• hash默认实现将对象地址转换为uintptr_t类型返回。
• isEqual:直接判断地址是否相同。
• member:根据汇编可以看到大概逻辑是先计算参数的hash，然后集合中的元素调用isEqual参数是hash值。

4.2.2.3 _unobserve:info:

[[_FBKVOSharedController sharedController] observe:object info:info];

4.2.3.1 sharedController

_FBKVOSharedController是个单例，有成员变量_infos:

 NSHashTable<_FBKVOInfo *> *_infos;

- (instancetype)init

{
self = [super init];
if (nil != self) {

    NSHashTable *infos = [NSHashTable alloc];
#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED

    _infos = [infos initWithOptions:NSPointerFunctionsWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
if ([NSHashTable respondsToSelector:@selector(weakObjectsHashTable)]) {

      _infos = [infos initWithOptions:NSPointerFunctionsWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];

    } else {
// silence deprecated warnings
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"

      _infos = [infos initWithOptions:NSPointerFunctionsZeroingWeakMemory|NSPointerFunctionsObjectPointerPersonality capacity:0];
#pragma clang diagnostic pop

    }

#endif
pthread_mutex_init(&_mutex, NULL);

  }
return self;

}

• infos的初始化是weak的，也就是它不影响_FBKVOInfo的引用计数。

4.2.3.2 observe: info:

- (void)observe:(id)object info:(nullable _FBKVOInfo *)info

{
if (nil == info) {
return;

  }

// register info
pthread_mutex_lock(&_mutex);

  [_infos addObject:info];
pthread_mutex_unlock(&_mutex);

// add observer
//被观察者调用官方kvo进行注册，context 传递的是 _FBKVOInfo 信息。

  [object addObserver:self forKeyPath:info->_keyPath options:info->_options context:(void *)info];

if (info->_state == _FBKVOInfoStateInitial) {
//状态变为Observing

    info->_state = _FBKVOInfoStateObserving;

  } else if (info->_state == _FBKVOInfoStateNotObserving) {
// this could happen when `NSKeyValueObservingOptionInitial` is one of the NSKeyValueObservingOptions,
// and the observer is unregistered within the callback block.
// at this time the object has been registered as an observer (in Foundation KVO),
// so we can safely unobserve it.
//当状态变为不在观察时移除

    [object removeObserver:self forKeyPath:info->_keyPath context:(void *)info];

  }

}

• 首先自己持有了传进来的info信息。
• observe: info:中调用系统kvo方法观察注册。context传递的是_FBKVOInfo信息。
• 对于系统而言观察者是_FBKVOSharedController。

4.2.3.3 unobserve: info:

- (void)unobserve:(id)object info:(nullable _FBKVOInfo *)info

{
if (nil == info) {
return;

  }

// unregister info
pthread_mutex_lock(&_mutex);

  [_infos removeObject:info];
pthread_mutex_unlock(&_mutex);

// remove observer
if (info->_state == _FBKVOInfoStateObserving) {

    [object removeObserver:self forKeyPath:info->_keyPath context:(void *)info];

  }

  info->_state = _FBKVOInfoStateNotObserving;

}

• 调用系统的removeObserver移除观察。

4.2.3.4 observeValueForKeyPath

既然是在4.2.3_FBKVOSharedController中进行的注册，那么系统的回调observeValueForKeyPath必然由它实现：

- (void)observeValueForKeyPath:(nullable NSString *)keyPath

                      ofObject:(nullable id)object

                        change:(nullable NSDictionary<NSString *, id> *)change

                       context:(nullable void *)context

{
NSAssert(context, @"missing context keyPath:%@ object:%@ change:%@", keyPath, object, change);


  _FBKVOInfo *info;


  {
// lookup context in registered infos, taking out a strong reference only if it exists
pthread_mutex_lock(&_mutex);

    info = [_infos member:(__bridge id)context];
pthread_mutex_unlock(&_mutex);

  }

if (nil != info) {

// take strong reference to controller

    FBKVOController *controller = info->_controller;
if (nil != controller) {

// take strong reference to observer
//观察者

      id observer = controller.observer;
if (nil != observer) {

// dispatch custom block or action, fall back to default action
if (info->_block) {

          NSDictionary<NSString *, id> *changeWithKeyPath = change;
// add the keyPath to the change dictionary for clarity when mulitple keyPaths are being observed
if (keyPath) {
//将keypath加入字典中

            NSMutableDictionary<NSString *, id> *mChange = [NSMutableDictionary dictionaryWithObject:keyPath forKey:FBKVONotificationKeyPathKey];

            [mChange addEntriesFromDictionary:change];

            changeWithKeyPath = [mChange copy];

          }

          info->_block(observer, object, changeWithKeyPath);

        } else if (info->_action) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"

          [observer performSelector:info->_action withObject:change withObject:object];
#pragma clang diagnostic pop

        } else {

          [observer observeValueForKeyPath:keyPath ofObject:object change:change context:info->_context];

        }

      }

    }

  }

}

• 从info中获取观察者，info信息是context传递过来的。
• 在_FBKVOInfo存在的情况下根据类型（block、action、系统原始回调）进行了回调。block回调的过程中添加了keyPath。

4.2.4 自动移除观察者

在FBKVOController的dealloc中调用了unobserveAll进行移除：

- (void)dealloc

{

  [self unobserveAll];
pthread_mutex_destroy(&_lock);

}

由于FBKVOController的实例是被观察者持有的，所以当观察者dealloc的时候FBKVOController实例也就dealloc了。在这里调用就相当于在观察者dealloc中调用了移除。

FBKVOController流程

五、通过gnustep探索

kvo与kvc相关的代码苹果并没有开源，对于它们的探索可以通过gnustep查看原理，gnustep中有一些苹果早期底层的实现。

5.1 addObserver

• setup()中是对一些表的初始化。
• replacementForClass创建并注册kvo类。
• 创建GSKVOInfo信息加入Map中。然后进行isa替换。
• 重写setter方法。

• 根据是否开启自动回调决定是否调用willChangeValueForKey以及didChangeValueForKey。

didChangeValueForKey