c++容器

std::atomic<ASDisplayNodeAtomicFlags> _atomicFlags;

c++类型：atomic_uint

std::atomic_uint _displaySentinel;

static std::atomic_bool storesUnflattenedLayouts = ATOMIC_VAR_INIT(NO);//#define ATOMIC_VAR_INIT(__v) {__v}，没明白ATOMIC_VAR_INIT有什么用

ASDN::RecursiveMutex __instanceLock__;

//结构体继承
/**
   Obj-C doesn't allow you to pass parameters to C++ ivar constructors.
   Provide a convenience to change the default from non-recursive to recursive.

   But wait! Recursive mutexes are a bad idea. Think twice before using one:

   http://www.zaval.org/resources/library/butenhof1.html
   http://www.fieryrobot.com/blog/2008/10/14/recursive-locks-will-kill-you/
   */
struct RecursiveMutex : Mutex
  {
    RecursiveMutex () : Mutex (true) {}
  };
//struct Mutex定义没看懂，里面有pthread_mach_thread_np，pthread_mutex_lock，pthread_mutex_init等使用

罕见的c++语法

std::shared_ptr<ASDisplayNodeLayout> _calculatedDisplayNodeLayout;
std::shared_ptr<ASDisplayNodeLayout> _pendingDisplayNodeLayout;
_calculatedDisplayNodeLayout = std::make_shared<ASDisplayNodeLayout>();
_pendingDisplayNodeLayout = nullptr;

初始化结构体

struct ASDisplayNodeFlags flags = {0};
flags.isInHierarchy = NO;
  flags.displaysAsynchronously = YES;
  flags.shouldAnimateSizeChanges = YES;
  flags.implementsDrawRect = ([c respondsToSelector:@selector(drawRect:withParameters:isCancelled:isRasterizing:)] ? 1 : 0);
  flags.implementsImageDisplay = ([c respondsToSelector:@selector(displayWithParameters:isCancelled:)] ? 1 : 0);
  if (instance) {
    flags.implementsDrawParameters = ([instance respondsToSelector:@selector(drawParametersForAsyncLayer:)] ? 1 : 0);
  } else {
    flags.implementsDrawParameters = ([c instancesRespondToSelector:@selector(drawParametersForAsyncLayer:)] ? 1 : 0);
  }

c结构体

struct ASDisplayNodeFlags {
    // public properties
    unsigned viewEverHadAGestureRecognizerAttached:1;
    unsigned layerBacked:1;
    unsigned displaysAsynchronously:1;
    unsigned rasterizesSubtree:1;
    unsigned shouldBypassEnsureDisplay:1;
    unsigned displaySuspended:1;
    unsigned shouldAnimateSizeChanges:1;
    
    // Wrapped view handling
    
    // The layer contents should not be cleared in case the node is wrapping a UIImageView.UIImageView is specifically
    // optimized for performance and does not use the usual way to provide the contents of the CALayer via the
    // CALayerDelegate method that backs the UIImageView.
    unsigned canClearContentsOfLayer:1;
    
    // Prevent calling setNeedsDisplay on a layer that backs a UIImageView. Usually calling setNeedsDisplay on a CALayer
    // triggers a recreation of the contents of layer unfortunately calling it on a CALayer that backs a UIImageView
    // it goes through the normal flow to assign the contents to a layer via the CALayerDelegate methods. Unfortunately
    // UIImageView does not do recreate the layer contents the usual way, it actually does not implement some of the
    // methods at all instead it throws away the contents of the layer and nothing will show up.
    unsigned canCallSetNeedsDisplayOfLayer:1;

    unsigned implementsDrawRect:1;
    unsigned implementsImageDisplay:1;
    unsigned implementsDrawParameters:1;

    // internal state
    unsigned isEnteringHierarchy:1;
    unsigned isExitingHierarchy:1;
    unsigned isInHierarchy:1;
    unsigned visibilityNotificationsDisabled:VISIBILITY_NOTIFICATIONS_DISABLED_BITS;
    unsigned isDeallocating:1;
  } _flags;

检查指定类是否重写了另一类的实例方法

BOOL ASSubclassOverridesSelector(Class superclass, Class subclass, SEL selector)
{
  if (superclass == subclass) return NO; // Even if the class implements the selector, it doesn't override itself.
  Method superclassMethod = class_getInstanceMethod(superclass, selector);
  Method subclassMethod = class_getInstanceMethod(subclass, selector);
  return (superclassMethod != subclassMethod);
}

以block的实现替换c类中的origSEL

IMP ASReplaceMethodWithBlock(Class c, SEL origSEL, id block)
{
  NSCParameterAssert(block);
  
  // Get original method
  Method origMethod = class_getInstanceMethod(c, origSEL);
  NSCParameterAssert(origMethod);
  
  // Convert block to IMP trampoline and replace method implementation
  IMP newIMP = imp_implementationWithBlock(block);
  
  // Try adding the method if not yet in the current class
  if (!class_addMethod(c, origSEL, newIMP, method_getTypeEncoding(origMethod))) {
    return method_setImplementation(origMethod, newIMP);
  } else {
    return method_getImplementation(origMethod);
  }
}

没看懂

__block IMP originalLayoutSpecThatFitsIMP = ASReplaceMethodWithBlock(self, @selector(_locked_layoutElementThatFits:), ^(ASDisplayNode *_self, ASSizeRange sizeRange) {
          NSArray *oldSubnodes = _self.subnodes;
          ASLayoutSpec *layoutElement = ((ASLayoutSpec *( *)(id, SEL, ASSizeRange))originalLayoutSpecThatFitsIMP)(_self, @selector(_locked_layoutElementThatFits:), sizeRange);
          NSArray *subnodes = _self.subnodes;
          ASDisplayNodeAssert(oldSubnodes.count == subnodes.count, @"Adding or removing nodes in layoutSpecBlock or layoutSpecThatFits: is not allowed and can cause unexpected behavior.");
          for (NSInteger i = 0; i < oldSubnodes.count; i++) {
              ASDisplayNodeAssert(oldSubnodes[i] == subnodes[i], @"Adding or removing nodes in layoutSpecBlock or layoutSpecThatFits: is not allowed and can cause unexpected behavior.");
          }
          return layoutElement;
      });

setFlag(Synchronous, ![layerClass isSubclassOfClass:[_ASDisplayLayer class]]);

__unused是何用？

__unused Class initializeSelf = self;

强制子类重写方法

#define ASDisplayNodeAssert(condition, desc, ...) NSAssert(condition, desc, ##__VA_ARGS__)
- (void)_staticInitialize
{
  ASDisplayNodeAssert(NO, @"_staticInitialize must be overridden");
}

神奇的语法

ASPrimitiveTraitCollection ASPrimitiveTraitCollectionMakeDefault()
{
  return (ASPrimitiveTraitCollection) {
    // Default values can be defined in here
    .userInterfaceIdiom = UIUserInterfaceIdiomUnspecified,
    .containerSize = CGSizeZero,
  };
}

检查是否主线程

static inline BOOL ASDisplayNodeThreadIsMain()
{
  return 0 != pthread_main_np();
}

runloop相关

// Self is guaranteed to outlive the observer.  Without the high cost of a weak pointer,
    // __unsafe_unretained allows us to avoid flagging the memory cycle detector.
    __unsafe_unretained __typeof__(self) weakSelf = self;
    void (^handlerBlock) (CFRunLoopObserverRef observer, CFRunLoopActivity activity) = ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {
      [weakSelf processQueue];
    };
//添加observer
    _runLoopObserver = CFRunLoopObserverCreateWithHandler(NULL, kCFRunLoopBeforeWaiting, true, 0, handlerBlock);
    CFRunLoopAddObserver(_runLoop, _runLoopObserver,  kCFRunLoopCommonModes);
    
    // It is not guaranteed that the runloop will turn if it has no scheduled work, and this causes processing of
    // the queue to stop. Attaching a custom loop source to the run loop and signal it if new work needs to be done
    CFRunLoopSourceContext sourceContext = {};
    sourceContext.perform = runLoopSourceCallback;
#if ASRunLoopQueueLoggingEnabled
    sourceContext.info = (__bridge void *)self;
#endif
    _runLoopSource = CFRunLoopSourceCreate(NULL, 0, &sourceContext);//添加source
    CFRunLoopAddSource(runloop, _runLoopSource, kCFRunLoopCommonModes);

#if ASRunLoopQueueLoggingEnabled
    _runloopQueueLoggingTimer = [NSTimer timerWithTimeInterval:1.0 target:self selector:@selector(checkRunLoop) userInfo:nil repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:_runloopQueueLoggingTimer forMode:NSRunLoopCommonModes];
#endif

- (void)dealloc
{
  if (CFRunLoopContainsSource(_runLoop, _runLoopSource, kCFRunLoopCommonModes)) {
    CFRunLoopRemoveSource(_runLoop, _runLoopSource, kCFRunLoopCommonModes);
  }
  CFRelease(_runLoopSource);
  _runLoopSource = nil;
  
  if (CFRunLoopObserverIsValid(_runLoopObserver)) {
    CFRunLoopObserverInvalidate(_runLoopObserver);
  }
  CFRelease(_runLoopObserver);
  _runLoopObserver = nil;
}

//手动标记source为待执行并唤醒runloop
    CFRunLoopSourceSignal(_runLoopSource);
    CFRunLoopWakeUp(_runLoop);

通过CFRunLoopGetMain保障在主线程中释放object

extern void ASPerformMainThreadDeallocation(_Nullable id object)
{
  /**
   * UIKit components must be deallocated on the main thread. We use this shared
   * run loop queue to gradually deallocate them across many turns of the main run loop.
   */
  static ASRunLoopQueue *queue;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    queue = [[ASRunLoopQueue alloc] initWithRunLoop:CFRunLoopGetMain() retainObjects:YES handler:nil];
    queue.batchSize = 10;
  });
  if (object != nil) {
    [queue enqueue:object];
  }
}

判断指定类是否需要在主线程中释放

BOOL ASClassRequiresMainThreadDeallocation(Class c)
{
  if (c == [UIImage class] || c == [UIColor class]) {
    return NO;
  }
  
  if ([c isSubclassOfClass:[UIResponder class]]
      || [c isSubclassOfClass:[CALayer class]]
      || [c isSubclassOfClass:[UIGestureRecognizer class]]) {
    return YES;
  }

  const char *name = class_getName(c);
  if (strncmp(name, "UI", 2) == 0 || strncmp(name, "AV", 2) == 0 || strncmp(name, "CA", 2) == 0) {
    return YES;
  }

  return NO;
}

ASDN::MutexLocker l(instanceLock)的作用是加锁、解锁。其内部实现基本原理是在构造函数中对instanceLock进行了加锁操作，在析构函数中对instanceLock进行了解锁操作。

- (BOOL)shouldAnimateSizeChanges
{
  ASDN::MutexLocker l(__instanceLock__);
  return _flags.shouldAnimateSizeChanges;
}

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