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
- 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;
}
}
- 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
- 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
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