objc4

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));
}