Android指针管理：RefBase,SP,WP （二）

（1）在Android中，RefBase结合了sp和wp，实现了一套通过引用计数的方法来控制对象声明周期的方法。

RefBase的定义在/frameworks/base/include/utils/RefBase.h，实现在/frameworks/base/libs/utils/RefBase.cpp。
wp的定义在/frameworks/base/include/utils/RefBase.h，
sp的定义在/frameworks/base/include/utils/StrongPointer.h中。

（2）weakref_impl是weakref_type 的子类

android_atomic_dec(&mCount) == 1  mCount减1，但是返回的是mCount减1之前的值。如果返回1，表示这次减过之后引用计数就是0了，就把对象delete掉。 

android_atomic_inc(&impl->mStrong)  mCount加1，但是返回的是mCount加1之前的值。

android_atomic_add(-INITIAL_STRONG_VALUE, &impl->mStrong);  加一个数，在此加的是（-INITIAL_STRONG_VALUE）

android_atomic_cmpxchg(curCount, curCount+1, &impl->mStrong) 表示如果impl->mStrong的值为curCount，则把impl->mString的值改为curCount+1

（3）对象声明周期的控制
enum {
    OBJECT_LIFETIME_STRONG  = 0x0000,
    OBJECT_LIFETIME_WEAK    = 0x0001,
    OBJECT_LIFETIME_MASK    = 0x0001
};

void    extendObjectLifetime(int32_t mode);

RefBase中，声明了一个枚举和extendObjectLifetime函数，来控制对象的生命周期。
void RefBase::extendObjectLifetime(int32_t mode)
{
    android_atomic_or(mode, &mRefs->mFlags);  // 用mode给weakref_impl的mFlags赋值     mFlags只在这一个地方赋值
}

（4）

incStrong中， 将强引用数与弱引用数同时 +1

decStrong中，将强引用数与弱引用数同时 -1

incWeak中，只有弱引用数 +1

decWeak中，只有弱引用数 -1

（5）

weakref_impl.mFlag == OBJECT_LIFETIME_STRONG时:
    强引用计数来控制对象的生命周期，弱引用对象控制weakref_impl的生命周期。
    强引用为0，对象被delete；弱引用为0时，weakref_impl被delete。
    记住：使用wp时，要有sp生成，否则可能会引起segment fault。
weakref_impl.mFlag == OBJECT_LIFETIME_WEAK时:
    由弱引用来控制对象和weakref_impl的生命周期。
    强引用为0无作用，弱引用为0时，对象和weakref_impl被同时delete。

 （6）

void RefBase::incStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->incWeak(id);
    
    refs->addStrongRef(id);
    //refs->mStrong的值+1，但是返回的c是+1之前的值   
    const int32_t c = android_atomic_inc(&refs->mStrong);　　　　
    ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFS
    ALOGD("incStrong of %p from %p: cnt=%d
", this, id, c);
#endif
    if (c != INITIAL_STRONG_VALUE)  {    //+1之前不是INITAL_STRONG_VALUE，返回
        return;
    }

    //refs->mStrong=  INITIAL_STRONG_VALUE + 1 - INITIAL_STRONG_VALUE = 1 最终为1
    android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
    refs->mBase->onFirstRef();
}

第一次创建强引用会回调RefBase的onFirstRef()方法，这个方法很重要，派生类可以重载次方法，做一些初始化操作。在audio system中，很多类重载此方法！！！

RefBase.h 源文件

#ifndef ANDROID_REF_BASE_H
#define ANDROID_REF_BASE_H

#include <cutils/atomic.h>

#include <stdint.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>

#include <utils/StrongPointer.h>
#include <utils/TypeHelpers.h>

// ---------------------------------------------------------------------------
namespace android {

class TextOutput;
TextOutput& printWeakPointer(TextOutput& to, const void* val);

// ---------------------------------------------------------------------------

#define COMPARE_WEAK(_op_)                                      
inline bool operator _op_ (const sp<T>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}                                                               
inline bool operator _op_ (const T* o) const {                  
    return m_ptr _op_ o;                                        
}                                                               
template<typename U>                                            
inline bool operator _op_ (const sp<U>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}                                                               
template<typename U>                                            
inline bool operator _op_ (const U* o) const {                  
    return m_ptr _op_ o;                                        
}

// ---------------------------------------------------------------------------

class ReferenceRenamer {
protected:
    // destructor is purposedly not virtual so we avoid code overhead from
    // subclasses; we have to make it protected to guarantee that it
    // cannot be called from this base class (and to make strict compilers
    // happy).
    ~ReferenceRenamer() { }
public:
    virtual void operator()(size_t i) const = 0;
};

// ---------------------------------------------------------------------------

class RefBase
{
public:
            void            incStrong(const void* id) const;
            void            decStrong(const void* id) const;
    
            void            forceIncStrong(const void* id) const;

            //! DEBUGGING ONLY: Get current strong ref count.
            int32_t         getStrongCount() const;

    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 weakref_type;
    class weakref_impl;
    
                            RefBase(const RefBase& o);
            RefBase&        operator=(const RefBase& o);

private:
    friend class ReferenceMover;

    static void renameRefs(size_t n, const ReferenceRenamer& renamer);

    static void renameRefId(weakref_type* ref,
            const void* old_id, const void* new_id);

    static void renameRefId(RefBase* ref,
            const void* old_id, const void* new_id);

        weakref_impl* const mRefs;
};

// ---------------------------------------------------------------------------

template <class T>
class LightRefBase
{
public:
    inline LightRefBase() : mCount(0) { }
    inline void incStrong(__attribute__((unused)) const void* id) const {
        android_atomic_inc(&mCount);
    }
    inline void decStrong(__attribute__((unused)) const void* id) const {
        if (android_atomic_dec(&mCount) == 1) {
            delete static_cast<const T*>(this);
        }
    }
    //! DEBUGGING ONLY: Get current strong ref count.
    inline int32_t getStrongCount() const {
        return mCount;
    }

    typedef LightRefBase<T> basetype;

protected:
    inline ~LightRefBase() { }

private:
    friend class ReferenceMover;
    inline static void renameRefs(size_t n, const ReferenceRenamer& renamer) { }
    inline static void renameRefId(T* ref,
            const void* old_id, const void* new_id) { }

private:
    mutable volatile int32_t mCount;
};

// ---------------------------------------------------------------------------

template <typename T>
class wp
{
public:
    typedef typename RefBase::weakref_type weakref_type;
    
    inline wp() : m_ptr(0) { }

    wp(T* other);
    wp(const wp<T>& other);
    wp(const sp<T>& other);
    template<typename U> wp(U* other);
    template<typename U> wp(const sp<U>& other);
    template<typename U> wp(const wp<U>& other);

    ~wp();
    
    // Assignment

    wp& operator = (T* other);
    wp& operator = (const wp<T>& other);
    wp& operator = (const sp<T>& other);
    
    template<typename U> wp& operator = (U* other);
    template<typename U> wp& operator = (const wp<U>& other);
    template<typename U> wp& operator = (const sp<U>& other);
    
    void set_object_and_refs(T* other, weakref_type* refs);

    // promotion to sp
    
    sp<T> promote() const;

    // Reset
    
    void clear();

    // Accessors
    
    inline  weakref_type* get_refs() const { return m_refs; }
    
    inline  T* unsafe_get() const { return m_ptr; }

    // Operators

    COMPARE_WEAK(==)
    COMPARE_WEAK(!=)
    COMPARE_WEAK(>)
    COMPARE_WEAK(<)
    COMPARE_WEAK(<=)
    COMPARE_WEAK(>=)

    inline bool operator == (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) && (m_refs == o.m_refs);
    }
    template<typename U>
    inline bool operator == (const wp<U>& o) const {
        return m_ptr == o.m_ptr;
    }

    inline bool operator > (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);
    }
    template<typename U>
    inline bool operator > (const wp<U>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);
    }

    inline bool operator < (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);
    }
    template<typename U>
    inline bool operator < (const wp<U>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);
    }
                         inline bool operator != (const wp<T>& o) const { return m_refs != o.m_refs; }
    template<typename U> inline bool operator != (const wp<U>& o) const { return !operator == (o); }
                         inline bool operator <= (const wp<T>& o) const { return !operator > (o); }
    template<typename U> inline bool operator <= (const wp<U>& o) const { return !operator > (o); }
                         inline bool operator >= (const wp<T>& o) const { return !operator < (o); }
    template<typename U> inline bool operator >= (const wp<U>& o) const { return !operator < (o); }

private:
    template<typename Y> friend class sp;
    template<typename Y> friend class wp;

    T*              m_ptr;
    weakref_type*   m_refs;
};

template <typename T>
TextOutput& operator<<(TextOutput& to, const wp<T>& val);

#undef COMPARE_WEAK

// ---------------------------------------------------------------------------
// No user serviceable parts below here.

template<typename T>
wp<T>::wp(T* other)
    : m_ptr(other)
{
    if (other) m_refs = other->createWeak(this);
}

template<typename T>
wp<T>::wp(const wp<T>& other)
    : m_ptr(other.m_ptr), m_refs(other.m_refs)
{
    if (m_ptr) m_refs->incWeak(this);
}

template<typename T>
wp<T>::wp(const sp<T>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = m_ptr->createWeak(this);
    }
}

template<typename T> template<typename U>
wp<T>::wp(U* other)
    : m_ptr(other)
{
    if (other) m_refs = other->createWeak(this);
}

template<typename T> template<typename U>
wp<T>::wp(const wp<U>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = other.m_refs;
        m_refs->incWeak(this);
    }
}

template<typename T> template<typename U>
wp<T>::wp(const sp<U>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = m_ptr->createWeak(this);
    }
}

template<typename T>
wp<T>::~wp()
{
    if (m_ptr) m_refs->decWeak(this);
}

template<typename T>
wp<T>& wp<T>::operator = (T* other)
{
    weakref_type* newRefs =
        other ? other->createWeak(this) : 0;
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = newRefs;
    return *this;
}

template<typename T>
wp<T>& wp<T>::operator = (const wp<T>& other)
{
    weakref_type* otherRefs(other.m_refs);
    T* otherPtr(other.m_ptr);
    if (otherPtr) otherRefs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = otherRefs;
    return *this;
}

template<typename T>
wp<T>& wp<T>::operator = (const sp<T>& other)
{
    weakref_type* newRefs =
        other != NULL ? other->createWeak(this) : 0;
    T* otherPtr(other.m_ptr);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = newRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (U* other)
{
    weakref_type* newRefs =
        other ? other->createWeak(this) : 0;
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = newRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const wp<U>& other)
{
    weakref_type* otherRefs(other.m_refs);
    U* otherPtr(other.m_ptr);
    if (otherPtr) otherRefs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = otherRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const sp<U>& other)
{
    weakref_type* newRefs =
        other != NULL ? other->createWeak(this) : 0;
    U* otherPtr(other.m_ptr);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = newRefs;
    return *this;
}

template<typename T>
void wp<T>::set_object_and_refs(T* other, weakref_type* refs)
{
    if (other) refs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = refs;
}

template<typename T>
sp<T> wp<T>::promote() const
{
    sp<T> result;
    if (m_ptr && m_refs->attemptIncStrong(&result)) {
        result.set_pointer(m_ptr);
    }
    return result;
}

template<typename T>
void wp<T>::clear()
{
    if (m_ptr) {
        m_refs->decWeak(this);
        m_ptr = 0;
    }
}

template <typename T>
inline TextOutput& operator<<(TextOutput& to, const wp<T>& val)
{
    return printWeakPointer(to, val.unsafe_get());
}

// ---------------------------------------------------------------------------

// this class just serves as a namespace so TYPE::moveReferences can stay
// private.
class ReferenceMover {
public:
    // it would be nice if we could make sure no extra code is generated
    // for sp<TYPE> or wp<TYPE> when TYPE is a descendant of RefBase:
    // Using a sp<RefBase> override doesn't work; it's a bit like we wanted
    // a template<typename TYPE inherits RefBase> template...

    template<typename TYPE> static inline
    void move_references(sp<TYPE>* d, sp<TYPE> const* s, size_t n) {

        class Renamer : public ReferenceRenamer {
            sp<TYPE>* d;
            sp<TYPE> const* s;
            virtual void operator()(size_t i) const {
                // The id are known to be the sp<>'s this pointer
                TYPE::renameRefId(d[i].get(), &s[i], &d[i]);
            }
        public:
            Renamer(sp<TYPE>* d, sp<TYPE> const* s) : s(s), d(d) { }
        };

        memmove(d, s, n*sizeof(sp<TYPE>));
        TYPE::renameRefs(n, Renamer(d, s));
    }


    template<typename TYPE> static inline
    void move_references(wp<TYPE>* d, wp<TYPE> const* s, size_t n) {

        class Renamer : public ReferenceRenamer {
            wp<TYPE>* d;
            wp<TYPE> const* s;
            virtual void operator()(size_t i) const {
                // The id are known to be the wp<>'s this pointer
                TYPE::renameRefId(d[i].get_refs(), &s[i], &d[i]);
            }
        public:
            Renamer(wp<TYPE>* d, wp<TYPE> const* s) : s(s), d(d) { }
        };

        memmove(d, s, n*sizeof(wp<TYPE>));
        TYPE::renameRefs(n, Renamer(d, s));
    }
};

// specialization for moving sp<> and wp<> types.
// these are used by the [Sorted|Keyed]Vector<> implementations
// sp<> and wp<> need to be handled specially, because they do not
// have trivial copy operation in the general case (see RefBase.cpp
// when DEBUG ops are enabled), but can be implemented very
// efficiently in most cases.

template<typename TYPE> inline
void move_forward_type(sp<TYPE>* d, sp<TYPE> const* s, size_t n) {
    ReferenceMover::move_references(d, s, n);
}

template<typename TYPE> inline
void move_backward_type(sp<TYPE>* d, sp<TYPE> const* s, size_t n) {
    ReferenceMover::move_references(d, s, n);
}

template<typename TYPE> inline
void move_forward_type(wp<TYPE>* d, wp<TYPE> const* s, size_t n) {
    ReferenceMover::move_references(d, s, n);
}

template<typename TYPE> inline
void move_backward_type(wp<TYPE>* d, wp<TYPE> const* s, size_t n) {
    ReferenceMover::move_references(d, s, n);
}


}; // namespace android

// ---------------------------------------------------------------------------

#endif // ANDROID_REF_BASE_H

去掉Debug和其他信息，简化之后的RefBase.h：

类RefBase：

class RefBase
{
public:
            void            incStrong(const void* id) const;
            void            decStrong(const void* id) const;
    
            void            forceIncStrong(const void* id) const;

            //! DEBUGGING ONLY: Get current strong ref count.
            int32_t         getStrongCount() const;
    
            weakref_type*   createWeak(const void* id) const;
            
            weakref_type*   getWeakRefs() const;

    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 weakref_type;
    class weakref_impl;
    
                            RefBase(const RefBase& o);
            RefBase&        operator=(const RefBase& o);

};

类LightRefBase：

template <class T>
class LightRefBase
{
public:
    inline LightRefBase() : mCount(0) { }
    inline void incStrong(__attribute__((unused)) const void* id) const {
        android_atomic_inc(&mCount);
    }
    inline void decStrong(__attribute__((unused)) const void* id) const {
        if (android_atomic_dec(&mCount) == 1) {
            delete static_cast<const T*>(this);
        }
    }
    //! DEBUGGING ONLY: Get current strong ref count.
    inline int32_t getStrongCount() const {
        return mCount;
    }

    typedef LightRefBase<T> basetype;

protected:
    inline ~LightRefBase() { }

private:
    friend class ReferenceMover;
    inline static void renameRefs(size_t n, const ReferenceRenamer& renamer) { }
    inline static void renameRefId(T* ref,
            const void* old_id, const void* new_id) { }

private:
    mutable volatile int32_t mCount;
};

类wp：

template <typename T>
class wp
{
public:
    typedef typename RefBase::weakref_type weakref_type;
    
    inline wp() : m_ptr(0) { }

    wp(T* other);
    wp(const wp<T>& other);
    wp(const sp<T>& other);
    template<typename U> wp(U* other);
    template<typename U> wp(const sp<U>& other);
    template<typename U> wp(const wp<U>& other);

    ~wp();
    
    // Assignment

    wp& operator = (T* other);
    wp& operator = (const wp<T>& other);
    wp& operator = (const sp<T>& other);
    
    template<typename U> wp& operator = (U* other);
    template<typename U> wp& operator = (const wp<U>& other);
    template<typename U> wp& operator = (const sp<U>& other);
    
    void set_object_and_refs(T* other, weakref_type* refs);

    // promotion to sp
    
    sp<T> promote() const;

    // Reset
    
    void clear();

    // Accessors
    
    inline  weakref_type* get_refs() const { return m_refs; }
    
    inline  T* unsafe_get() const { return m_ptr; }

    // Operators

    COMPARE_WEAK(==)
    COMPARE_WEAK(!=)
    COMPARE_WEAK(>)
    COMPARE_WEAK(<)
    COMPARE_WEAK(<=)
    COMPARE_WEAK(>=)

    inline bool operator == (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) && (m_refs == o.m_refs);
    }
    template<typename U>
    inline bool operator == (const wp<U>& o) const {
        return m_ptr == o.m_ptr;
    }

    inline bool operator > (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);
    }
    template<typename U>
    inline bool operator > (const wp<U>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs > o.m_refs) : (m_ptr > o.m_ptr);
    }

    inline bool operator < (const wp<T>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);
    }
    template<typename U>
    inline bool operator < (const wp<U>& o) const {
        return (m_ptr == o.m_ptr) ? (m_refs < o.m_refs) : (m_ptr < o.m_ptr);
    }
                         inline bool operator != (const wp<T>& o) const { return m_refs != o.m_refs; }
    template<typename U> inline bool operator != (const wp<U>& o) const { return !operator == (o); }
                         inline bool operator <= (const wp<T>& o) const { return !operator > (o); }
    template<typename U> inline bool operator <= (const wp<U>& o) const { return !operator > (o); }
                         inline bool operator >= (const wp<T>& o) const { return !operator < (o); }
    template<typename U> inline bool operator >= (const wp<U>& o) const { return !operator < (o); }

private:
    template<typename Y> friend class sp;
    template<typename Y> friend class wp;

    T*              m_ptr;
    weakref_type*   m_refs;
};

类wp函数实现：

template<typename T>
wp<T>::wp(T* other)
    : m_ptr(other)
{
    if (other) m_refs = other->createWeak(this);
}

template<typename T>
wp<T>::wp(const wp<T>& other)
    : m_ptr(other.m_ptr), m_refs(other.m_refs)
{
    if (m_ptr) m_refs->incWeak(this);
}

template<typename T>
wp<T>::wp(const sp<T>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = m_ptr->createWeak(this);
    }
}

template<typename T> template<typename U>
wp<T>::wp(U* other)
    : m_ptr(other)
{
    if (other) m_refs = other->createWeak(this);
}

template<typename T> template<typename U>
wp<T>::wp(const wp<U>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = other.m_refs;
        m_refs->incWeak(this);
    }
}

template<typename T> template<typename U>
wp<T>::wp(const sp<U>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = m_ptr->createWeak(this);
    }
}

template<typename T>
wp<T>::~wp()
{
    if (m_ptr) m_refs->decWeak(this);
}

template<typename T>
wp<T>& wp<T>::operator = (T* other)
{
    weakref_type* newRefs =
        other ? other->createWeak(this) : 0;
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = newRefs;
    return *this;
}

template<typename T>
wp<T>& wp<T>::operator = (const wp<T>& other)
{
    weakref_type* otherRefs(other.m_refs);
    T* otherPtr(other.m_ptr);
    if (otherPtr) otherRefs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = otherRefs;
    return *this;
}

template<typename T>
wp<T>& wp<T>::operator = (const sp<T>& other)
{
    weakref_type* newRefs =
        other != NULL ? other->createWeak(this) : 0;
    T* otherPtr(other.m_ptr);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = newRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (U* other)
{
    weakref_type* newRefs =
        other ? other->createWeak(this) : 0;
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = newRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const wp<U>& other)
{
    weakref_type* otherRefs(other.m_refs);
    U* otherPtr(other.m_ptr);
    if (otherPtr) otherRefs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = otherRefs;
    return *this;
}

template<typename T> template<typename U>
wp<T>& wp<T>::operator = (const sp<U>& other)
{
    weakref_type* newRefs =
        other != NULL ? other->createWeak(this) : 0;
    U* otherPtr(other.m_ptr);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = otherPtr;
    m_refs = newRefs;
    return *this;
}

template<typename T>
void wp<T>::set_object_and_refs(T* other, weakref_type* refs)
{
    if (other) refs->incWeak(this);
    if (m_ptr) m_refs->decWeak(this);
    m_ptr = other;
    m_refs = refs;
}

template<typename T>
sp<T> wp<T>::promote() const
{
    sp<T> result;
    if (m_ptr && m_refs->attemptIncStrong(&result)) {
        result.set_pointer(m_ptr);
    }
    return result;
}

template<typename T>
void wp<T>::clear()
{
    if (m_ptr) {
        m_refs->decWeak(this);
        m_ptr = 0;
    }
}

RefBase.cpp 源文件

简化之后的RefBase.cpp：

继承自weakref_type的类weakref_impl：

class RefBase::weakref_impl : public RefBase::weakref_type
{
public:
    volatile int32_t    mStrong;
    volatile int32_t    mWeak;
    RefBase* const      mBase;
    volatile int32_t    mFlags;

    weakref_impl(RefBase* base)
        : mStrong(INITIAL_STRONG_VALUE)
        , mWeak(0)
        , mBase(base)
        , mFlags(0)
    {
    }

    void addStrongRef(const void* /*id*/) { }
    void removeStrongRef(const void* /*id*/) { }
    void renameStrongRefId(const void* /*old_id*/, const void* /*new_id*/) { }
    void addWeakRef(const void* /*id*/) { }
    void removeWeakRef(const void* /*id*/) { }
    void renameWeakRefId(const void* /*old_id*/, const void* /*new_id*/) { }
    void printRefs() const { }
    void trackMe(bool, bool) { }

};

RefBase类的函数实现：

#define INITIAL_STRONG_VALUE (1<<28)


void RefBase::incStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->incWeak(id);
    
    refs->addStrongRef(id);
    const int32_t c = android_atomic_inc(&refs->mStrong);
    ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFS
    ALOGD("incStrong of %p from %p: cnt=%d
", this, id, c);
#endif
    if (c != INITIAL_STRONG_VALUE)  {
        return;
    }

    android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
    refs->mBase->onFirstRef();
}

void RefBase::decStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->removeStrongRef(id);
    const int32_t c = android_atomic_dec(&refs->mStrong);
#if PRINT_REFS
    ALOGD("decStrong of %p from %p: cnt=%d
", this, id, c);
#endif
    ALOG_ASSERT(c >= 1, "decStrong() called on %p too many times", refs);
    if (c == 1) {
        refs->mBase->onLastStrongRef(id);
        if ((refs->mFlags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
            delete this;
        }
    }
    refs->decWeak(id);
}

void RefBase::forceIncStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->incWeak(id);
    
    refs->addStrongRef(id);
    const int32_t c = android_atomic_inc(&refs->mStrong);
    ALOG_ASSERT(c >= 0, "forceIncStrong called on %p after ref count underflow",
               refs);
#if PRINT_REFS
    ALOGD("forceIncStrong of %p from %p: cnt=%d
", this, id, c);
#endif

    switch (c) {
    case INITIAL_STRONG_VALUE:
        android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
        // fall through...
    case 0:
        refs->mBase->onFirstRef();
    }
}

int32_t RefBase::getStrongCount() const
{
    return mRefs->mStrong;
}

RefBase::weakref_type* RefBase::createWeak(const void* id) const
{
    mRefs->incWeak(id);
    return mRefs;
}

RefBase::weakref_type* RefBase::getWeakRefs() const
{
    return mRefs;
}

RefBase::RefBase()
    : mRefs(new weakref_impl(this))
{
}

RefBase::~RefBase()
{
    if (mRefs->mStrong == INITIAL_STRONG_VALUE) {
        // we never acquired a strong (and/or weak) reference on this object.
        delete mRefs;
    } else {
        // life-time of this object is extended to WEAK or FOREVER, in
        // which case weakref_impl doesn't out-live the object and we
        // can free it now.
        if ((mRefs->mFlags & OBJECT_LIFETIME_MASK) != OBJECT_LIFETIME_STRONG) {
            // It's possible that the weak count is not 0 if the object
            // re-acquired a weak reference in its destructor
            if (mRefs->mWeak == 0) {
                delete mRefs;
            }
        }
    }
    // for debugging purposes, clear this.
    const_cast<weakref_impl*&>(mRefs) = NULL;
}

void RefBase::extendObjectLifetime(int32_t mode)
{
    android_atomic_or(mode, &mRefs->mFlags);
}

void RefBase::onFirstRef()
{
}

void RefBase::onLastStrongRef(const void* /*id*/)
{
}

bool RefBase::onIncStrongAttempted(uint32_t flags, const void* id)
{
    return (flags&FIRST_INC_STRONG) ? true : false;
}

void RefBase::onLastWeakRef(const void* /*id*/)
{
}

  RefBase::weakref_type 类的函数实现：

RefBase* RefBase::weakref_type::refBase() const
{
    return static_cast<const weakref_impl*>(this)->mBase;
}

void RefBase::weakref_type::incWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    impl->addWeakRef(id);
    const int32_t c = android_atomic_inc(&impl->mWeak);
    ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}


void RefBase::weakref_type::decWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    impl->removeWeakRef(id);
    const int32_t c = android_atomic_dec(&impl->mWeak);
    ALOG_ASSERT(c >= 1, "decWeak called on %p too many times", this);
    if (c != 1) return;

    if ((impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_STRONG) {
        // This is the regular lifetime case. The object is destroyed
        // when the last strong reference goes away. Since weakref_impl
        // outlive the object, it is not destroyed in the dtor, and
        // we'll have to do it here.
        if (impl->mStrong == INITIAL_STRONG_VALUE) {
            // Special case: we never had a strong reference, so we need to
            // destroy the object now.
            delete impl->mBase;
        } else {
            // ALOGV("Freeing refs %p of old RefBase %p
", this, impl->mBase);
            delete impl;
        }
    } else {
        // less common case: lifetime is OBJECT_LIFETIME_{WEAK|FOREVER}
        impl->mBase->onLastWeakRef(id);
        if ((impl->mFlags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_WEAK) {
            // this is the OBJECT_LIFETIME_WEAK case. The last weak-reference
            // is gone, we can destroy the object.
            delete impl->mBase;
        }
    }
}

bool RefBase::weakref_type::attemptIncStrong(const void* id)
{
    incWeak(id);
    
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    int32_t curCount = impl->mStrong;

    ALOG_ASSERT(curCount >= 0,
            "attemptIncStrong called on %p after underflow", this);

    while (curCount > 0 && curCount != INITIAL_STRONG_VALUE) {
        // we're in the easy/common case of promoting a weak-reference
        // from an existing strong reference.
        if (android_atomic_cmpxchg(curCount, curCount+1, &impl->mStrong) == 0) {
            break;
        }
        // the strong count has changed on us, we need to re-assert our
        // situation.
        curCount = impl->mStrong;
    }
    
    if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {
        // we're now in the harder case of either:
        // - there never was a strong reference on us
        // - or, all strong references have been released
        if ((impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_STRONG) {
            // this object has a "normal" life-time, i.e.: it gets destroyed
            // when the last strong reference goes away
            if (curCount <= 0) {
                // the last strong-reference got released, the object cannot
                // be revived.
                decWeak(id);
                return false;
            }

            // here, curCount == INITIAL_STRONG_VALUE, which means
            // there never was a strong-reference, so we can try to
            // promote this object; we need to do that atomically.
            while (curCount > 0) {
                if (android_atomic_cmpxchg(curCount, curCount + 1,
                        &impl->mStrong) == 0) {
                    break;
                }
                // the strong count has changed on us, we need to re-assert our
                // situation (e.g.: another thread has inc/decStrong'ed us)
                curCount = impl->mStrong;
            }

            if (curCount <= 0) {
                // promote() failed, some other thread destroyed us in the
                // meantime (i.e.: strong count reached zero).
                decWeak(id);
                return false;
            }
        } else {
            // this object has an "extended" life-time, i.e.: it can be
            // revived from a weak-reference only.
            // Ask the object's implementation if it agrees to be revived
            if (!impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id)) {
                // it didn't so give-up.
                decWeak(id);
                return false;
            }
            // grab a strong-reference, which is always safe due to the
            // extended life-time.
            curCount = android_atomic_inc(&impl->mStrong);
        }

        // If the strong reference count has already been incremented by
        // someone else, the implementor of onIncStrongAttempted() is holding
        // an unneeded reference.  So call onLastStrongRef() here to remove it.
        // (No, this is not pretty.)  Note that we MUST NOT do this if we
        // are in fact acquiring the first reference.
        if (curCount > 0 && curCount < INITIAL_STRONG_VALUE) {
            impl->mBase->onLastStrongRef(id);
        }
    }
    
    impl->addStrongRef(id);

#if PRINT_REFS
    ALOGD("attemptIncStrong of %p from %p: cnt=%d
", this, id, curCount);
#endif

    // now we need to fix-up the count if it was INITIAL_STRONG_VALUE
    // this must be done safely, i.e.: handle the case where several threads
    // were here in attemptIncStrong().
    curCount = impl->mStrong;
    while (curCount >= INITIAL_STRONG_VALUE) {
        ALOG_ASSERT(curCount > INITIAL_STRONG_VALUE,
                "attemptIncStrong in %p underflowed to INITIAL_STRONG_VALUE",
                this);
        if (android_atomic_cmpxchg(curCount, curCount-INITIAL_STRONG_VALUE,
                &impl->mStrong) == 0) {
            break;
        }
        // the strong-count changed on us, we need to re-assert the situation,
        // for e.g.: it's possible the fix-up happened in another thread.
        curCount = impl->mStrong;
    }

    return true;
}

bool RefBase::weakref_type::attemptIncWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);

    int32_t curCount = impl->mWeak;
    ALOG_ASSERT(curCount >= 0, "attemptIncWeak called on %p after underflow",
               this);
    while (curCount > 0) {
        if (android_atomic_cmpxchg(curCount, curCount+1, &impl->mWeak) == 0) {
            break;
        }
        curCount = impl->mWeak;
    }

    if (curCount > 0) {
        impl->addWeakRef(id);
    }

    return curCount > 0;
}

int32_t RefBase::weakref_type::getWeakCount() const
{
    return static_cast<const weakref_impl*>(this)->mWeak;
}

void RefBase::weakref_type::printRefs() const
{
    static_cast<const weakref_impl*>(this)->printRefs();
}

void RefBase::weakref_type::trackMe(bool enable, bool retain)
{
    static_cast<weakref_impl*>(this)->trackMe(enable, retain);
}

sp类声明 （StrongPointer.h）

/*
 * Copyright (C) 2005 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef ANDROID_STRONG_POINTER_H
#define ANDROID_STRONG_POINTER_H

#include <cutils/atomic.h>

#include <stdint.h>
#include <sys/types.h>
#include <stdlib.h>

// ---------------------------------------------------------------------------
namespace android {

template<typename T> class wp;

// ---------------------------------------------------------------------------

#define COMPARE(_op_)                                           
inline bool operator _op_ (const sp<T>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}                                                               
inline bool operator _op_ (const T* o) const {                  
    return m_ptr _op_ o;                                        
}                                                               
template<typename U>                                            
inline bool operator _op_ (const sp<U>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}                                                               
template<typename U>                                            
inline bool operator _op_ (const U* o) const {                  
    return m_ptr _op_ o;                                        
}                                                               
inline bool operator _op_ (const wp<T>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}                                                               
template<typename U>                                            
inline bool operator _op_ (const wp<U>& o) const {              
    return m_ptr _op_ o.m_ptr;                                  
}

// ---------------------------------------------------------------------------

template<typename T>
class sp {
public:
    inline sp() : m_ptr(0) { }

    sp(T* other);
    sp(const sp<T>& other);
    template<typename U> sp(U* other);
    template<typename U> sp(const sp<U>& other);

    ~sp();

    // Assignment

    sp& operator = (T* other);
    sp& operator = (const sp<T>& other);

    template<typename U> sp& operator = (const sp<U>& other);
    template<typename U> sp& operator = (U* other);

    //! Special optimization for use by ProcessState (and nobody else).
    void force_set(T* other);

    // Reset

    void clear();

    // Accessors

    inline  T&      operator* () const  { return *m_ptr; }
    inline  T*      operator-> () const { return m_ptr;  }
    inline  T*      get() const         { return m_ptr; }

    // Operators

    COMPARE(==)
    COMPARE(!=)
    COMPARE(>)
    COMPARE(<)
    COMPARE(<=)
    COMPARE(>=)

private:    
    template<typename Y> friend class sp;
    template<typename Y> friend class wp;
    void set_pointer(T* ptr);
    T* m_ptr;
};

#undef COMPARE

// ---------------------------------------------------------------------------
// No user serviceable parts below here.

template<typename T>
sp<T>::sp(T* other)
        : m_ptr(other) {
    if (other)
        other->incStrong(this);
}

template<typename T>
sp<T>::sp(const sp<T>& other)
        : m_ptr(other.m_ptr) {
    if (m_ptr)
        m_ptr->incStrong(this);
}

template<typename T> template<typename U>
sp<T>::sp(U* other)
        : m_ptr(other) {
    if (other)
        ((T*) other)->incStrong(this);
}

template<typename T> template<typename U>
sp<T>::sp(const sp<U>& other)
        : m_ptr(other.m_ptr) {
    if (m_ptr)
        m_ptr->incStrong(this);
}

template<typename T>
sp<T>::~sp() {
    if (m_ptr)
        m_ptr->decStrong(this);
}

template<typename T>
sp<T>& sp<T>::operator =(const sp<T>& other) {
    T* otherPtr(other.m_ptr);
    if (otherPtr)
        otherPtr->incStrong(this);
    if (m_ptr)
        m_ptr->decStrong(this);
    m_ptr = otherPtr;
    return *this;
}

template<typename T>
sp<T>& sp<T>::operator =(T* other) {
    if (other)
        other->incStrong(this);
    if (m_ptr)
        m_ptr->decStrong(this);
    m_ptr = other;
    return *this;
}

template<typename T> template<typename U>
sp<T>& sp<T>::operator =(const sp<U>& other) {
    T* otherPtr(other.m_ptr);
    if (otherPtr)
        otherPtr->incStrong(this);
    if (m_ptr)
        m_ptr->decStrong(this);
    m_ptr = otherPtr;
    return *this;
}

template<typename T> template<typename U>
sp<T>& sp<T>::operator =(U* other) {
    if (other)
        ((T*) other)->incStrong(this);
    if (m_ptr)
        m_ptr->decStrong(this);
    m_ptr = other;
    return *this;
}

template<typename T>
void sp<T>::force_set(T* other) {
    other->forceIncStrong(this);
    m_ptr = other;
}

template<typename T>
void sp<T>::clear() {
    if (m_ptr) {
        m_ptr->decStrong(this);
        m_ptr = 0;
    }
}

template<typename T>
void sp<T>::set_pointer(T* ptr) {
    m_ptr = ptr;
}

}; // namespace android

// ---------------------------------------------------------------------------

#endif // ANDROID_STRONG_POINTER_H