文件Base64以后会翻倍的涨，例如一张52kb的图片 base64以后string文件大小为185kb，在通过string转回图片为135kb 图片转文字：
UIImage *_originImage = [UIImage imageNamed:@"0.jpg"];NSData *_data = UIImageJPEGRepresentation(_originImage, 1.0f);NSString *_encodedImageStr = [_data base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];[_encodedImageStr writeToFile:@"/Users/Singer/Documents/0.txt" atomically:YES encoding:NSUTF8StringEncoding error:nil];NSLog(@"===Encoded image:\n%@", _encodedImageStr);
文字转图片：
NSString *imageStr = [NSString stringWithContentsOfFile:@"/Users/Singer/Documents/0.txt" encoding:NSUTF8StringEncoding error:nil];NSData *d = [[NSData alloc]initWithBase64EncodedString:imageStr options:NSDataBase64DecodingIgnoreUnknownCharacters];UIImage *image = [UIImage imageWithData:d];UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(50, 50, 200, 200)];imageView.image = image;[self.view addSubview:imageView];[d writeToFile:@"/Users/Singer/Documents/1111.jpg" atomically:YES];
