iOS-weak实现原理#

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//初始化weak表
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/**
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 * Initialize a fresh weak pointer to some object location.
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 * It would be used for code like:
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 *
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 * (The nil case)
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 * __weak id weakPtr;
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 * (The non-nil case)
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 * NSObject *o = ...;
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 * __weak id weakPtr = o;
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 *
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 * @param addr Address of __weak ptr.
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 * @param val Object ptr.
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 */
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id objc_initWeak(id *addr, id val)
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{
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    *addr = 0;
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    if (!val) return nil;
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    return objc_storeWeak(addr, val); // 存储weak对象
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}

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/**
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 * This function stores a new value into a __weak variable. It would
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 * be used anywhere a __weak variable is the target of an assignment.
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 *
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 * @param location The address of the weak pointer itself
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 * @param newObj The new object this weak ptr should now point to
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 *
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 * @return \e newObj
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 */
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id
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objc_storeWeak(id *location, id newObj)
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{
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    id oldObj;
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    SideTable *oldTable;
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    SideTable *newTable;
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    spinlock_t *lock1;
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#if SIDE_TABLE_STRIPE > 1
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    spinlock_t *lock2;
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#endif
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    // Acquire locks for old and new values.
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    // Order by lock address to prevent lock ordering problems.
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    // Retry if the old value changes underneath us.
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 retry:
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    oldObj = *location;
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    oldTable = SideTable::tableForPointer(oldObj);
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    newTable = SideTable::tableForPointer(newObj);
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    lock1 = &newTable->slock;
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#if SIDE_TABLE_STRIPE > 1
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    lock2 = &oldTable->slock;
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    if (lock1 > lock2) {
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        spinlock_t *temp = lock1;
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        lock1 = lock2;
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        lock2 = temp;
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    }
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    if (lock1 != lock2) spinlock_lock(lock2);
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#endif
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    spinlock_lock(lock1);
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    if (*location != oldObj) {
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        spinlock_unlock(lock1);
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#if SIDE_TABLE_STRIPE > 1
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        if (lock1 != lock2) spinlock_unlock(lock2);
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#endif
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        goto retry;
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    }
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    weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
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    newObj = weak_register_no_lock(&newTable->weak_table, newObj, location);
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    // weak_register_no_lock returns nil if weak store should be rejected
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    // Set is-weakly-referenced bit in refcount table.
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    if (newObj  &&  !newObj->isTaggedPointer()) {
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        newObj->setWeaklyReferenced_nolock();
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    }
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    // Do not set *location anywhere else. That would introduce a race.
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    *location = newObj;
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    spinlock_unlock(lock1);
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#if SIDE_TABLE_STRIPE > 1
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    if (lock1 != lock2) spinlock_unlock(lock2);
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#endif
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    return newObj;
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}