- (NSData *)returnData:(NSString *)urlString{
    dispatch_queue_t concurrentQueue = 
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

    dispatch_sync(concurrentQueue,^{   // here return type showing error

        NSString *strUrl = urlString;
        if([strUrl length] == 0)
            return nil; // from this point i want to stop the execution of the block and return nil .

        // otherwise only continue the left operations



    });         
}

解决方法@H_301_6@

- (NSData *)returnData:(NSString *)urlString {
    dispatch_queue_t concurrentQueue = 
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);

    // Declare your local data outside the block.
    // `__block` specifies that the variable can be modified from within the block.
    __block NSData *localData = nil;

    dispatch_sync(concurrentQueue,^{
        // Do something with `localData`...
        localData = somethingWonderful;
    });

    // `localData` now contains your wonderful data.
    return localData;
}

块的格式(参数/返回类型)在函数的签名中指定.在这种情况下,dispatch_sync使用没有返回类型且没有参数的块.如果您想要使用更多变量,则需要在块外部声明它们,如上面的代码所示.