一 关系图

spwp.png

二 各种场景应用

1)初始化

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

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

其中 mStrong的初始值是INITIAL_STRONG_VALUE=0X10000000，#define INITIAL_STRONG_VALUE (1<<28)
而mWeak的初始值为0，一个weakref_impl 对象被创建

2. 只有sp引用
   {
   sp<A> spA(new A);

}

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

void RefBase::incStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->incWeak(id);
    
    refs->addStrongRef(id);
    const int32_t c = refs->mStrong.fetch_add(1, std::memory_order_relaxed);
    ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFS
    ALOGD("incStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
    if (c != INITIAL_STRONG_VALUE)  {
        return;
    }

    int32_t old __unused = refs->mStrong.fetch_sub(INITIAL_STRONG_VALUE, std::memory_order_relaxed);
    // A decStrong() must still happen after us.
    ALOG_ASSERT(old > INITIAL_STRONG_VALUE, "0x%x too small", old);
    refs->mBase->onFirstRef();
}

void RefBase::weakref_type::incWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    impl->addWeakRef(id);
    const int32_t c __unused = impl->mWeak.fetch_add(1,
            std::memory_order_relaxed);
    ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}

在构造函数执行完毕后，mStrong和mWeak的值为1
当spA在区域执行完毕后，调用析构函数

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

void RefBase::decStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->removeStrongRef(id);
    const int32_t c = refs->mStrong.fetch_sub(1, std::memory_order_release);
#if PRINT_REFS
    ALOGD("decStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
    LOG_ALWAYS_FATAL_IF(BAD_STRONG(c), "decStrong() called on %p too many times",
            refs);
    if (c == 1) {
        std::atomic_thread_fence(std::memory_order_acquire);
        refs->mBase->onLastStrongRef(id);
        int32_t flags = refs->mFlags.load(std::memory_order_relaxed);
        if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
            delete this;
            // The destructor does not delete refs in this case.
        }
    }
    // Note that even with only strong reference operations, the thread
    // deallocating this may not be the same as the thread deallocating refs.
    // That's OK: all accesses to this happen before its deletion here,
    // and all accesses to refs happen before its deletion in the final decWeak.
    // The destructor can safely access mRefs because either it's deleting
    // mRefs itself, or it's running entirely before the final mWeak decrement.
    //
    // Since we're doing atomic loads of `flags`, the static analyzer assumes
    // they can change between `delete this;` and `refs->decWeak(id);`. This is
    // not the case. The analyzer may become more okay with this patten when
    // https://bugs.llvm.org/show_bug.cgi?id=34365 gets resolved. NOLINTNEXTLINE
    refs->decWeak(id);
}

mStrong为1，最后一次引用，释放掉spA,其中调用refBase的析构函数

RefBase::~RefBase()
{
    int32_t flags = mRefs->mFlags.load(std::memory_order_relaxed);
    // Life-time of this object is extended to WEAK, in
    // which case weakref_impl doesn't out-live the object and we
    // can free it now.
    if ((flags & OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_WEAK) {
        // It's possible that the weak count is not 0 if the object
        // re-acquired a weak reference in its destructor
        if (mRefs->mWeak.load(std::memory_order_relaxed) == 0) {
            delete mRefs;
        }
    } else if (mRefs->mStrong.load(std::memory_order_relaxed)
            == INITIAL_STRONG_VALUE) {
        // We never acquired a strong reference on this object.
        LOG_ALWAYS_FATAL_IF(mRefs->mWeak.load() != 0,
                "RefBase: Explicit destruction with non-zero weak "
                "reference count");
        // TODO: Always report if we get here. Currently MediaMetadataRetriever
        // C++ objects are inconsistently managed and sometimes get here.
        // There may be other cases, but we believe they should all be fixed.
        delete mRefs;
    }
    // For debugging purposes, clear mRefs.  Ineffective against outstanding wp's.
    const_cast<weakref_impl*&>(mRefs) = NULL;
}

当调用decWeak时，由于flags为OBJECT_LIFETIME_STRONG，此时mStrong为0，调用delete impl释放掉第1步中的Impl

void RefBase::weakref_type::decWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    impl->removeWeakRef(id);
    const int32_t c = impl->mWeak.fetch_sub(1, std::memory_order_release);
    LOG_ALWAYS_FATAL_IF(BAD_WEAK(c), "decWeak called on %p too many times",
            this);
    if (c != 1) return;
    atomic_thread_fence(std::memory_order_acquire);

    int32_t flags = impl->mFlags.load(std::memory_order_relaxed);
    if ((flags&OBJECT_LIFETIME_MASK) == OBJECT_LIFETIME_STRONG) {
        // This is the regular lifetime case. The object is destroyed
        // when the last strong reference goes away. Since weakref_impl
        // outlives the object, it is not destroyed in the dtor, and
        // we'll have to do it here.
        if (impl->mStrong.load(std::memory_order_relaxed)
                == INITIAL_STRONG_VALUE) {
            // Decrementing a weak count to zero when object never had a strong
            // reference.  We assume it acquired a weak reference early, e.g.
            // in the constructor, and will eventually be properly destroyed,
            // usually via incrementing and decrementing the strong count.
            // Thus we no longer do anything here.  We log this case, since it
            // seems to be extremely rare, and should not normally occur. We
            // used to deallocate mBase here, so this may now indicate a leak.
            ALOGW("RefBase: Object at %p lost last weak reference "
                    "before it had a strong reference", impl->mBase);
        } else {
            // ALOGV("Freeing refs %p of old RefBase %p\n", this, impl->mBase);
            delete impl;
        }
    } else {
        // This is the OBJECT_LIFETIME_WEAK case. The last weak-reference
        // is gone, we can destroy the object.
        impl->mBase->onLastWeakRef(id);
        delete impl->mBase;
    }
}

3. 只有wp弱引用

{
wp<A> wpA(new A);
}

调用构造函数

template<typename T>
wp<T>::wp(const sp<T>& other)
    : m_ptr(other.m_ptr)
{
    if (m_ptr) {
        m_refs = m_ptr->createWeak(this);
    }
}
RefBase::weakref_type* RefBase::createWeak(const void* id) const
{
    mRefs->incWeak(id);
    return mRefs;
}

void RefBase::weakref_type::incWeak(const void* id)
{
    weakref_impl* const impl = static_cast<weakref_impl*>(this);
    impl->addWeakRef(id);
    const int32_t c __unused = impl->mWeak.fetch_add(1,
            std::memory_order_relaxed);
    ALOG_ASSERT(c >= 0, "incWeak called on %p after last weak ref", this);
}

通过代码分析，只是给mWeak计数器+1，而mStrong 则是初始值INITIAL_STRONG_VALUE，当wpA超过作用域被回收的时候，调用析构函数

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

根据上面的代码，调用delete impl->mBase;释放掉wpA，同时RefBase的析构函数被调用,delete mRefs，释放weakref_impl

4. 同时有wp 和sp指针

{
sp<A> spA(new A)
wp<A> wpA(spA);
}

在构造期间 mStong = 1, mWeak = 2，在释放时先调用wpA的析构函数，mWeak=1,mStrong=1,然后调用spA的析构函数，同2）流程
5）WP指针调用类方法
由于WP没有重载"->"，不能直接调用类方法，在调用decStrong中，Object可能被释放，
我们需要将wp指针升级为sp指针

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;
}

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

    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 (impl->mStrong.compare_exchange_weak(curCount, curCount+1,
                std::memory_order_relaxed)) {
            break;
        }
        // the strong count has changed on us, we need to re-assert our
        // situation. curCount was updated by compare_exchange_weak.
    }
    
    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
        int32_t flags = impl->mFlags.load(std::memory_order_relaxed);
        if ((flags&OBJECT_LIFETIME_MASK) == 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 (impl->mStrong.compare_exchange_weak(curCount, curCount+1,
                        std::memory_order_relaxed)) {
                    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 has been updated.
            }

            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 = impl->mStrong.fetch_add(1, std::memory_order_relaxed);
            // 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\n", this, id, curCount);
#endif

    // curCount is the value of mStrong before we incremented it.
    // 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 > INITIAL_STRONG_VALUE is OK, and can happen if we're doing
    // this in the middle of another incStrong.  The subtraction is handled
    // by the thread that started with INITIAL_STRONG_VALUE.
    if (curCount == INITIAL_STRONG_VALUE) {
        impl->mStrong.fetch_sub(INITIAL_STRONG_VALUE,
                std::memory_order_relaxed);
    }

    return true;
}

first:当curCount > 0 & & curCount! = INITIAL_STRONG_VALUE，说明已经有sp指针指向，mStrong 加1
second:如果没有sp指针的话，分如下几种情况
Flag = OBJECT_LIFETIME_STRONG，curCount=0，说明object已经释放掉，此时强制转化，返回false
third: 如果Flag = OBJECT_LIFETIME_STRONG, and curCount = INITIAL_STRONG_VALUE，没有sp引用，此时可以将mStrong加1
four:如果Flag = OBJECT_LIFETIME_WEAK, and curCount <= 0 || curCount = = INITIAL_STRONG_VALUE，此种情况下mStrong加1