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:
- 调用
calloc
方法,为类实例分配内存 - 调用obj->initInstanceIsa(cls, dtor)方法,初始化obj的isa
- 返回obj
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
中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
alloc
和new
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:
- 取出当前对象的isa.bits值
- isa.bits分别赋值给
oldisa
和newisa
- 根据isa_t的标志位
newisa.nonpointer
,来判断runtime是否只开启了isa优化。 - 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 - 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。 - 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
- 最后,调用
StoreExclusive
,更新对象的isa.bits
。
__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:- 取出引用对象对应的SideTable节点
SideTable *newTable
; - 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。 - 更新对象isa的
weakly_referenced bit
标志位,表明该对象被弱引用了。 - 将weak pointer指向对象
- 返回对象
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.
}
在
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_t
从weak_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
[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.似乎是说,当调用
If you don’t want a temporary object, you can also create an empty dictionary using alloc and init.
[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:- hot page存在且未满,调用
page->add(obj)
- hot page存在但已满, 调用
autoreleaseFullPage(obj, page)
- hot page不存在,调用
autoreleaseNoPage(obj)
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:
- 如果当前page存储的object已经不满半页,则讲page的child释放
- 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。
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_object
的rootRetainCount()
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:
- 判断object是否使用了isa优化
- 如果使用了isa优化,先取出
1 + bits.extra_rc
- 再判断是否需要读取side talbe(
if (bits.has_sidetable_rc)
) - 如果需要,则加上side table 中存储的retain count
- 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。
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:
- 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。 - 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:
- 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。 - 造成isa.extra_rc下溢出其实有两个原因:
borrow from side table
ordeallocate
。要注意对这两个下溢出原因的不同处理。
((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
来调用的。
我们来看一下
NSObject
的dealloc
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
【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。
推荐阅读
- EffectiveObjective-C2.0|EffectiveObjective-C2.0 笔记 - 第二部分
- 深入理解|深入理解 Android 9.0 Crash 机制(二)
- iOS|iOS runtime应用整理
- Objective-c
- iOS开发需要掌握的原理
- 轻量模块注意力机制ECA-Net(注意力模块+一维卷积)
- Java中的反射
- k8s|k8s(六)(配置管理与集群安全机制)
- 垃圾回收机制(第十二天)
- 在Objective-C的Category中使用属性的懒加载