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NSLog(@"echo>>>>>>%@",echo);
NSLog(@"echo>>>>>>%d",echo==nil);
NSLog(@"echo>>>>>>%d",[echo length]);
016-06-14 15:13:29.739 OA_App[4990:1558049] echo>>>>>><null>
2016-06-14 15:13:29.740 OA_App[4990:1558049] echo>>>>>>0 //这里不应该是1 true吗?
2016-06-14 15:13:29.740 OA_App[4990:1558049] -[NSNull length]: unrecognized selector sent to instance 0x34c15850