Objective-C|Objective-C runtime机制(8)——OC对象从创建到销毁

在我们前面的几章中,分析了OC的runtime一些底层的数据结构以及实现机制。今天,我们就从一个OC对象的生命周期的角度,来解析在runtime底层是如何实现的。
我们创建一个对象(或对象引用)有几种方式?

Student *student = [[Student alloc] init]; Student *student2 = [Student new]; __weak Student *weakStudent = [Student new]; NSDictionary *dict = [[NSDictionary alloc] init]; NSDictionary *autoreleaseDict = [NSDictionary dictionary];

有很多种方式,我们就来依次看一下这些方式的背后实现。
alloc 要创建一个对象,第一步就是需要为对象分配内存。在创建内存时,我们会调用alloc方法。查看runtime的NSObject +alloc方法实现:
+ (id)alloc { return _objc_rootAlloc(self); }

// Base class implementation of +alloc. cls is not nil. // Calls [cls allocWithZone:nil]. id _objc_rootAlloc(Class cls) { return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/); }

alloc方法会将self作为参数传入_objc_rootAlloc(Class cls) 方法中,注意,因为alloc是一个类方法,因此此时的self是一个Class类型。
最终该方法会落脚到callAlloc方法。
static ALWAYS_INLINE id callAlloc(Class cls, bool checkNil, bool allocWithZone=false)

static ALWAYS_INLINE id callAlloc(Class cls, bool checkNil, bool allocWithZone=false) { if (slowpath(checkNil && !cls)) return nil; #if __OBJC2__ if (fastpath(!cls->ISA()->hasCustomAWZ())) { // No alloc/allocWithZone implementation. Go straight to the allocator. // fixme store hasCustomAWZ in the non-meta class and // add it to canAllocFast's summary if (fastpath(cls->canAllocFast())) { // 如果可以fast alloc,走这里 // No ctors, raw isa, etc. Go straight to the metal. bool dtor = cls->hasCxxDtor(); id obj = (id)calloc(1, cls->bits.fastInstanceSize()); // 直接调用 calloc方法,申请1块大小为bits.fastInstanceSize()的内存 if (slowpath(!obj)) return callBadAllocHandler(cls); obj->initInstanceIsa(cls, dtor); return obj; } else { // 如果不可以fast alloc,走这里, // Has ctor or raw isa or something. Use the slower path. id obj = class_createInstance(cls, 0); // (1)需要读取cls 的class_ro_t 中的instanceSize,并使之大于16 byte, Because : CF requires all objects be at least 16 bytes. (2)initInstanceIsa if (slowpath(!obj)) return callBadAllocHandler(cls); return obj; } } #endif// No shortcuts available. if (allocWithZone) return [cls allocWithZone:nil]; return [cls alloc]; }

在callAlloc方法里面,做了三件事:
  1. 调用calloc方法,为类实例分配内存
  2. 调用obj->initInstanceIsa(cls, dtor)方法,初始化obj的isa
  3. 返回obj
在第一件事中,调用calloc方法,你需要提供需要申请内存的大小。在OC中有两条分支:
(1)can alloc fast
(2)can't alloc fast
对于可以alloc fast的类,应该是经过编译器优化的类。这种类的实例大小直接被放到了bits
struct class_data_bits_t {// Values are the FAST_ flags above. uintptr_t bits; ... }

而不需要通过bits找到class_rw_t->class_ro_t->instanceSize。省略了这一条查找路径,而是直接读取位值,其创建实例的速度自然比不能alloc fast的类要快。
而对于不能alloc fast的类,则会进入第二条路径,代码会通过上面所说的通过bits找到class_rw_t->class_ro_t->instanceSize来确定需要申请内存的大小。
当申请了对象的内存后,还需要初始化类实例对象的isa成员变量:
obj->initInstanceIsa(cls, hasCxxDtor);

inline void objc_object::initInstanceIsa(Class cls, bool hasCxxDtor) { assert(!cls->instancesRequireRawIsa()); assert(hasCxxDtor == cls->hasCxxDtor()); initIsa(cls, true, hasCxxDtor); }

inline void objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) { assert(!isTaggedPointer()); if (!nonpointer) { // 如果没有启用isa 优化,则直接将cls赋值给isa.cls,来表明当前object 是哪个类的实例 isa.cls = cls; } else { // 如果启用了isa 优化,则初始化isa的三个内容(1) isa基本的内容,包括nonpointer置1以及设置OC magic vaule (2)置位has_cxx_dtor (3) 记录对象所属类的信息。 通过 newisa.shiftcls = (uintptr_t)cls >> 3; assert(!DisableNonpointerIsa); assert(!cls->instancesRequireRawIsa()); isa_t newisa(0); #if SUPPORT_INDEXED_ISA assert(cls->classArrayIndex() > 0); newisa.bits = ISA_INDEX_MAGIC_VALUE; // isa.magic is part of ISA_MAGIC_VALUE // isa.nonpointer is part of ISA_MAGIC_VALUE newisa.has_cxx_dtor = hasCxxDtor; newisa.indexcls = (uintptr_t)cls->classArrayIndex(); #else newisa.bits = ISA_MAGIC_VALUE; // isa.magic is part of ISA_MAGIC_VALUE // isa.nonpointer is part of ISA_MAGIC_VALUE newisa.has_cxx_dtor = hasCxxDtor; newisa.shiftcls = (uintptr_t)cls >> 3; #endif// This write must be performed in a single store in some cases // (for example when realizing a class because other threads // may simultaneously try to use the class). // fixme use atomics here to guarantee single-store and to // guarantee memory order w.r.t. the class index table // ...but not too atomic because we don't want to hurt instantiation isa = newisa; } }

结合代码注释,以及我们在Objective-C runtime机制(5)——iOS 内存管理中提到的关于isa的描述,应该可以理解isa初始化的逻辑。
init 我们再来看一下init方法:
- (id)init { return _objc_rootInit(self); }

id _objc_rootInit(id obj) { // In practice, it will be hard to rely on this function. // Many classes do not properly chain -init calls. return obj; }

实现很简单,就是将自身返回,没有做任何其他操作。
__strong
Student *student = [[Student alloc] init]; Student *student2 = [Student new];

在等号的左边,我们通过allocnew的方式创建了两个OC对象。而在右面,我们通过Student *的方式来引用这些对象。
在OC中,对对象所有的引用都是有所有权修饰符的,所有权修饰符会告诉编译器,该如何处理对象的引用关系。如果代码中没有显示指明所有权修饰符,则默认为__strong所有权。
因此上面代码实际是:
__strong Student *student = [[Student alloc] init]; __strong Student *student2 = [Student new];

对于new方法,苹果的文档解释为:
Allocates a new instance of the receiving class, sends it an initmessage, and returns the initialized object.
其实就是alloc + init 方法的简写。因此,这里的两种创建实例对象的方式可以理解是一个。
那么,当所有权修饰符是__strong时,runtime是如何管理对象引用的呢?
runtime会通过 void objc_storeStrong(id *location, id obj) 方法来处理__strong 引用。 这里的location就是引用指针,即Student *student,而obj就是被引用的对象,即Student实例
void objc_storeStrong(id *location, id obj) { id prev = *location; if (obj == prev) { return; } objc_retain(obj); //1.retain obj *location = obj; //2.将location 指向 obj objc_release(prev); //3. release location之前指向的obj }

代码逻辑很简单,主要是调用了objc_retain和objc_release两个方法。
我们分别来看一下它们的实现。
objc_retain
id objc_retain(id obj) { if (!obj) return obj; if (obj->isTaggedPointer()) return obj; return obj->retain(); }inline id objc_object::retain() { assert(!isTaggedPointer()); if (fastpath(!ISA()->hasCustomRR())) { return rootRetain(); }return ((id(*)(objc_object *, SEL))objc_msgSend)(this, SEL_retain); }

可以看到,objc_retain方法最终会调到objc_object类的rootRetain方法:
ALWAYS_INLINE id objc_object::rootRetain() { return rootRetain(false, false); }

ALWAYS_INLINE id objc_object::rootRetain(bool tryRetain, bool handleOverflow) { if (isTaggedPointer()) return (id)this; bool sideTableLocked = false; bool transcribeToSideTable = false; isa_t oldisa; isa_t newisa; do { transcribeToSideTable = false; oldisa = LoadExclusive(&isa.bits); newisa = oldisa; if (slowpath(!newisa.nonpointer)) {// 如果没有采用isa优化, 则返回sidetable记录的内容, 用slowpath表明这不是一个大概率事件 ClearExclusive(&isa.bits); if (!tryRetain && sideTableLocked) sidetable_unlock(); if (tryRetain) return sidetable_tryRetain() ? (id)this : nil; else return sidetable_retain(); } // don't check newisa.fast_rr; we already called any RR overrides if (slowpath(tryRetain && newisa.deallocating)) { ClearExclusive(&isa.bits); if (!tryRetain && sideTableLocked) sidetable_unlock(); return nil; } // 采用了isa优化,做extra_rc++,同时检查是否extra_rc溢出,若溢出,则extra_rc减半,并将另一半转存至sidetable uintptr_t carry; newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++if (slowpath(carry)) { // 有carry值,表示extra_rc 溢出 // newisa.extra_rc++ overflowed if (!handleOverflow) {// 如果不处理溢出情况,则在这里会递归调用一次,再进来的时候,handleOverflow会被rootRetain_overflow设置为true,从而进入到下面的溢出处理流程 ClearExclusive(&isa.bits); return rootRetain_overflow(tryRetain); } // Leave half of the retain counts inline and // prepare to copy the other half to the side table. if (!tryRetain && !sideTableLocked) sidetable_lock(); // 进行溢出处理:逻辑很简单,先在extra_rc中留一半计数,同时把has_sidetable_rc设置为true,表明借用了sidetable,然后把另一半放到sidetable中 sideTableLocked = true; transcribeToSideTable = true; newisa.extra_rc = RC_HALF; newisa.has_sidetable_rc = true; } } while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits))); // 将oldisa 替换为 newisa,并赋值给isa.bits(更新isa_t), 如果不成功,do while再试一遍if (slowpath(transcribeToSideTable)) { //isa的extra_rc溢出,将一半的refer count值放到sidetable中 // Copy the other half of the retain counts to the side table. sidetable_addExtraRC_nolock(RC_HALF); }if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock(); return (id)this; }

这一段rootRetain方法,在我们之前的文章Objective-C runtime机制(5)——iOS 内存管理已经做过分析。
我们就在总结一下rootRetain方法的流程:
  1. 取出当前对象的isa.bits值
  2. isa.bits分别赋值给oldisanewisa
  3. 根据isa_t的标志位newisa.nonpointer,来判断runtime是否只开启了isa优化。
  4. 如果newisa.nonpointer为0,则走老的流程,调用sidetable_retain方法,在SideTable中找到this对应的节点,side table refcntStorage + 1
  5. 如果newisa.nonpointer为1,则在newisa.extra_rc上做引用计数+1操作。同时,需要判断是否计数溢出。
  6. 如果newisa.extra_rc溢出,则进行溢出处理:newisa.extra_rc计数减半,将计数的另一半放到SideTable中。并设置newisa.has_sidetable_rc = true,表明引用计数借用了SideTable
  7. 最后,调用StoreExclusive,更新对象的isa.bits
总结:
__strong引用会使得被引用对象计数+1,同时,会使得之前的饮用对象计数-1。
__weak
__weak Student *weakStudent = [Student new];

当使用__weak所有权修饰符来引用对象时?会发生什么呢?
当weakStudent弱引用Student对象时,会调用objc_initWeak方法。当weakStudent超出其作用域要销毁时,会调用objc_destoryWeak方法。
我们分别看一下它们的实现:
/** * Initialize a fresh weak pointer to some object location. * It would be used for code like: * * (The nil case) * __weak id weakPtr; * (The non-nil case) * NSObject *o = ...; * __weak id weakPtr = o; * * This function IS NOT thread-safe with respect to concurrent * modifications to the weak variable. (Concurrent weak clear is safe.) * * @param location Address of __weak ptr. * @param newObj Object ptr. */// @param location __weak 指针的地址 // @param newObj 被弱引用的对象指针 // @return __weak 指针id objc_initWeak(id *location, id newObj) { if (!newObj) { *location = nil; return nil; }return storeWeak (location, (objc_object*)newObj); }

template static id storeWeak(id *location, objc_object *newObj) { assert(haveOld||haveNew); if (!haveNew) assert(newObj == nil); Class previouslyInitializedClass = nil; id oldObj; SideTable *oldTable; SideTable *newTable; // Acquire locks for old and new values. // Order by lock address to prevent lock ordering problems. // Retry if the old value changes underneath us. retry: if (haveOld) { // 如果weak ptr之前弱引用过一个obj,则将这个obj所对应的SideTable取出,赋值给oldTable oldObj = *location; oldTable = &SideTables()[oldObj]; } else { oldTable = nil; // 如果weak ptr之前没有弱引用过一个obj,则oldTable = nil } if (haveNew) { // 如果weak ptr要weak引用一个新的obj,则将该obj对应的SideTable取出,赋值给newTable newTable = &SideTables()[newObj]; } else { newTable = nil; // 如果weak ptr不需要引用一个新obj,则newTable = nil }// 加锁操作,防止多线程中竞争冲突 SideTable::lockTwo(oldTable, newTable); // location 应该与 oldObj 保持一致,如果不同,说明当前的 location 已经处理过 oldObj 可是又被其他线程所修改 if (haveOld&&*location != oldObj) { SideTable::unlockTwo(oldTable, newTable); goto retry; }// Prevent a deadlock between the weak reference machinery // and the +initialize machinery by ensuring that no // weakly-referenced object has an un-+initialized isa. if (haveNew&&newObj) { Class cls = newObj->getIsa(); if (cls != previouslyInitializedClass&& !((objc_class *)cls)->isInitialized())// 如果cls还没有初始化,先初始化,再尝试设置weak { SideTable::unlockTwo(oldTable, newTable); _class_initialize(_class_getNonMetaClass(cls, (id)newObj)); // If this class is finished with +initialize then we're good. // If this class is still running +initialize on this thread // (i.e. +initialize called storeWeak on an instance of itself) // then we may proceed but it will appear initializing and // not yet initialized to the check above. // Instead set previouslyInitializedClass to recognize it on retry. previouslyInitializedClass = cls; // 这里记录一下previouslyInitializedClass, 防止改if分支再次进入goto retry; // 重新获取一遍newObj,这时的newObj应该已经初始化过了 } }// Clean up old value, if any. if (haveOld) { weak_unregister_no_lock(&oldTable->weak_table, oldObj, location); // 如果weak_ptr之前弱引用过别的对象oldObj,则调用weak_unregister_no_lock,在oldObj的weak_entry_t中移除该weak_ptr地址 }// Assign new value, if any. if (haveNew) { // 如果weak_ptr需要弱引用新的对象newObj // (1) 调用weak_register_no_lock方法,将weak ptr的地址记录到newObj对应的weak_entry_t中 newObj = (objc_object *) weak_register_no_lock(&newTable->weak_table, (id)newObj, location, crashIfDeallocating); // weak_register_no_lock returns nil if weak store should be rejected// (2) 更新newObj的isa的weakly_referenced bit标志位 // Set is-weakly-referenced bit in refcount table. if (newObj&&!newObj->isTaggedPointer()) { newObj->setWeaklyReferenced_nolock(); }// Do not set *location anywhere else. That would introduce a race. // (3)*location 赋值,也就是将weak ptr直接指向了newObj。可以看到,这里并没有将newObj的引用计数+1 *location = (id)newObj; // 将weak ptr指向object } else { // No new value. The storage is not changed. }// 解锁,其他线程可以访问oldTable, newTable了 SideTable::unlockTwo(oldTable, newTable); return (id)newObj; // 返回newObj,此时的newObj与刚传入时相比,weakly-referenced bit位置1 }

可以看到,storeWeak函数会根据haveOld参数来决定是否需要处理weak 指针之前弱引用的对象。我们这里的weakStudent是第一次弱引用对象(a fresh weak pointer),因此,haveOld = false。关于haveOld = false的情况,我们稍后分析。
当haveOld = false时,storeWeak函数做的事情如下:
  1. 取出引用对象对应的SideTable节点SideTable *newTable;
  2. 调用weak_register_no_lock方法,将weak pointer的地址记录到对象对应的weak_entry_t中。
  3. 更新对象isa的weakly_referenced bit标志位,表明该对象被弱引用了。
  4. 将weak pointer指向对象
  5. 返回对象
关于weak_register_no_lock以及weak相关的数据结构,我们在Objective-C runtime机制(6)——weak引用的底层实现原理有相关探讨,就不再复述。
下面看另一种情况:
__weak Son *son = [Son new]; son = [Son new];

当weakStudent再次指向另一个对象时,则不会调用objc_initWeak方法,而是会调用objc_storeWeak方法:
/** * This function stores a new value into a __weak variable. It would * be used anywhere a __weak variable is the target of an assignment. * * @param location The address of the weak pointer itself * @param newObj The new object this weak ptr should now point to * * @return \e newObj */ id objc_storeWeak(id *location, id newObj) { return storeWeak (location, (objc_object *)newObj); }

其实还是调用了storeWeak方法,只不过DontHaveOld参数换成了DoHaveOld
当传入DoHaveOld时,storeWeak会进入分支:
// Clean up old value, if any. if (haveOld) { weak_unregister_no_lock(&oldTable->weak_table, oldObj, location); // 如果weak_ptr之前弱引用过别的对象oldObj,则调用weak_unregister_no_lock,在oldObj的weak_entry_t中移除该weak_ptr地址 }

void weak_unregister_no_lock(weak_table_t *weak_table, id referent_id, id *referrer_id) { objc_object *referent = (objc_object *)referent_id; objc_object **referrer = (objc_object **)referrer_id; weak_entry_t *entry; if (!referent) return; if ((entry = weak_entry_for_referent(weak_table, referent))) { // 查找到referent所对应的weak_entry_t remove_referrer(entry, referrer); // 在referent所对应的weak_entry_t的hash数组中,移除referrer// 移除元素之后, 要检查一下weak_entry_t的hash数组是否已经空了 bool empty = true; if (entry->out_of_line()&&entry->num_refs != 0) { empty = false; } else { for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) { if (entry->inline_referrers[i]) { empty = false; break; } } }if (empty) { // 如果weak_entry_t的hash数组已经空了,则需要将weak_entry_t从weak_table中移除 weak_entry_remove(weak_table, entry); } }// Do not set *referrer = nil. objc_storeWeak() requires that the // value not change. }

weak_unregister_no_lock方法中,将weak pointer的地址从对象的weak_entry_t中移除,同时会判断weak_entry_t是否已经空了,如果空了,则需要把weak_entry_tweak_table中移除。
总结:
__weak引用对象时,会在对象的weak_entry_t中登记该weak pointer的地址(这也就是为什么当对象释放时,weak pointer会被置为nil)。如果weak pointer之前已经弱引用过其他对象,则要先将weak pointer地址从其他对象的weak_entry_t中移除,同时,需要对weak_entry_t进行判空逻辑。
autorelease
NSDictionary *dict = [[NSDictionary alloc] init]; NSDictionary *autoreleaseDict = [NSDictionary dictionary];

当我们创建NSDictionary对象时,有这么两种方式。那么,这两种方式有什么区别呢?
在ARC时代,若方法名以下列词语开头,则其返回对象归调用者所有(意为需调用者负责释放内存,但对ARC来说,其实并没有手动release的必要)
  • alloc
  • new
  • copy
  • mutableCopy
而不使用这些词语开头的方法,如[NSDictionary dictionary]
根据苹果官方文档,当调用[NSDictionary dictionary]时:
This method is declared primarily for use with mutable subclasses of NSDictionary.
If you don’t want a temporary object, you can also create an empty dictionary using alloc and init.
似乎是说,当调用[NSDictionary dictionary]的形式时,会产生一个临时的对象。类似的,还有[NSArray array], [NSData data]
关于这种形式生成的变量,则表示“方法所返回的对象并不归调用者所有”。在这种情况下,返回的对象会自动释放。
其实我们可以理解为:当调用dictionary形式生成对象时,NSDictionary对象的引用计数管理,就不需要用户参与了(这在MRC时代有很大的区别,但是对于ARC来说,其实和alloc形式没有太大的区别了)。用[NSDictionary dictionary]其实相当于代码
[[NSDictionary alloc] init] autorelease];

这里会将NSDictionary对象交给了autorelease pool来管理。
事实是这样的吗?我们查看[NSDictionary dictionary]的汇编代码(Product->Perform Action->Assemble),可以看到,编译器会调用objc_retainAutoreleasedReturnValue方法。而objc_retainAutoreleasedReturnValue又是什么鬼?这其实是编译器的一个优化,前面我们说[NSDictionary dictionary]会在方法内部为NSDictionary实例调用autorelease,而如果这时候在外面用一个强引用来引用这个NSDictionary对象的话,还是需要调用一个retain,而此时,的autorelease和retain其实是可以相互抵消的。于是,编译器就给了一个优化,不是直接调用autorelease方法,而是调用objc_retainAutoreleasedReturnValue来做这样的判断,如果autorelease后面紧跟了retain,则将autorelease和retain都抵消掉,不再代码里面出现。(详见《Effective Objective-C 2.0》 P126)。
OK,上面是一些题外话,我们回到autorelease的主题上来。在ARC时代,我们通过如下形式使用autorelease:
@autorelease { // do your code }

实质上,编译器会将如上形式的代码转换为:
objc_autoreleasePoolPush(); // do your code objc_autoreleasePoolPop();

查看它们在runtime中的定义:
void * objc_autoreleasePoolPush(void) { return AutoreleasePoolPage::push(); } static inline void *push() { id *dest; if (DebugPoolAllocation) { // Each autorelease pool starts on a new pool page. dest = autoreleaseNewPage(POOL_BOUNDARY); } else { dest = autoreleaseFast(POOL_BOUNDARY); } assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY); return dest; }

static inline id *autoreleaseFast(id obj) { AutoreleasePoolPage *page = hotPage(); if (page && !page->full()) { return page->add(obj); } else if (page) { return autoreleaseFullPage(obj, page); } else { return autoreleaseNoPage(obj); } }

可以看到,当push到autorelease时,最终会调用到autoreleaseFast, 在autoreleaseFast中,会首先取出当前线程的hotPage,根据当前hotPage的三种状态:
  1. hot page存在且未满,调用page->add(obj)
  2. hot page存在但已满, 调用autoreleaseFullPage(obj, page)
  3. hot page不存在,调用 autoreleaseNoPage(obj)
关于这三个方法的实现细节,我们在Objective-C runtime机制(5)——iOS 内存管理有详细的分析。
当需要pop autorelease pool时,则会调用objc_autoreleasePoolPop()
void objc_autoreleasePoolPop(void *ctxt) { AutoreleasePoolPage::pop(ctxt); }static inline void pop(void *token) { AutoreleasePoolPage *page; id *stop; if (token == (void*)EMPTY_POOL_PLACEHOLDER) { // Popping the top-level placeholder pool. if (hotPage()) { // Pool was used. Pop its contents normally. // Pool pages remain allocated for re-use as usual. pop(coldPage()->begin()); } else { // Pool was never used. Clear the placeholder. setHotPage(nil); } return; }page = pageForPointer(token); stop = (id *)token; if (*stop != POOL_BOUNDARY) { if (stop == page->begin()&&!page->parent) { // Start of coldest page may correctly not be POOL_BOUNDARY: // 1. top-level pool is popped, leaving the cold page in place // 2. an object is autoreleased with no pool } else { // 这是为了兼容旧的SDK,看来在新的SDK里面,token 可能的取值只有两个:(1)POOL_BOUNDARY, (2)page->begin() && !page->parent也就是第一个page // Error. For bincompat purposes this is not // fatal in executables built with old SDKs. return badPop(token); } }if (PrintPoolHiwat) printHiwat(); page->releaseUntil(stop); // 对token之前的object,每一个都调用objc_release方法// memory: delete empty children if (DebugPoolAllocation&&page->empty()) { // special case: delete everything during page-per-pool debugging AutoreleasePoolPage *parent = page->parent; page->kill(); setHotPage(parent); } else if (DebugMissingPools&&page->empty()&&!page->parent) { // special case: delete everything for pop(top) // when debugging missing autorelease pools page->kill(); setHotPage(nil); } else if (page->child) { // hysteresis: keep one empty child if page is more than half full if (page->lessThanHalfFull()) { page->child->kill(); } else if (page->child->child) { page->child->child->kill(); } } }

在Pop中,会根据传入的token,调用 page->releaseUntil(stop) 方法,对每一个存储于page上的object调用objc_release(obj)方法。
之后,还会根据当前page的状态:page->lessThanHalfFull()或其他,来决定其child的处理方式:
  1. 如果当前page存储的object已经不满半页,则讲page的child释放
  2. 如果当前page存储的object仍满半页,则保留一个空的child,并且将空child之后的所有child都释放掉。
retain count 当我们需要获取对象的引用计数时,在ARC下可以调用如下方法:
CFGetRetainCount((__bridge CFTypeRef)(obj))

这是CF的方法调用,而在runtime中,我们可以调用NSObject的方法:
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;

通过注释,可以知道在ARC环境下,该方法是不可用的,但是不影响我们了解它的具体实现。
- (NSUInteger)retainCount { return ((id)self)->rootRetainCount(); }

方法里面讲self转为id类型,即objc_object类型,然后调用objc_objectrootRetainCount()方法。
inline uintptr_t objc_object::rootRetainCount() { //case 1: 如果是tagged pointer,则直接返回this,因为tagged pointer是不需要引用计数的 if (isTaggedPointer()) return (uintptr_t)this; // 将objcet对应的sidetable上锁 sidetable_lock(); isa_t bits = LoadExclusive(&isa.bits); ClearExclusive(&isa.bits); // case 2: 如果采用了优化的isa指针 if (bits.nonpointer) { uintptr_t rc = 1 + bits.extra_rc; // 先读取isa.extra_rc if (bits.has_sidetable_rc) { // 如果使用了sideTable来存储retain count, 还需要读取sidetable中的数据 rc += sidetable_getExtraRC_nolock(); // 总引用计数= rc + sidetable } sidetable_unlock(); return rc; } // case 3:如果没采用优化的isa指针,则直接返回sidetable中的值 sidetable_unlock(); return sidetable_retainCount(); }

获取retain count的方法很简单:
  1. 判断object是否使用了isa优化
  2. 如果使用了isa优化,先取出1 + bits.extra_rc
  3. 再判断是否需要读取side talbe( if (bits.has_sidetable_rc)
  4. 如果需要,则加上side table 中存储的retain count
  5. 如果没有使用isa优化,则直接读取side table 中的retain count,并加1,作为引用计数。
还有一种特殊的情况是,如果object pointer是tagged pointer,则不参与任何操作。
release 当object需要引用计数减一时,会调用release方法。
objc_object::rootRelease() { return rootRelease(true, false); }ALWAYS_INLINE bool objc_object::rootRelease(bool performDealloc, bool handleUnderflow) { if (isTaggedPointer()) return false; bool sideTableLocked = false; isa_t oldisa; isa_t newisa; retry: do { oldisa = LoadExclusive(&isa.bits); newisa = oldisa; if (slowpath(!newisa.nonpointer)) { // 慢路径 : 如果没有开启isa优化,则到sidetable中引用计数减一 ClearExclusive(&isa.bits); // 空方法 if (sideTableLocked) sidetable_unlock(); return sidetable_release(performDealloc); } // don't check newisa.fast_rr; we already called any RR overrides uintptr_t carry; newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry); // extra_rc-- if (slowpath(carry)) { // 如果下溢出,则goto underflow // don't ClearExclusive() goto underflow; } } while (slowpath(!StoreReleaseExclusive(&isa.bits, oldisa.bits, newisa.bits))); // 修改isa bits(如果不成功,则进入while循环,再试一把,直到成功为止)if (slowpath(sideTableLocked)) sidetable_unlock(); return false; // 如果没有溢出,则在这里就会返回false(表明引用计数不等于0,没有dealloc)// 只有isa.extra_rc-1 下溢出后,才会进入下面的代码。下溢出有两种情况: // 1. borrow from side table . isa.extra_rc 有从side table存储。这是假溢出,只需要将side table中的RC_HALF移回到isa.extra_rc即可。并返回false // 2. deallocate。 这种情况是真下溢出。此时isa.extra_rc < 0,且没有newisa.has_sidetable_rc 没有想side table 借位。说明object引用计数==0,(1) 设置newisa.deallocating = true; //(2)触发object 的dealloc方法, (3)并返回true,表明对象deallocation // // Really deallocate. //if (slowpath(newisa.deallocating)) { //ClearExclusive(&isa.bits); //if (sideTableLocked) sidetable_unlock(); //return overrelease_error(); //// does not actually return //} //newisa.deallocating = true; //if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry; // //if (slowpath(sideTableLocked)) sidetable_unlock(); // //__sync_synchronize(); //if (performDealloc) { //((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc); //} //return true; // // //////////////////////////////////////////////////////////////////////////////////////////////////////// underflow: // newisa.extra_rc-- underflowed: borrow from side table or deallocate// abandon newisa to undo the decrement newisa = oldisa; if (slowpath(newisa.has_sidetable_rc)) {// 如果借用了 sideTable 做 rc,走这里 if (!handleUnderflow) { ClearExclusive(&isa.bits); return rootRelease_underflow(performDealloc); }// Transfer retain count from side table to inline storage.if (!sideTableLocked) { ClearExclusive(&isa.bits); // ClearExclusive 是一个空函数 sidetable_lock(); sideTableLocked = true; // Need to start over to avoid a race against // the nonpointer -> raw pointer transition. goto retry; }// 如果extra_rc 减1后,其值carryout(小于0),则要处理side table,如果之前有在side talbe中借位RC_HALF,则把这RC_HALF在拿回来到extrc_rc中,并保留side table剩下的值 // Try to remove some retain counts from the side table. size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF); // To avoid races, has_sidetable_rc must remain set // even if the side table count is now zero.if (borrowed > 0) { // Side table retain count decreased. // Try to add them to the inline count. newisa.extra_rc = borrowed - 1; // redo the original decrement too bool stored = StoreReleaseExclusive(&isa.bits, oldisa.bits, newisa.bits); if (!stored) { // Inline update failed. // Try it again right now. This prevents livelock on LL/SC // architectures where the side table access itself may have // dropped the reservation. isa_t oldisa2 = LoadExclusive(&isa.bits); isa_t newisa2 = oldisa2; if (newisa2.nonpointer) { uintptr_t overflow; newisa2.bits = addc(newisa2.bits, RC_ONE * (borrowed-1), 0, &overflow); if (!overflow) { stored = StoreReleaseExclusive(&isa.bits, oldisa2.bits, newisa2.bits); } } }if (!stored) { // Inline update failed. // Put the retains back in the side table. sidetable_addExtraRC_nolock(borrowed); // 如果更新 isa extra_rc 失败,则把side table中的数再放回去 (好尴尬),然后再试一把 goto retry; }// Decrement successful after borrowing from side table. // This decrement cannot be the deallocating decrement - the side // table lock and has_sidetable_rc bit ensure that if everyone // else tried to -release while we worked, the last one would block. sidetable_unlock(); return false; } else { // Side table is empty after all. Fall-through to the dealloc path. } }// Really deallocate.if (slowpath(newisa.deallocating)) { ClearExclusive(&isa.bits); if (sideTableLocked) sidetable_unlock(); return overrelease_error(); // does not actually return } newisa.deallocating = true; if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry; if (slowpath(sideTableLocked)) sidetable_unlock(); __sync_synchronize(); if (performDealloc) { ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc); } return true; }

这里的逻辑主要有两块:
  1. 如果没有使用isa.extra_rc作引用计数,则调用sidetable_release,该方法会到side table中做计数减一,同时,会check 计数是否为0,如果为0,则调用对象的dealloc方法。
  2. 如果使用了isa.extra_rc作引用计数,则在isa.extra_rc中做引用计数减一,同时需要判断是否下溢出(carry > 0)
newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry); // extra_rc--

这里要注意处理下溢出的逻辑:
  1. 首先,下溢出是针对isa.extra_rc来说的。也就是启用了isa优化引用计数才会走到 underflow: 代码段。
  2. 造成isa.extra_rc下溢出其实有两个原因:borrow from side table or deallocate。要注意对这两个下溢出原因的不同处理。
dealloc 当对象引用计数为0时,会调用对象的dealloc方法,这在上面的release方法中,是通过
((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);

来调用的。
我们来看一下NSObjectdealloc方法是怎样实现的:
- (void)dealloc { _objc_rootDealloc(self); }void _objc_rootDealloc(id obj) { assert(obj); obj->rootDealloc(); }

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)) { // 如果没有weak引用 & 没有关联对象 & 没有c++析构 & 没有side table借位 // 就直接free assert(!sidetable_present()); free(this); } else { object_dispose((id)this); } }

id object_dispose(id obj) { if (!obj) return nil; objc_destructInstance(obj); // step 1. 先调用runtime的objc_destructInstance free(obj); // step 2. free 掉这个objreturn nil; }

/*********************************************************************** * 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); // 调用C++析构函数 if (assoc) _object_remove_assocations(obj); // 移除所有的关联对象,并将其自身从Association Manager的map中移除 obj->clearDeallocating(); // 清理相关的引用 }return obj; }

在对象dealloc的过程中,会根据当前对象isa_t的各个标志位,来做对应的清理工作,清理完毕后,会调用free(obj)来释放内存。
清理工作会在objc_destructInstance方法中进行,主要包括:
  • 如果有C++析构函数,调用C++析构
  • 如果有关联对象,调用_object_remove_assocations(obj)将关联在该对象的对象移除
  • 调用obj->clearDeallocating()方法,主要是(1)将weak 引用置为nil,并在weak_table_t中删除对象节点。(2)如果有side table计数借位,则side table中对应的节点移除
总结 本篇文章从[[NSObject alloc] init]方法说起,讲解了alloc,init背后的实现逻辑,以及OC中的所有权修饰符__strong, __weak。并讲述了autoreleasepool的背后实现。同时,分析了retain 和 release引用计数相关函数。
【Objective-C|Objective-C runtime机制(8)——OC对象从创建到销毁】最终,我们分析了对象dealloc所做的清理工作。

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