atomic

atomic原子性，是property的关键字之一，会对属性的get，set操作加锁，保证读写的安全

例子🌰：

@interface ViewController ()

@property (nonatomic, copy)NSString *testAtomicStr;

@end


@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    
    dispatch_queue_t queueGlobal = dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT);
    for (int i = 0; i < 1000; i ++) {
        dispatch_async(queueGlobal, ^{
            self.testAtomicStr = [NSString stringWithFormat:@"testAtomicStr%d",i];
            NSLog(@"atomic test in asyn: %@",self.testAtomicStr);
            
        });
    }
    
}

上面代码运行会crash：Thread 9: EXC_BAD_ACCESS (code=EXC_I386_GPFLT)

原因：

set操作会先release旧值再赋值新值，在多线程下，release的值可能被其他线程读取，造成空指针错误

修复：

把nonatomic改为atomic就行，atomic可以理解为在set，get操作时候加锁，效果类似下面代码，但不完全一样，因为atomic的锁只针对set，get操作

    dispatch_queue_t queueGlobal = dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT);
    for (int i = 0; i < 1000; i ++) {
        dispatch_async(queueGlobal, ^{
            @synchronized (self) {
                self.testAtomicStr = [NSString stringWithFormat:@"testAtomicStr%d",i];
                NSLog(@"atomic test in asyn: %@",self.testAtomicStr);
            }
            
        });
    }

那atomic就是安全的吗？

答案是否定的

例子🌰

@interface ViewController ()

@property (atomic, copy)NSString *testAtomicStr;

@end


@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    
    dispatch_queue_t queueGlobal = dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT);
    for (int i = 0; i < 1000; i ++) {
        dispatch_async(queueGlobal, ^{
            self.testAtomicStr = [NSString stringWithFormat:@"testAtomicStr%d",i];
            NSLog(@"atomic test in asyn: %@",self.testAtomicStr);
            
        });
    }
    
}

上面代码我们预期结果应该是下标0-999按顺序输出，实际结果是不符合预期的👇👇👇

2023-08-15 10:48:51.326968+0800 CTMVVMDemo1113[17653:2381770] atomic test in asyn: testAtomicStr1
2023-08-15 10:48:51.326968+0800 CTMVVMDemo1113[17653:2381764] atomic test in asyn: testAtomicStr1
2023-08-15 10:48:51.326975+0800 CTMVVMDemo1113[17653:2381766] atomic test in asyn: testAtomicStr3
2023-08-15 10:48:51.326968+0800 CTMVVMDemo1113[17653:2381769] atomic test in asyn: testAtomicStr2
2023-08-15 10:48:51.326991+0800 CTMVVMDemo1113[17653:2381768] atomic test in asyn: testAtomicStr4
2023-08-15 10:48:51.327031+0800 CTMVVMDemo1113[17653:2381765] atomic test in asyn: testAtomicStr5
2023-08-15 10:48:51.327089+0800 CTMVVMDemo1113[17653:2381773] atomic test in asyn: testAtomicStr6
2023-08-15 10:48:51.327140+0800 CTMVVMDemo1113[17653:2381770] atomic test in asyn: testAtomicStr8
2023-08-15 10:48:51.327163+0800 CTMVVMDemo1113[17653:2381774] atomic test in asyn: testAtomicStr7
2023-08-15 10:48:51.327169+0800 CTMVVMDemo1113[17653:2381775] atomic test in asyn: testAtomicStr9
2023-08-15 10:48:51.327175+0800 CTMVVMDemo1113[17653:2381764] atomic test in asyn: testAtomicStr10

结论atomic不完全安全，我们要结合具体场景合理使用