第一行，是C++的 using命令，这里是使用AS命名空间下的MutexLocker

using AS::MutexLocker;

ASLayoutElementContext的初始化方法，相应的参数都用参数名字来表示，十分好理解

#pragma mark - ASLayoutElementContext

- (instancetype)init
{
  if (self = [super init]) {
    _transitionID = ASLayoutElementContextDefaultTransitionID;
  }
  return self;
}

第一个定义的常量，默认设置为NAN，一种特殊的浮点类型

CGFloat const ASLayoutElementParentDimensionUndefined = NAN;

其他常量定义看起来都是十分清晰的，用命名代表数值

CGSize const ASLayoutElementParentSizeUndefined = {ASLayoutElementParentDimensionUndefined, ASLayoutElementParentDimensionUndefined};

int32_t const ASLayoutElementContextInvalidTransitionID = 0;
int32_t const ASLayoutElementContextDefaultTransitionID = ASLayoutElementContextInvalidTransitionID + 1;

本地线程，这里用了__unsafe_unretained
有个疑问，为什么不用__weak?

#ifdef __i386__
  #define AS_TLS_AVAILABLE 0
#else
  #define AS_TLS_AVAILABLE 1

这里分成两种模式来进行定义，i386和非i386

#if AS_TLS_AVAILABLE

static _Thread_local __unsafe_unretained ASLayoutElementContext *tls_context;

void ASLayoutElementPushContext(ASLayoutElementContext *context)
{
  // NOTE: It would be easy to support nested contexts – just use an NSMutableArray here.
  ASDisplayNodeCAssertNil(tls_context, @"Nested ASLayoutElementContexts aren't supported.");
  
  tls_context = (__bridge ASLayoutElementContext *)(__bridge_retained CFTypeRef)context;
}

这里用了ASDisplayNodeCAssertNil的宏命令
定义如下，内部调用了NSException的方法，##__VA_ARGS__的作用是省略掉多余的参数

#define ASDisplayNodeCAssert(condition, desc, ...) NSCAssert(condition, desc, ##__VA_ARGS__)
#define ASDisplayNodeCAssertNil(condition, desc, ...) ASDisplayNodeCAssert((condition) == nil, desc, ##__VA_ARGS__)

这里定义了一个获取当前Context的函数，让外部可以通过该方法拿到本地线程？

ASLayoutElementContext *ASLayoutElementGetCurrentContext()
{
  // Don't retain here. Caller will retain if it wants to!
  return tls_context;
}

释放本地线程方法

void ASLayoutElementPopContext()
{
  ASDisplayNodeCAssertNotNil(tls_context, @"Attempt to pop context when there wasn't a context!");
  CFRelease((__bridge CFTypeRef)tls_context);
  tls_context = nil;
}

#else

静态方法，获取线程的key,里面使用了单例模式

static pthread_key_t ASLayoutElementContextKey() {
  static pthread_key_t k;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    pthread_key_create(&k, NULL);
  });
  return k;
}

这里用到了线程储存方法pthread_getspecific，pthread_setspecific
用到了const auto命令，拷贝变量，但不允许修改，auto表示可以推断类型

void ASLayoutElementPushContext(ASLayoutElementContext *context)
{
  // NOTE: It would be easy to support nested contexts – just use an NSMutableArray here.
  ASDisplayNodeCAssertNil(pthread_getspecific(ASLayoutElementContextKey()), @"Nested ASLayoutElementContexts aren't supported.");
  
  const auto cfCtx = (__bridge_retained CFTypeRef)context;
  pthread_setspecific(ASLayoutElementContextKey(), cfCtx);
}

获取当前线程的Context

ASLayoutElementContext *ASLayoutElementGetCurrentContext()
{
  // Don't retain here. Caller will retain if it wants to!
  const auto ctxPtr = pthread_getspecific(ASLayoutElementContextKey());
  return (__bridge ASLayoutElementContext *)ctxPtr;
}

移除Context

void ASLayoutElementPopContext()
{
  const auto ctx = (CFTypeRef)pthread_getspecific(ASLayoutElementContextKey());
  ASDisplayNodeCAssertNotNil(ctx, @"Attempt to pop context when there wasn't a context!");
  CFRelease(ctx);
  pthread_setspecific(ASLayoutElementContextKey(), NULL);
}

原先在头文件定义的内容，在实现文件赋值定义

#pragma mark - ASLayoutElementStyle

NSString * const ASLayoutElementStyleWidthProperty = @"ASLayoutElementStyleWidthProperty";
此处省略一些...

此处用宏定义了两个函数，对于宏的理解，可以参考一下某大神的博客
宏定义的黑魔法

#define ASLayoutElementStyleSetSizeWithScope(x)                                    \
  ({                                                                               \
    __instanceLock__.lock();                                                       \
    const ASLayoutElementSize oldSize = _size.load();                              \
    ASLayoutElementSize newSize = oldSize;                                         \
    {x};                                                                           \
    BOOL changed = !ASLayoutElementSizeEqualToLayoutElementSize(oldSize, newSize); \
    if (changed) {                                                                 \
      _size.store(newSize);                                                        \
    }                                                                              \
    __instanceLock__.unlock();                                                     \
    changed;                                                                       \
  })

#define ASLayoutElementStyleCallDelegate(propertyName)\
do {\
  [self propertyDidChange:propertyName];\
  [_delegate style:self propertyDidChange:propertyName];\
} while(0)

未完待续……