RxSwift-KVO底层探索（上）

KVO 在我们实际开发之中运用非常之多，很多开发者都知道原理！但是这些原理是如何来的，一般都是浅尝辄止。这个篇章我会从 Swift 入手分析，探索 KVO 底层源码.希望让读者真正掌握这一块底层，知其然而知其所以然！

KVO简介

首先我们从 KVO 的三部曲开始

// 1: 添加观察
person.addObserver(self, forKeyPath: "name", options: .new, context: nil)
// 2: 观察响应回调
override func observeValue(forKeyPath keyPath:, of object:, change: , context:）{}
// 3: 移除观察
person.removeObserver(self, forKeyPath: "name")
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其实我们也知道，就是平时在开发的时候，我们也可以通过计算型属性也可以直接观察

var name: String = ""{
    willSet{
        print(newValue)
    }
    didSet{
        print(oldValue)
    }
}
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问题来了：这两者有什么关系？

KVO与计算型属性的关系

下面我们开始分析，首先感谢苹果开源精神，在 Github 可以直接下载，我们通过 Swift 源码展开分析

public func willChangeValue<Value>(for keyPath: __owned KeyPath<Self, Value>) {
    (self as! NSObject).willChangeValue(forKey: _bridgeKeyPathToString(keyPath))
}

public func willChange<Value>(_ changeKind: NSKeyValueChange, valuesAt indexes: IndexSet, for keyPath: __owned KeyPath<Self, Value>) {
    (self as! NSObject).willChange(changeKind, valuesAt: indexes, forKey: _bridgeKeyPathToString(keyPath))
}

public func willChangeValue<Value>(for keyPath: __owned KeyPath<Self, Value>, withSetMutation mutation: NSKeyValueSetMutationKind, using set: Set<Value>) -> Void {
    (self as! NSObject).willChangeValue(forKey: _bridgeKeyPathToString(keyPath), withSetMutation: mutation, using: set)
}

public func didChangeValue<Value>(for keyPath: __owned KeyPath<Self, Value>) {
    (self as! NSObject).didChangeValue(forKey: _bridgeKeyPathToString(keyPath))
}

public func didChange<Value>(_ changeKind: NSKeyValueChange, valuesAt indexes: IndexSet, for keyPath: __owned KeyPath<Self, Value>) {
    (self as! NSObject).didChange(changeKind, valuesAt: indexes, forKey: _bridgeKeyPathToString(keyPath))
}

public func didChangeValue<Value>(for keyPath: __owned KeyPath<Self, Value>, withSetMutation mutation: NSKeyValueSetMutationKind, using set: Set<Value>) -> Void {
    (self as! NSObject).didChangeValue(forKey: _bridgeKeyPathToString(keyPath), withSetMutation: mutation, using: set)
}
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• willChangeValue 和 didChangeValue 作为数据改变的两个重要的方法
• 我们通过这两个方法继续展开分析

class Target : NSObject, NSKeyValueObservingCustomization {
    // This dynamic property is observed by KVO
    @objc dynamic var objcValue: String
    @objc dynamic var objcValue2: String {
        willSet {
            willChangeValue(for: \.objcValue2)
        }
        didSet {
            didChangeValue(for: \.objcValue2)
        }
    }
}
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NSObject
NSKeyValueObservingCustomization

public protocol NSKeyValueObservingCustomization : NSObjectProtocol {
    static func keyPathsAffectingValue(for key: AnyKeyPath) -> Set<AnyKeyPath>
    static func automaticallyNotifiesObservers(for key: AnyKeyPath) -> Bool
}
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• 这也是我们两个非常重要的方法，平时在开发也是很有利的方法， keyPathsAffectingValue 能够建立 keyPath 的依赖，例如两个属性的变化同时影响一个重要属性的改变： 进度 = 下载量 / 总量
• automaticallyNotifiesObservers 自动开关
• 很明显我们的计算型属性在 willSet 里面就调用 willChangeValue ， didSet 调用 didChangeValue ，的确我们计算型属性是和我们KVO相关方法是有所关联，这里也直接证明！
• OK，我们探索完这个问题，我们摸着这条线继续探索 KVO底层 ！

KVO底层

这里说明一下，本篇章的贴出的源码没有给大家省略，目的是想让大家认真阅读，自己对照学习。当然可能中间我也忽略过一些细节，源码直接贴出来方便自己理解

添加观察

person.addObserver(self, forKeyPath: "name", options: .new, context: nil)
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现在我们开始探索底层源码：

- (void) addObserver: (NSObject*)anObserver
	  forKeyPath: (NSString*)aPath
	     options: (NSKeyValueObservingOptions)options
	     context: (void*)aContext
{
  GSKVOInfo             *info;
  GSKVOReplacement      *r;
  NSKeyValueObservationForwarder *forwarder;
  NSRange               dot;

  setup();
  [kvoLock lock];

  // Use the original class
  r = replacementForClass([self class]);

  /*
   * Get the existing observation information, creating it (and changing
   * the receiver to start key-value-observing by switching its class)
   * if necessary.
   */
  info = (GSKVOInfo*)[self observationInfo];
  if (info == nil)
    {
      info = [[GSKVOInfo alloc] initWithInstance: self];
      [self setObservationInfo: info];
      object_setClass(self, [r replacement]);
    }

  /*
   * Now add the observer.
   */
  dot = [aPath rangeOfString:@"."];
  if (dot.location != NSNotFound)
    {
      forwarder = [[NSKeyValueObservationForwarder alloc]
        initWithKeyPath: aPath
	       ofObject: self
	     withTarget: anObserver
		context: aContext];
      [info addObserver: anObserver
             forKeyPath: aPath
                options: options
                context: forwarder];
    }
  else
    {
      [r overrideSetterFor: aPath];
      [info addObserver: anObserver
             forKeyPath: aPath
                options: options
                context: aContext];
    }

  [kvoLock unlock];
}
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• 中间 replacementForClass 做了一些处理：

  • 创建了一个动态子类名字："NSKVONotifing_原类的名字"
  • 添加了 class、set、dealloc 方法
  • 原类的 isa 与动态 isa 切换

• 由原来的观察者进行迁移到 GSKVOInfo

- (void) addObserver: (NSObject*)anObserver
	  forKeyPath: (NSString*)aPath
	     options: (NSKeyValueObservingOptions)options
	     context: (void*)aContext
{
  GSKVOPathInfo         *pathInfo;
  GSKVOObservation      *observation;
  unsigned              count;

  if ([anObserver respondsToSelector:
    @selector(observeValueForKeyPath:ofObject:change:context:)] == NO)
    {
      return;
    }
  [iLock lock];
  pathInfo = (GSKVOPathInfo*)NSMapGet(paths, (void*)aPath);
  if (pathInfo == nil)
    {
      pathInfo = [GSKVOPathInfo new];
      // use immutable object for map key
      aPath = [aPath copy];
      NSMapInsert(paths, (void*)aPath, (void*)pathInfo);
      [pathInfo release];
      [aPath release];
    }

  observation = nil;
  pathInfo->allOptions = 0;
  count = [pathInfo->observations count];
  while (count-- > 0)
    {
      GSKVOObservation      *o;

      o = [pathInfo->observations objectAtIndex: count];
      if (o->observer == anObserver)
        {
          o->context = aContext;
          o->options = options;
          observation = o;
        }
      pathInfo->allOptions |= o->options;
    }
  if (observation == nil)
    {
      observation = [GSKVOObservation new];
      GSAssignZeroingWeakPointer((void**)&observation->observer,
	(void*)anObserver);
      observation->context = aContext;
      observation->options = options;
      [pathInfo->observations addObject: observation];
      [observation release];
      pathInfo->allOptions |= options;
    }

  if (options & NSKeyValueObservingOptionInitial)
    {
      /* If the NSKeyValueObservingOptionInitial option is set,
       * we must send an immediate notification containing the
       * existing value in the NSKeyValueChangeNewKey
       */
      [pathInfo->change setObject: [NSNumber numberWithInt: 1]
                           forKey:  NSKeyValueChangeKindKey];
      if (options & NSKeyValueObservingOptionNew)
        {
          id    value;

          value = [instance valueForKeyPath: aPath];
          if (value == nil)
            {
              value = null;
            }
          [pathInfo->change setObject: value
                               forKey: NSKeyValueChangeNewKey];
        }
      [anObserver observeValueForKeyPath: aPath
                                ofObject: instance
                                  change: pathInfo->change
                                 context: aContext];
    }
  [iLock unlock];
}
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• 判断我们的观察者是否能够响应： observeValueForKeyPath:ofObject:change:context: 方法。常规操作，没有回调，响应就没有什么意义了！
• 通过获取 pathInfo 来保存 KVO信息
• 中间对 context & options 的处理数据
• NSKeyValueObservingOptionInitial 就会主动发起一次 KVO 响应: observeValueForKeyPath

观察属性变化的时候

- (void) willChangeValueForKey: (NSString*)aKey
{
  GSKVOPathInfo *pathInfo;
  GSKVOInfo     *info;

  info = (GSKVOInfo *)[self observationInfo];
  if (info == nil)
    {
      return;
    }

  pathInfo = [info lockReturningPathInfoForKey: aKey];
  if (pathInfo != nil)
    {
      if (pathInfo->recursion++ == 0)
        {
          id    old = [pathInfo->change objectForKey: NSKeyValueChangeNewKey];

          if (old != nil)
            {
              /* We have set a value for this key already, so the value
               * we set must now be the old value and we don't need to
               * refetch it.
               */
              [pathInfo->change setObject: old
                                   forKey: NSKeyValueChangeOldKey];
              [pathInfo->change removeObjectForKey: NSKeyValueChangeNewKey];
            }
          else if (pathInfo->allOptions & NSKeyValueObservingOptionOld)
            {
              /* We don't have an old value set, so we must fetch the
               * existing value because at least one observation wants it.
               */
              old = [self valueForKey: aKey];
              if (old == nil)
                {
                  old = null;
                }
              [pathInfo->change setObject: old
                                   forKey: NSKeyValueChangeOldKey];
            }
          [pathInfo->change setValue:
            [NSNumber numberWithInt: NSKeyValueChangeSetting]
            forKey: NSKeyValueChangeKindKey];

          [pathInfo notifyForKey: aKey ofInstance: [info instance] prior: YES];
        }
      [info unlock];
    }
  [self willChangeValueForDependentsOfKey: aKey];
}
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pathInfo
pathInfo->change
[pathInfo notifyForKey: aKey ofInstance: [info instance] prior: YES];

- (void) notifyForKey: (NSString *)aKey ofInstance: (id)instance prior: (BOOL)f
{
  unsigned      count;
  id            oldValue;
  id            newValue;

  if (f == YES)
    {
      if ((allOptions & NSKeyValueObservingOptionPrior) == 0)
        {
          return;   // Nothing to do.
        }
      [change setObject: [NSNumber numberWithBool: YES]
                 forKey: NSKeyValueChangeNotificationIsPriorKey];
    }
  else
    {
      [change removeObjectForKey: NSKeyValueChangeNotificationIsPriorKey];
    }

  oldValue = [[change objectForKey: NSKeyValueChangeOldKey] retain];
  if (oldValue == nil)
    {
      oldValue = null;
    }
  newValue = [[change objectForKey: NSKeyValueChangeNewKey] retain];
  if (newValue == nil)
    {
      newValue = null;
    }

  /* Retain self so that we won't be deallocated during the
   * notification process.
   */
  [self retain];
  count = [observations count];
  while (count-- > 0)
    {
      GSKVOObservation  *o = [observations objectAtIndex: count];

      if (f == YES)
        {
          if ((o->options & NSKeyValueObservingOptionPrior) == 0)
            {
              continue;
            }
        }
      else
        {
          if (o->options & NSKeyValueObservingOptionNew)
            {
              [change setObject: newValue
                         forKey: NSKeyValueChangeNewKey];
            }
        }

      if (o->options & NSKeyValueObservingOptionOld)
        {
          [change setObject: oldValue
                     forKey: NSKeyValueChangeOldKey];
        }

      [o->observer observeValueForKeyPath: aKey
                                 ofObject: instance
                                   change: change
                                  context: o->context];
    }

  [change setObject: oldValue forKey: NSKeyValueChangeOldKey];
  [oldValue release];
  [change setObject: newValue forKey: NSKeyValueChangeNewKey];
  [newValue release];
  [self release];
}
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[o->observer observeValueForKeyPath: aKey ofObject: instance change: change context: o->context];

移除观察者

移除观察的流程相对来说，比较简单了，但是优秀的我还是愿意和大家一起探索

- (void) removeObserver: (NSObject*)anObserver forKeyPath: (NSString*)aPath
{
  GSKVOInfo	*info;
  id            forwarder;

  /*
   * Get the observation information and remove this observation.
   */
  info = (GSKVOInfo*)[self observationInfo];
  forwarder = [info contextForObserver: anObserver ofKeyPath: aPath];
  [info removeObserver: anObserver forKeyPath: aPath];
  if ([info isUnobserved] == YES)
    {
      /*
       * The instance is no longer being observed ... so we can
       * turn off key-value-observing for it.
       */
      object_setClass(self, [self class]);
      IF_NO_GC(AUTORELEASE(info);)
      [self setObservationInfo: nil];
    }
  if ([aPath rangeOfString:@"."].location != NSNotFound)
    [forwarder finalize];
}
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• 拿回我们 observationInfo 就是我们信息收集者
• 利用 NSMapRemove(paths, (void*)aPath) 移除
• 动态子类的 isa 和原类的 isa 切换回来
• 把当前设置的 info 置空

OK 完美解析了KVO底层源码！我们在探索完KVO底层实现才能说是真正的掌握了，而不是通过面试宝典背下结论，那是没有什么意义! 在真正的高手对决间一眼就能看出，中间忽略了一些小细节，比如set的多种情况， setNumber 类型， setInt 类型, setLong 类型....我相信聪明的你一样可以解析读懂！ 就问此时此刻还有谁？45度仰望天空，该死！我这无处安放的魅力！