atomic

@property (atomic,assign) int age;

意味著這個(gè)屬性是原子性操作,也就是保證setter和getter是線程同步的.

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);
}

源碼上來看如果是atomic,賦值的前后加了自旋鎖spinlock

它并不能保證使用屬性的過程是線程安全的.

        MyPerson *person = [MyPerson new];
        person.datas = [NSMutableArray new];
        
        //如果這邊不加鎖,多線程情況下并不能保證線程同步.atomic僅保證了getter方法的線程同步.addObject方法不是線程同步
        //加鎖
        [person.datas addObject:@"1"];
        [person.datas addObject:@"2"];
        [person.datas addObject:@"3"];
        //解鎖

IO操作

讀寫安全方案

• 同一時(shí)間.只能有一個(gè)線程進(jìn)行寫的操作
• 同一時(shí)間,允許有多個(gè)線程進(jìn)行讀的操作
• 同一時(shí)間,不允許既有寫的操作又有讀的操作

pthread_rwlock:讀寫鎖
dispatch_barrier_async 異步柵欄調(diào)用

@interface ViewController ()

@property (nonatomic,assign) pthread_rwlock_t lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    pthread_rwlock_init(&_lock, NULL);
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    for (int i = 0; i < 10; i++) {
        
        dispatch_async(queue, ^{
            [self read];
        });
        
        dispatch_async(queue, ^{
            [self write];
        });
    }
}


- (void)read
{
    pthread_rwlock_rdlock(&_lock);
    sleep(1);
    NSLog(@"%s",__func__);
    
    pthread_rwlock_unlock(&_lock);
}

- (void)write
{
    pthread_rwlock_wrlock(&_lock);
    sleep(1);
    NSLog(@"%s",__func__);
    
    pthread_rwlock_unlock(&_lock);
    
}

@end

dispatch_barrier_sync

• 這個(gè)函數(shù)傳入的并發(fā)隊(duì)列必須是通過dispatch_queue_create創(chuàng)建的.
• 如果傳入的是個(gè)串行or全局的并發(fā)隊(duì)列,效果等同于dispatch_async

    dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);
    for (int i = 0; i < 50; i++) {
        
        dispatch_async(queue2, ^{
            [self read];
        });
        
        
        dispatch_async(queue2, ^{
            [self read];
        });
        
        
        dispatch_async(queue2, ^{
            [self read];
        });
        
        
        
        dispatch_barrier_sync(queue2, ^{
            [self write];
        });
    }


- (void)read
{
//    pthread_rwlock_rdlock(&_lock);
    sleep(1);
    NSLog(@"%s",__func__);
    
//    pthread_rwlock_unlock(&_lock);
}

- (void)write
{
//    pthread_rwlock_wrlock(&_lock);
    sleep(1);
    NSLog(@"%s",__func__);
    
//    pthread_rwlock_unlock(&_lock);
    
}