iOS如何改变图片的尺寸？

+(UIImage *)image:(UIImage*)image byScalingToSize:(CGSize)targetSize {
UIImage *sourceImage = image;
UIImage *newImage = nil;

UIGraphicsBeginImageContext(targetSize);

CGRect thumbnailRect = CGRectZero;
thumbnailRect.origin = CGPointZero;
thumbnailRect.size.width  = targetSize.width;
thumbnailRect.size.height = targetSize.height;

[sourceImage drawInRect:thumbnailRect];

newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

return newImage ;
}