Swift源码之引用计数管理一

• 标识Object-C或者Swift的内存管理标志类

  typedef unsigned long   uintptr_t;
  /// 标识引用计数管理
  class WeakReferenceBits {
    
    /// 标识
    enum : uintptr_t {
      /// ObCject-C class
        NativeMarkerMask  = 0
      /// Swift class
      NativeMarkerValue = 0
    }
    
    /// 存储的bits位
    uintptr_t bits;
    
    /// 获取 side-table
    HeapObjectSideTableEntry *getNativeOrNull() const {
      assert(isNativeOrNull());
      if (bits == 0)
        return nullptr;
      else
        return
          reinterpret_cast<HeapObjectSideTableEntry *>(bits & ~NativeMarkerMask);
    }
    
    /// 设置side-table
    void setNativeOrNull(HeapObjectSideTableEntry *newValue) {
      assert((uintptr_t(newValue) & NativeMarkerMask) == 0);
      if (newValue)
        bits = uintptr_t(newValue) | NativeMarkerValue;
      else
        bits = 0;
    }
  };
  

• WeakReference的负责管理WeakReferenceBits

  // Redeclare APIs from the Objective-C runtime.
  // These functions are not available through public headers, but are guaranteed
  // to exist on OS X >= 10.9 and iOS >= 7.0.
  
  OBJC_EXPORT id objc_retain(id);
  OBJC_EXPORT void objc_release(id);
  OBJC_EXPORT id _objc_rootAutorelease(id);
  OBJC_EXPORT void objc_moveWeak(id*, id*);
  OBJC_EXPORT void objc_copyWeak(id*, id*);
  OBJC_EXPORT id objc_initWeak(id*, id);
  OBJC_EXPORT void objc_destroyWeak(id*);
  OBJC_EXPORT id objc_loadWeakRetained(id*);
  
  /// 引用计数管理
  class WeakReference {
    union {
    ///  atomic 仅仅是为了保证线程安全加的锁,实际操作还是 WeakReferenceBits
      std::atomic<WeakReferenceBits> nativeValue;
      
    /// 如果是OBJC类型
    #if SWIFT_OBJC_INTEROP
      id nonnativeValue;
    #endif
    };
    
     /// 区分OBJC和Swift的class
    void initWithNativeness(void *object, bool isNative) {
      if (isNative)
        /// swift-class
        nativeInit(static_cast<HeapObject *>(object));
      else
        /// Objc-class
        nonnativeInit(static_cast<id>(object));
    }
    
     void unknownInit(void *object) {
     /// 判断是否是Objc类型
      if (isObjCTaggedPointerOrNull(object)) {
        nonnativeValue = static_cast<id>(object);
      } else {
        /// 判断是否使用Swift原生引用计数管理对象
        bool isNative = objectUsesNativeSwiftReferenceCounting(object);
        initWithNativeness(object, isNative);
      }
    }
    
    /// OBJC-Class类型
    void nonnativeInit(id object) {
      /// OC的initWeak方法被触发
      objc_initWeak(&nonnativeValue, object);
    }
    
    /// Swift类的引用计数
    void nativeInit(HeapObject *object) {
      /// 判断是否存在side-Table
      auto side = object ? object->refCounts.formWeakReference() : nullptr;
      nativeValue.store(WeakReferenceBits(side), std::memory_order_relaxed);
    }
    
    /// 释放机制
    void destroyWithNativeness(bool isNative) {
      if (isNative)
        nativeDestroy();
      else
        nonnativeDestroy();
    }
    
    /// 原生Swift-class释放
    void nativeDestroy() {
      auto oldBits = nativeValue.load(std::memory_order_relaxed);
      nativeValue.store(nullptr, std::memory_order_relaxed);
      destroyOldNativeBits(oldBits);
    }
    
    /// 释放Swift-class
    void destroyOldNativeBits(WeakReferenceBits oldBits) {
      auto oldSide = oldBits.getNativeOrNull();
      if (oldSide)
        
        /// 这里下一篇文章讲 decrementWeak
        oldSide->decrementWeak();
    }
    
    /// OBJC-class释放
    void nonnativeDestroy() {
      objc_destroyWeak(&nonnativeValue);
    }
  }