在某些場(chǎng)景下為了提高效率，就需要使用多讀單寫，
這就需要我們使用到GCD中的兩個(gè)柵欄函數(shù)

dispatch_barrier_sync(queue,  ^{
     //需等待這里的block任務(wù)執(zhí)行完，才能執(zhí)行后面異步任務(wù)
 })

dispatch_barrier_async(queue,  ^{
   //不需要等待block任務(wù)執(zhí)行完，就能執(zhí)行后面異步任務(wù)
})

這兩者區(qū)別是
1、barrier_sync會(huì)阻塞它之后的任務(wù)的入隊(duì)，必須等到 barrier_sync 任務(wù)執(zhí)行完畢，才會(huì)把后面的異步任務(wù)添加到并發(fā)隊(duì)列中，
2、而 barrier_async不需要等自身的 block 執(zhí)行完成就可以把后面的任務(wù)添加到隊(duì)列中。

舉例：
以讀取圖片緩存為例來說明：

@property(nonatomic,strong)NSMutableDictionary *imageDic;//假設(shè)用于保存圖片緩存
@property(nonatomic,strong)dispatch_queue_t concurrent_queue;//并發(fā)隊(duì)列

-(void)initProperties{
    //創(chuàng)建一個(gè)并發(fā)隊(duì)列
    _concurrent_queue = dispatch_queue_create("mutliRead_singleWrite", DISPATCH_QUEUE_CONCURRENT);
    //創(chuàng)建字典
    _imageDic = [NSMutableDictionary dictionary];
}

//可以多線程進(jìn)行讀
-(UIImage *)getImageWithKey:(NSString *)url{
//    __block UIImage *image;
//    dispatch_async(self.concurrent_queue, ^{
//        image = [self.imageDic valueForKey:url];
//    });
    
    UIImage *image;
    image = [self.imageDic valueForKey:url];
    return image;
}
//只能單線程進(jìn)行寫
-(void)setImageWithKey:(NSString *)key withImg:(UIImage *)image{
    
    dispatch_sync(self.concurrent_queue, ^{
        [self.imageDic setValue:image forKey:key];
    });
}

//防止多線程同時(shí)訪問資源，每次只能讓一個(gè)線程訪問進(jìn)行讀寫
-(UIImage *)getSafeImage:(NSString *)url{
    UIImage *image;
    @synchronized (self.imageDic) {
        image = [self.imageDic valueForKey:url];
    }
    return image;
}

-(void)setSafeImageWithKey:(NSString *)key withImg:(UIImage *)image{
    @synchronized (self.imageDic) {
        [self.imageDic setValue:image forKey:key];
    }
}