

假设你有一个包含多层嵌套的自定义对象，例如一个`Person`对象，`Person`对象包含一个`Address`对象，`Address`对象又包含`City`等属性。如何通过KVO来监听`City`属性的变化？并且如果在对象关系动态变化（如`Address`对象被替换）的情况下，如何保证KVO的正常工作？

Answer 1

1. 基本KVO监听City属性变化
   • 首先，确保相关类继承自NSObject（在Objective - C中，KVO基于NSObject的键值观察机制）。
   • 在持有Person对象的类中，注册KVO监听：

   - (void)viewDidLoad {
       [super viewDidLoad];
       Person *person = [[Person alloc] init];
       [person addObserver:self forKeyPath:@"address.city" options:NSKeyValueObservingOptionNew context:nil];
   }
   - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
       if ([keyPath isEqualToString:@"address.city"]) {
           NSString *newCity = change[NSKeyValueChangeNewKey];
           NSLog(@"City has changed to: %@", newCity);
       }
   }
   - (void)dealloc {
       [self.person removeObserver:self forKeyPath:@"address.city"];
   }
   

   • 在Swift中：

   class ViewController: UIViewController {
       var person: Person!
       override func viewDidLoad() {
           super.viewDidLoad()
           person = Person()
           person.addObserver(self, forKeyPath: "address.city", options:.new, context: nil)
       }
       override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
           if keyPath == "address.city" {
               if let newCity = change?[.newKey] as? String {
                   print("City has changed to: \(newCity)")
               }
           }
       }
       deinit {
           person.removeObserver(self, forKeyPath: "address.city")
       }
   }
   

2. 处理对象关系动态变化（如Address对象被替换）
   • Objective - C：
     • 当Address对象可能被替换时，除了监听address.city，还需要监听address本身的变化。
     • 注册对address的监听：

     - (void)viewDidLoad {
         [super viewDidLoad];
         Person *person = [[Person alloc] init];
         [person addObserver:self forKeyPath:@"address.city" options:NSKeyValueObservingOptionNew context:nil];
         [person addObserver:self forKeyPath:@"address" options:NSKeyValueObservingOptionNew context:nil];
     }
     - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
         if ([keyPath isEqualToString:@"address"]) {
             // 移除旧的`address.city`监听
             Address *oldAddress = change[NSKeyValueChangeOldKey];
             if (oldAddress) {
                 [oldAddress removeObserver:self forKeyPath:@"city"];
             }
             // 添加新的`address.city`监听
             Address *newAddress = change[NSKeyValueChangeNewKey];
             if (newAddress) {
                 [newAddress addObserver:self forKeyPath:@"city" options:NSKeyValueObservingOptionNew context:nil];
             }
         } else if ([keyPath isEqualToString:@"address.city"]) {
             NSString *newCity = change[NSKeyValueChangeNewKey];
             NSLog(@"City has changed to: %@", newCity);
         }
     }
     - (void)dealloc {
         [self.person removeObserver:self forKeyPath:@"address.city"];
         [self.person removeObserver:self forKeyPath:@"address"];
     }
     

   • Swift：

     class ViewController: UIViewController {
         var person: Person!
         override func viewDidLoad() {
             super.viewDidLoad()
             person = Person()
             person.addObserver(self, forKeyPath: "address.city", options:.new, context: nil)
             person.addObserver(self, forKeyPath: "address", options:.new, context: nil)
         }
         override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
             if keyPath == "address" {
                 if let oldAddress = change?[.oldKey] as? Address {
                     oldAddress.removeObserver(self, forKeyPath: "city")
                 }
                 if let newAddress = change?[.newKey] as? Address {
                     newAddress.addObserver(self, forKeyPath: "city", options:.new, context: nil)
                 }
             } else if keyPath == "address.city" {
                 if let newCity = change?[.newKey] as? String {
                     print("City has changed to: \(newCity)")
                 }
             }
         }
         deinit {
             person.removeObserver(self, forKeyPath: "address.city")
             person.removeObserver(self, forKeyPath: "address")
         }
     }
     

这样在City属性变化以及Address对象被替换时，都能通过KVO正常监听并处理变化。