Android上C++对象的wp提升为sp的过程

wp对象调用promote方法返回sp对象，如果sp指向的对象 已经销毁，promote返回NULL

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;

}

可以将wp提升为sp的三种情况：

1、  没有sp指向目标对象且mStrong == INITIAL_STRONG_VALUE

2、  没有sp指向目标对象且mStrong == 0 且mFlags == OBJECT_LIFETIME_WEAK

3、有sp指向目标对象

attemptIncStrong()代码说明了上面的三种情况

bool RefBase::weakref_type::attemptIncStrong(const void* id)

{

    incWeak(id);

  

    weakref_impl* const impl = static_cast<weakref_impl*>(this);

  

    int32_t curCount = impl->mStrong;

    LOG_ASSERT(curCount >= 0, "attemptIncStrong called on %p after underflow",

               this);

    while (curCount > 0 && curCount != INITIAL_STRONG_VALUE) {

        if (android_atomic_cmpxchg(curCount, curCount+1, &impl->mStrong) == 0) {

            break;

        }

        curCount = impl->mStrong;

    }

  

    if (curCount <= 0 || curCount == INITIAL_STRONG_VALUE) {

        bool allow;

        if (curCount == INITIAL_STRONG_VALUE) {

            // Attempting to acquire first strong reference...  this is allowed

            // if the object does NOT have a longer lifetime (meaning the

            // implementation doesn't need to see this), or if the implementation

            // allows it to happen.

            allow = (impl->mFlags&OBJECT_LIFETIME_WEAK) != OBJECT_LIFETIME_WEAK

                  || impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id);

        } else {

            // Attempting to revive the object...  this is allowed

            // if the object DOES have a longer lifetime (so we can safely

            // call the object with only a weak ref) and the implementation

            // allows it to happen.

            allow = (impl->mFlags&OBJECT_LIFETIME_WEAK) == OBJECT_LIFETIME_WEAK

                  && impl->mBase->onIncStrongAttempted(FIRST_INC_STRONG, id);

        }

        if (!allow) {

            decWeak(id);

            return false;

        }

        curCount = android_atomic_inc(&impl->mStrong);

 

        …

        }

    }

  

    impl->addStrongRef(id);

    …

    return true;

}