iOS是通過(guò)引用計(jì)數(shù)來(lái)管理對(duì)象的生命周期的，當(dāng)引用計(jì)數(shù)起為0的時(shí)候會(huì)調(diào)用delloc函數(shù)釋放該對(duì)象泻帮，在64bit中添寺，引用計(jì)數(shù)可以直接存儲(chǔ)在優(yōu)化過(guò)的isa指針中，也可能存儲(chǔ)在SideTable類中:

image.png

• refcnts 是一個(gè)存放著對(duì)象引用計(jì)數(shù)的散列表，當(dāng)isa中存儲(chǔ)不下的時(shí)候會(huì)存儲(chǔ)在refcnts中捞蛋。執(zhí)行SideTable_release操作的時(shí)候引用計(jì)數(shù)會(huì)減一，引用計(jì)數(shù)器為0的時(shí)候會(huì)通過(guò)message_send執(zhí)行delloc函數(shù)柬姚，會(huì)在refcnts表中找到對(duì)象并釋放拟杉。

• weak_table 一個(gè)全局的weak 引用哈希表：存放弱引用指針，會(huì)在弱引用的對(duì)象釋放的時(shí)候量承，通過(guò)該弱引用對(duì)象的地址找到弱引用指針搬设，釋放并至為nil穴店；

面試

• runtime 是怎么實(shí)現(xiàn)weak置nil的
• weak修飾的釋放則自動(dòng)被置為nil的實(shí)現(xiàn)原理
• ARC幫我們做了什么?

delloc方法釋放對(duì)象

當(dāng)一個(gè)對(duì)象的引用計(jì)數(shù)為0的時(shí)候會(huì)執(zhí)行delloc函數(shù)：

// Replaced by NSZombies
- (void)dealloc {
    _objc_rootDealloc(self);
}

void
_objc_rootDealloc(id obj)
{
    assert(obj);

    obj->rootDealloc();
}

然后會(huì)通過(guò)對(duì)象的isa指針判斷對(duì)象是否有弱引用，C++析構(gòu)函數(shù)拿穴，關(guān)聯(lián)對(duì)象泣洞，如果沒(méi)有直接執(zhí)行free()釋放該對(duì)象，如果有執(zhí)行object_dispose();

inline void
objc_object::rootDealloc()
{
    if (isTaggedPointer()) return;  // fixme necessary?

    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
        object_dispose((id)this);
    }
}

object_dispose在執(zhí)行free(obj)釋放對(duì)象前默色，會(huì)執(zhí)行objc_destructInstance(obj)釋放關(guān)聯(lián)對(duì)象球凰，弱引用，C++析構(gòu)函數(shù)

/***********************************************************************
* object_dispose
* fixme
* Locking: none
**********************************************************************/
id 
object_dispose(id obj)
{
    if (!obj) return nil;

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}

釋放弱引用腿宰，C++析構(gòu)函數(shù)呕诉，然后執(zhí)行clearDeallocating釋放關(guān)聯(lián)對(duì)象；

/***********************************************************************
* objc_destructInstance
* Destroys an instance without freeing memory. 
* Calls C++ destructors.
* Calls ARC ivar cleanup.
* Removes associative references.
* Returns `obj`. Does nothing if `obj` is nil.
**********************************************************************/
void *objc_destructInstance(id obj) 
{
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating();
    }

    return obj;
}

根據(jù)isa.nonpointer判斷isa類型是否是64位優(yōu)化后的指針吃度；

inline void 
objc_object::clearDeallocating()
{
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();
    }
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
        // Slow path for non-pointer isa with weak refs and/or side table data.
        clearDeallocating_slow();
    }

    assert(!sidetable_present());
}

weak_clear_no_lock(&table.weak_table, (id)this);獲取全局weak_table表甩挫， 并把對(duì)象地址傳入，通過(guò)對(duì)象地址&table 算出散列表中的下標(biāo)椿每，然后清除引用的對(duì)象捶闸；

// Slow path of clearDeallocating() 
// for objects with nonpointer isa
// that were ever weakly referenced 
// or whose retain count ever overflowed to the side table.
NEVER_INLINE void
objc_object::clearDeallocating_slow()
{
    assert(isa.nonpointer  &&  (isa.weakly_referenced || isa.has_sidetable_rc));

    SideTable& table = SideTables()[this];
    table.lock();
    if (isa.weakly_referenced) {
        weak_clear_no_lock(&table.weak_table, (id)this);
    }
    if (isa.has_sidetable_rc) {
        table.refcnts.erase(this);
    }
    table.unlock();
}

通過(guò)這個(gè)方法獲取entry
weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
然后執(zhí)行 weak_entry_remove(weak_table, entry) 從weak_table表中移除，然后執(zhí)行free（）釋放該對(duì)象拖刃；

/** 
 * Called by dealloc; nils out all weak pointers that point to the 
 * provided object so that they can no longer be used.
 * 
 * @param weak_table 
 * @param referent The object being deallocated. 
 */
void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    
    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }
    
    weak_entry_remove(weak_table, entry);
}

/** 
 * Return the weak reference table entry for the given referent. 
 * If there is no entry for referent, return NULL. 
 * Performs a lookup.
 *
 * @param weak_table 
 * @param referent The object. Must not be nil.
 * 
 * @return The table of weak referrers to this object. 
 */
static weak_entry_t *
weak_entry_for_referent(weak_table_t *weak_table, objc_object *referent)
{
    assert(referent);

    weak_entry_t *weak_entries = weak_table->weak_entries;

    if (!weak_entries) return nil;

    size_t begin = hash_pointer(referent) & weak_table->mask;
    size_t index = begin;
    size_t hash_displacement = 0;
    while (weak_table->weak_entries[index].referent != referent) {
        index = (index+1) & weak_table->mask;
        if (index == begin) bad_weak_table(weak_table->weak_entries);
        hash_displacement++;
        if (hash_displacement > weak_table->max_hash_displacement) {
            return nil;
        }
    }
    
    return &weak_table->weak_entries[index];
}

面試

• weak修飾的釋放則自動(dòng)被置為nil的實(shí)現(xiàn)原理？

1贪绘，創(chuàng)建person對(duì)象兑牡， 并被person2弱引用了，所以person2這個(gè)弱引用指針就會(huì)放在全局的weak_table表中税灌；

2均函，當(dāng)釋放要person對(duì)象的時(shí)候，通過(guò)runtime去weak_table表中找到person的弱引用person2菱涤，然后釋放person2這個(gè)弱引用指針苞也，并且設(shè)置person2=nil，然后再釋放person對(duì)象;

  __strong Person *person1;
  __weak Person *person2;
  __unsafe_unretained Person *person3;
    
    NSLog(@"111");
    
    {
        Person *person = [[Person alloc] init];
        
        person2 = person;
    }
    
    NSLog(@"222 - %@", person2);


 111
 [Person dealloc]
 222 - (null)

特別補(bǔ)充一點(diǎn)粘秆，如有需要會(huì)通過(guò)erase函數(shù)擦掉SideTable表中person的引用計(jì)數(shù)：

 if (isa.has_sidetable_rc) {
        table.refcnts.erase(this);
    }

• ARC幫我們做了什么?

ARC 其實(shí)就是LLVM + runtime如迟，LLVM會(huì)在編譯階段自動(dòng)幫我們添加[object release]，然后會(huì)在runtime階段根據(jù)引用計(jì)數(shù)判斷攻走，當(dāng)引用計(jì)數(shù)為0的時(shí)候會(huì)通過(guò)isa指針的信息抹除對(duì)象的一些引用殷勘，這些都是runtime的功勞。