一文理解Android系统中强指针的实现

class RefBase
{
public:
            //定义了成员变量用于维护强引用对象的引用计数
            void            incStrong(const void* id) const;
            //定义了成员变量用于维护强引用对象的引用计数
            void            decStrong(const void* id) const;
          
            void            forceIncStrong(const void* id) const;

            //获取强指针计数的数量.
            int32_t         getStrongCount() const;
    //这个类主要实现计数器的
    class weakref_type
    {
    public:
        RefBase*            refBase() const;

        void                incWeak(const void* id);
        void                decWeak(const void* id);

        // acquires a strong reference if there is already one.
        bool                attemptIncStrong(const void* id);

        // acquires a weak reference if there is already one.
        // This is not always safe. see ProcessState.cpp and BpBinder.cpp
        // for proper use.
        bool                attemptIncWeak(const void* id);

        //! DEBUGGING ONLY: Get current weak ref count.
        int32_t             getWeakCount() const;

        //! DEBUGGING ONLY: Print references held on object.
        void                printRefs() const;

        //! DEBUGGING ONLY: Enable tracking for this object.
        // enable -- enable/disable tracking
        // retain -- when tracking is enable, if true, then we save a stack trace
        //           for each reference and dereference; when retain == false, we
        //           match up references and dereferences and keep only the
        //           outstanding ones.

        void                trackMe(bool enable, bool retain);
    };

            weakref_type*   createWeak(const void* id) const;

            weakref_type*   getWeakRefs() const;

            //! DEBUGGING ONLY: Print references held on object.
    inline  void            printRefs() const { getWeakRefs()->printRefs(); }

            //! DEBUGGING ONLY: Enable tracking of object.
    inline  void            trackMe(bool enable, bool retain)
    {
        getWeakRefs()->trackMe(enable, retain);
    }

    typedef RefBase basetype;

protected:
                            RefBase();
    virtual                 ~RefBase();

    //! Flags for extendObjectLifetime()
    enum {
        OBJECT_LIFETIME_STRONG  = 0x0000,
        OBJECT_LIFETIME_WEAK    = 0x0001,
        OBJECT_LIFETIME_MASK    = 0x0001
    };

            void            extendObjectLifetime(int32_t mode);

    //! Flags for onIncStrongAttempted()
    enum {
        FIRST_INC_STRONG = 0x0001
    };

    virtual void            onFirstRef();
    virtual void            onLastStrongRef(const void* id);
    virtual bool            onIncStrongAttempted(uint32_t flags, const void* id);
    virtual void            onLastWeakRef(const void* id);

private:
    friend class ReferenceMover;
    static void moveReferences(void* d, void const* s, size_t n,
            const ReferenceConverterBase& caster);

private:
    friend class weakref_type;
    //通过类对象来获取计数器数据。
    class weakref_impl;

                            RefBase(const RefBase& o);
            RefBase&        operator=(const RefBase& o);

        weakref_impl* const mRefs;
};