KVO 实现原理

测试 Demo

KVOExplorer

添加观察者前

• 实例化一个 Person 对象，在添加观察者前，打印变量

  (lldb) po [personA _ivarDescription]
  <Person: 0x60000377d9e0>:
  in Person:
      _name (NSString*): nil
      _age (long): 0
  in NSObject:
      isa (Class): Person (isa, 0x10e8b36f0)

• 查看方法列表

  (lldb) pmethods personA -a
  Class Methods:
  No methods were found
  
  Instance Methods:
  - (void).cxx_destruct 0x10e8aee10
  - (id)name 0x10e8aed60
  - (void)setName:(id)arg0  0x10e8aed90
  - (long long)age 0x10e8aedd0
  - (void)setAge:(long long)arg0  0x10e8aedf0

添加观察者后

• 打印实例变量，添加观察者后 isa 指针指向了新了对象 NSKVONotifying_Person

  (lldb) po [personA _ivarDescription]
  <Person: 0x60000377d9e0>:
  in Person:
      _name (NSString*): nil
      _age (long): 0
  in NSObject:
      isa (Class): NSKVONotifying_Person (isa, 0x600000548630)

• 查看 NSKVONotifying_Person 父类

  (lldb) po class_getSuperclass((id)0x600000548630)
  Perso

• 打印方法列表

  (lldb) pmethods personA -a
  Class Methods:
  No methods were found
  
  Instance Methods:
  - (void)setName:(id)arg0  0x7fff258dba8b
  - (Class)class 0x7fff258da515
  - (void)dealloc 0x7fff258da27a
  - (bool)_isKVOA 0x7fff258da272

• 打印 IMP

  (lldb) print (IMP)0x7fff258dba8b
  (IMP) $78 = 0x00007fff258dba8b (Foundation`_NSSetObjectValueAndNotify)
  (lldb) print (IMP)0x7fff258da515
  (IMP) $79 = 0x00007fff258da515 (Foundation`NSKVOClass)
  (lldb) print (IMP)0x7fff258da27a
  (IMP) $80 = 0x00007fff258da27a (Foundation`NSKVODeallocate)
  (lldb) print (IMP)0x7fff258da272
  (IMP) $81 = 0x00007fff258da272 (Foundation`NSKVOIsAutonotifying)

为变量赋值

• 直接访问属性赋值，会触发 KVO

  personAB.name = @"John";

• 使用 KVC 为属性赋值，也会触发 KVO

  [personAB setValue:@"Jack" forKey:@"name"];

• 直接访问私有变量赋值，不会触发 KVO

  //Person.h
  @interface Person : NSObject{
      NSString *_nickname;
  }
  
  //Person.m
  - (void)setNickName:(NSString *)nickname{
      _nickname = nickname;
  }
  
  //ViewController.m
  [personAB setNickName:@"Air Jonh"];

• 使用 KVC 为私有变量赋值，会触发 KVO

  [personAB setValue:@"Big Boss" forKey:@"_nickname"];

• 调用堆栈

  * frame #0: 0x000000010e8ae9c2 KVOExplorer`-[ViewController observeValueForKeyPath:ofObject:change:context:](self=0x00007f913a409d70, _cmd="observeValueForKeyPath:ofObject:change:context:", keyPath=@"name", object=0x000060000377da20, change=0x000060000227f980, context=0x0000000000000000) at ViewController.m:90:18
      frame #1: 0x00007fff258de357 Foundation`NSKeyValueNotifyObserver + 329
      frame #2: 0x00007fff258e18f3 Foundation`NSKeyValueDidChange + 502
      frame #3: 0x00007fff258e11f5 Foundation`-[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:] + 741
      frame #4: 0x00007fff258e1af2 Foundation`-[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 68
      frame #5: 0x00007fff258dbb98 Foundation`_NSSetObjectValueAndNotify + 269
      frame #6: 0x000000010e8ae693 KVOExplorer`-[ViewController viewDidLoad](self=0x00007f913a409d70, _cmd="viewDidLoad") at ViewController.m:79:5

移除观察者后

• 打印实例变量

  (lldb) po [personA _ivarDescription]
  <Person: 0x60000377d9e0>:
  in Person:
      _name (NSString*): nil
      _age (long): 0
  in NSObject:
      isa (Class): Person (isa, 0x10e8b36f0)

• 打印方法列表

  (lldb) pmethods personA -a
  Class Methods:
  No methods were found
  
  Instance Methods:
  - (void).cxx_destruct 0x10e8aee10
  - (id)name 0x10e8aed60
  - (void)setName:(id)arg0  0x10e8aed90
  - (long long)age 0x10e8aedd0
  - (void)setAge:(long long)arg0  0x10e8aedf0

NSKVONotifying_<xxx>

当定义一个 NSKVONotifying_Person 类时，系统无法动态创建 NSKVONotifying_Persson 类，导致 KVO 失效，错误如下：

 [general] KVO failed to allocate class pair for name NSKVONotifying_Person, automatic key-value observing will not work for this class

参考资料

• 详解键值观察（KVO）及其实现机理
• mikeash.com: Friday Q&A 2009-01-23
• Introduction to Key-Value Observing Programming Guide
• Do I have to removeObserver in KVO manually