android sp(strong pointer) 和 wp(weak pointer)

在android中使用了sp(strong pointer) 和 wp(weak pointer)实现类似于java内存回收机制。

实现代码：

frameworks/native/libs/utils/RefBase.cpp

frameworks/native/include/utils/RefBase.h

frameworks/native/include/utils/StrongPointer.h

强引用为0时，实际的对象会被删除

弱引用为0时，weakref_impl会被删除

强引用

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

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

void RefBase::incStrong(const void* id) const
{
weakref_impl* const refs = mRefs;

//弱引用计数器增1
refs->incWeak(id);

//这个只在DEBUG版本追踪时有用，现在可以不用看
refs->addStrongRef(id);

//强引用计数器增1

const int32_t c = android_atomic_inc(&refs->mStrong);
ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);

//INITIAL_STRONG_VALUE的值(1<<28) ，当refs->mStrong第一次创建完后，以后就直接return了

if (c != INITIAL_STRONG_VALUE) {
return;
}

//第一次构造时，会最终使refs->mStrong == 1
android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
refs->mBase->onFirstRef();
}

void RefBase::decStrong(const void* id) const
{
weakref_impl* const refs = mRefs;

//和addStrongRef相对应
refs->removeStrongRef(id);

//强引用计数器减1

const int32_t c = android_atomic_dec(&refs->mStrong);

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) {
// 当refs->mStrong == 0时，自杀删除对象

delete this;
}
}
//弱引用计数器减1

refs->decWeak(id);
}

RefBase::~RefBase()
{
if (mRefs->mStrong == INITIAL_STRONG_VALUE) {
// we never acquired a strong (and/or weak) reference on this object.
//没有使用强引用计数，直接删除weakref_impl

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;
}

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