OC代碼

在main.m文件中定義ZYPerson類，分別有atomic修飾的屬性name和nonatomic修飾的屬性gender

@interface ZYPerson : NSObject

@property (atomic, copy) NSString *name;
@property (nonatomic, copy) NSString *gender;

@end

@implementation ZYPerson

@end

通過終端命令將main.m里的內(nèi)容轉(zhuǎn)換成c++代碼
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m

setter

先來看main.cpp中name和gender的setter方法

static void _I_ZYPerson_setName_(ZYPerson * self, SEL _cmd, NSString *name) {
    objc_setProperty (
                      self,
                      _cmd,
                      __OFFSETOFIVAR__(struct ZYPerson, _name),
                      (id)name,
                      1,
                      1);
}

static void _I_ZYPerson_setGender_(ZYPerson * self, SEL _cmd, NSString *gender) {
    objc_setProperty (
                      self,
                      _cmd,
                      __OFFSETOFIVAR__(struct ZYPerson, _gender),
                      (id)gender,
                      0,
                      1);
}

發(fā)現(xiàn)objc_setProperty中只有第5個(gè)參數(shù)不同，猜測跟atomic有關(guān)。
去runtime源碼中搜索方法objc_setProperty，可以發(fā)現(xiàn)：

• 第5個(gè)參數(shù)的確是用來判斷atomic的。
• 第6個(gè)參數(shù)shouldCopy，從實(shí)現(xiàn)來看是用來判斷copy關(guān)鍵字的

#define MUTABLE_COPY 2
void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy) 
{
    bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
    bool mutableCopy = (shouldCopy == MUTABLE_COPY);
    reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}

objc_setProperty內(nèi)部調(diào)用了真正的屬性賦值方法reallySetProperty，前6個(gè)參數(shù)和objc_setProperty一致，最后一個(gè)參數(shù)用來判斷shouldCopy是否等于MUTABLE_COPY=2。

static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
    if (offset == 0) {
        object_setClass(self, newValue);
        return;
    }

    id oldValue;
    id *slot = (id*) ((char*)self + offset);

    if (copy) {
        newValue = [newValue copyWithZone:nil];
    } else if (mutableCopy) {
        newValue = [newValue mutableCopyWithZone:nil];
    } else {
        if (*slot == newValue) return;
        newValue = objc_retain(newValue);
    }

    if (!atomic) {
        oldValue = *slot;
        *slot = newValue;
    } else {
        spinlock_t& slotlock = PropertyLocks[slot];
        slotlock.lock();
        oldValue = *slot;
        *slot = newValue;        
        slotlock.unlock();
    }

    objc_release(oldValue);
}

可以看出：不管是copy、mutableCopy、strong、retain關(guān)鍵詞修飾的OC對象，newValue都會(huì)retain一次，copy是操作內(nèi)部retain的。

• nonatomic：1. 是將新的屬性值直接賦值給對應(yīng)屬性； 2. release原值
• atomic：在賦值前后進(jìn)行spinlock_t加鎖/解鎖操作，其他操作跟nonatomic是一致的。

using spinlock_t = mutex_tt<LOCKDEBUG>;
class mutex_tt : nocopy_t {
    os_unfair_lock mLock;
}

查找源碼發(fā)現(xiàn)spinlock_t是os_unfair_lock，是自旋鎖。

getter

同樣getter方法也是如此，atomic修飾的屬性進(jìn)行加鎖處理，說明atomic屬性在setter和getter方法中是利用自旋鎖保證線程安全的。

static NSString * _I_ZYPerson_name(ZYPerson * self, SEL _cmd) { 
    typedef NSString * _TYPE;
return (_TYPE)objc_getProperty(
    self, 
    _cmd, 
    __OFFSETOFIVAR__(struct ZYPerson, _name), 
    1); 
}

id objc_getProperty(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
    if (offset == 0) {
        return object_getClass(self);
    }

    // Retain release world
    id *slot = (id*) ((char*)self + offset);
    if (!atomic) return *slot;
        
    // Atomic retain release world
    spinlock_t& slotlock = PropertyLocks[slot];
    slotlock.lock();
    id value = objc_retain(*slot);
    slotlock.unlock();
    
    // for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
    return objc_autoreleaseReturnValue(value);
}

注意：atomic保證setter和getter方法是線程安全的，但是不能保證屬性在使用過程中的安全。即person.name=@"001"是安全的，但是不能保證[person.name stringByAppendingString: @"test"];也是安全的。

閱讀源碼過程中涉及到的其他方法

判斷一個(gè)實(shí)例對象是否是TaggedPointer

#if (TARGET_OS_OSX || TARGET_OS_IOSMAC) && __x86_64__
    // 64-bit Mac - tag bit is LSB
#   define OBJC_MSB_TAGGED_POINTERS 0
#else
    // Everything else - tag bit is MSB
#   define OBJC_MSB_TAGGED_POINTERS 1
#endif

#if OBJC_MSB_TAGGED_POINTERS
#   define _OBJC_TAG_MASK (1UL<<63)
#else
#   define _OBJC_TAG_MASK 1UL
#endif

static inline bool 
_objc_isTaggedPointer(const void * _Nullable ptr)
{
    return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}

修改對象的isa指針指向新的Class對象

Class object_setClass(id obj, Class cls)
{
    if (!obj) return nil;
    if (!cls->isFuture()  &&  !cls->isInitialized()) {
        lookUpImpOrNil(nil, @selector(initialize), cls, LOOKUP_INITIALIZE);
    }

    return obj->changeIsa(cls);
}