OC代码
在main.m文件中定义ZYPerson类，分别有atomic修饰的属性name和nonatomic修饰的属性gender
@interface ZYPerson : NSObject
@property (atomic, copy) NSString *name;
@property (nonatomic, copy) NSString *gender;
@end
@implementation ZYPerson
@end
通过终端命令将main.m里的内容转换成c++代码
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m
setter
先来看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);
}
发现objc_setProperty中只有第5个参数不同，猜测跟atomic有关。
去runtime源码中搜索方法objc_setProperty，可以发现：
- 第5个参数的确是用来判断atomic的。
- 第6个参数shouldCopy，从实现来看是用来判断copy关键字的
#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内部调用了真正的属性赋值方法reallySetProperty，前6个参数和objc_setProperty一致，最后一个参数用来判断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关键词修饰的OC对象，newValue都会retain一次，copy是操作内部retain的。
- nonatomic：1. 是将新的属性值直接赋值给对应属性； 2. release原值
- atomic：在赋值前后进行spinlock_t加锁/解锁操作，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;
}
查找源码发现spinlock_t是os_unfair_lock，是自旋锁。
getter
同样getter方法也是如此，atomic修饰的属性进行加锁处理，说明atomic属性在setter和getter方法中是利用自旋锁保证线程安全的。
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保证setter和getter方法是线程安全的，但是不能保证属性在使用过程中的安全。即person.name=@"001"是安全的，但是不能保证[person.name stringByAppendingString: @"test"];也是安全的。
阅读源码过程中涉及到的其他方法
判断一个实例对象是否是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;
}
修改对象的isa指针指向新的Class对象
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);
}
