线程安全的计数

#import <libkern/OSAtomic.h>
/// a thread safe incrementing counter.
@interface _YYTextSentinel : NSObject
/// Returns the current value of the counter.
@property (atomic, readonly) int32_t value;
/// Increase the value atomically. @return The new value.
- (int32_t)increase;
@end

@implementation _YYTextSentinel {
    int32_t _value;
}
- (int32_t)value {
    return _value;
}
- (int32_t)increase {
    return OSAtomicIncrement32(&_value);
}
@end