OC高效率52之不要使用dispatch_get_current_queue

#import "ViewController.h"

@interface ViewController ()
@property (nonatomic ,strong) NSString *someString;
@property dispatch_queue_t syncQueue;
@end

@implementation ViewController
/**
 *  获取方法可能会死锁
 */
-(NSString *)someString{
    __block NSString *localSomeString;
    dispatch_sync(_syncQueue, ^{
        localSomeString = self.someString;
    });
    return localSomeString;
}

-(void)setSomeString:(NSString *)someString{
    dispatch_async(_syncQueue, ^{
        self.someString = someString;
    });

}
- (void)viewDidLoad {
    [super viewDidLoad];
    //dispatch_get_current_queue 判断当前代码正在那个队列上执行，iOS6.0后已弃用
    //因为队列间有层级关系，因此“检查当前队列是否执行同步派发所用的队列”这种办法并不总有效。
    
    /**
     *  ”队列特有数据“
     *
     *  @return 任意数据以键值对的形式关联到队列里
     */
    dispatch_queue_t queueA = dispatch_queue_create("queueA.Jie", NULL);
    dispatch_queue_t queueB = dispatch_queue_create("queueB.Zou", NULL);
    
    dispatch_set_target_queue(queueA, queueB);
    
    static int kQueueSpecific;
    CFStringRef queueSpecificValue = CFSTR("queueA");
    dispatch_queue_set_specific(queueA, &queueSpecificValue, (void *)queueSpecificValue, (dispatch_function_t)CFRelease);
        //queueA 待设置数据的队列  &queueSpecificValue  (void *)queueSpecificValue 键和值   (dispatch_function_t)CFRelease 析构函数（只能带一个指针参数且返回值必须是void）调用CFRelease“参数”以清理旧值
    dispatch_sync(queueB, ^{
        dispatch_block_t block = ^{NSLog(@"No deadLock");};
        CFStringRef retriedValue = dispatch_get_specific(&kQueueSpecific);
        if (retriedValue){
            block();
        }else{
        dispatch_sync(queueA, ^{
            
        });
        }
    });
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end