NSData和UIImage，NSString的相互转换

废话少说，上demo：

//NSData转换为UIImage

NSData *imageData = [NSData dataWithContentsOfFile: imagePath];

UIImage *image = [UIImage imageWithData: imageData];

//UIImage转换为NSData

NSData *imageData = UIImagePNGRepresentation(image);

//当我们需要将数据保存至文件或者进行网络传送时需将数据进行转换，转成01字符

//NSString转换为NSData

NSData *stringData = [string dataUsingEncoding:NSUTF8StringEncoding];

//NSData转换为NSString

NSString *string = [[NSString alloc] initWithData:stringData encoding:NSUTF8StringEncoding];