从NSObject说起
@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
Class isa OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}
点击Class 我们可以明显看到,iOS中的Class是一个objc_class结构体
typedef struct objc_class *Class;
runtime.h里关于objc_class的声明,其中我们可以清楚看到部分属性
struct objc_class {
///isa指针
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
///父类指针
Class _Nullable super_class OBJC2_UNAVAILABLE;
const char * _Nonnull name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
///属性列表
struct objc_ivar_list * _Nullable ivars OBJC2_UNAVAILABLE;
///方法列表
struct objc_method_list * _Nullable * _Nullable methodLists OBJC2_UNAVAILABLE;
///缓存数据
struct objc_cache * _Nonnull cache OBJC2_UNAVAILABLE;
///协议列表
struct objc_protocol_list * _Nullable protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
之后的底层实现就需要到官网去下载objc4源码查看具体实现
以上代码在objc里的实现,而objc_object 就是我们开发见到的id对象
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
};
/// A pointer to an instance of a class.
typedef struct objc_object *id;
可以在objc-pivate.h里看到具体实现,包括MRC的retain,release方法,而其中只有一个属性,就是isa,
struct objc_object {
private:
isa_t isa;
public:
Class getIsa();
void initIsa(Class cls /*nonpointer=false*/);
void initClassIsa(Class cls /*nonpointer=maybe*/);
void initProtocolIsa(Class cls /*nonpointer=maybe*/);
void initInstanceIsa(Class cls, bool hasCxxDtor);
id retain();
void release();
isa_t的声明,可以看到isa_t是一个共用体,包含了cls 和bits两个属性
union isa_t {
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#if defined(ISA_BITFIELD)
struct {
ISA_BITFIELD; // defined in isa.h
};
#endif
};
objc_class 部分声明,可以看到class继承与objc_object,本质也是对象
struct objc_class : objc_object {
Class superclass; //父类指针
cache_t cache; // 指针和 vtable
class_data_bits_t bits; // 存储类的方法、属性和遵循的协议等信息的地方
}
NSObject.mm里的retain操作
如果是Tagged Pointer
__attribute__((aligned(16), flatten, noinline))
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();
}
//objc_object执行retain
return ((id(*)(objc_object *, SEL))objc_msgSend)(this, @selector(retain));
}
//rootRetain方法
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)) {
ClearExclusive(&isa.bits);
if (rawISA()->isMetaClass()) return (id)this;
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;
}
uintptr_t carry;
newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++
if (slowpath(carry)) {
// newisa.extra_rc++ overflowed
if (!handleOverflow) {
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();
sideTableLocked = true;
transcribeToSideTable = true;
newisa.extra_rc = RC_HALF;
newisa.has_sidetable_rc = true;
}
} while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));
if (slowpath(transcribeToSideTable)) {
//复制保留计数的另一半到 sidetable.
sidetable_addExtraRC_nolock(RC_HALF);
}
if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
return (id)this;
}
release操作
__attribute__((aligned(16), flatten, noinline))
void
objc_release(id obj)
{
if (!obj) return;
if (obj->isTaggedPointer()) return;
return obj->release();
}
inline void
objc_object::release()
{
ASSERT(!isTaggedPointer());
if (fastpath(!ISA()->hasCustomRR())) {
rootRelease();
return;
}
((void(*)(objc_object *, SEL))objc_msgSend)(this, @selector(release));
}
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
if (isTaggedPointer()) return false;
bool sideTableLocked = false;
isa_t oldisa;
isa_t newisa;
retry:
do {
//加载isa
oldisa = LoadExclusive(&isa.bits);
newisa = oldisa;
if (slowpath(!newisa.nonpointer)) {
ClearExclusive(&isa.bits);
if (rawISA()->isMetaClass()) return false;
if (sideTableLocked) sidetable_unlock();
return sidetable_release(performDealloc);
}
// don't check newisa.fast_rr; we already called any RR overrides
uintptr_t carry;
//将 isa 中的引用计数减一
newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry); // extra_rc--
if (slowpath(carry)) {
// don't ClearExclusive()
goto underflow;
}
} while (slowpath(!StoreReleaseExclusive(&isa.bits,
oldisa.bits, newisa.bits)));
//调用 StoreReleaseExclusive 方法保存新的 isa
if (slowpath(sideTableLocked)) sidetable_unlock();
return false;
underflow:
// newisa.extra_rc-- 溢出: 从 side table 移除或dealloc
// abandon newisa to undo the decrement
newisa = oldisa;
//如果存在clear
if (slowpath(newisa.has_sidetable_rc)) {
if (!handleUnderflow) {
ClearExclusive(&isa.bits);
return rootRelease_underflow(performDealloc);
}
// Transfer retain count from side table to inline storage.
if (!sideTableLocked) {//清空bit
ClearExclusive(&isa.bits);
sidetable_lock();
sideTableLocked = true;
// Need to start over to avoid a race against
// the nonpointer -> raw pointer transition.
goto retry;
}
// 试着把side table上的引用保留计数
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) {
//为了避免冲突，has_sidetable_rc必须保持设置，即使边表计数现在是零。
newisa.extra_rc = borrowed - 1; // redo the original decrement too
bool stored = StoreReleaseExclusive(&isa.bits,
oldisa.bits, newisa.bits);
if (!stored) {
// Inline 更新失败。
// 重新尝试.这可以防止LL/SC上的锁
// 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) {
// 更新失败
// 将 计数 返还给 side table.
sidetable_addExtraRC_nolock(borrowed);
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();
__c11_atomic_thread_fence(__ATOMIC_ACQUIRE);
if (performDealloc) {
((void(*)(objc_object *, SEL))objc_msgSend)(this, @selector(dealloc));
}
return true;
}
autorelease
__attribute__((aligned(16), flatten, noinline))
id
objc_autorelease(id obj)
{
if (!obj) return obj;
if (obj->isTaggedPointer()) return obj;
return obj->autorelease();
}
inline id
objc_object::autorelease()
{
ASSERT(!isTaggedPointer());
if (fastpath(!ISA()->hasCustomRR())) {
return rootAutorelease();
}
return ((id(*)(objc_object *, SEL))objc_msgSend)(this, @selector(autorelease));
}
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