iOS 后台返回的json数据中包含json字符串

摘要：json字符串转对象-----先把json字符串UTF8编码成data，再用data转成对象

NSArray * arr = [self jsonObject:model.MESSAGE_CONTENT];

• (id)jsonObject:(NSString *)str {
  str = [str stringByReplacingOccurrencesOfString:@"\n" withString:@"\n"];
  str = [str stringByReplacingOccurrencesOfString:@"\r" withString:@"\r"];

  NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];
  return [self jsonObjectFromData:data];
  }

• (id)jsonObjectFromData:(NSData *)data {
  NSError *error = nil;
  return [NSJSONSerialization JSONObjectWithData:data
  options:0
  error:&error];
  }