NSData轉換成NSString：

NSString *msg = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

NSString轉換成NSData

NSData* xmlData = [str dataUsingEncoding:NSUTF8StringEncoding];

UIImage轉換成NSString

NSData *data = UIImageJPEGRepresentation(img, 1.0);

NSString *pictureDataString = [data base64Encoding];

NSString轉換成UIImage

NSData *aData = [NSData dataWithBase64EncodedString:str];
UIImage *image = [UIImage imageWithData:aData];