iOS上直接縮小UIImageView的大小會產生鋸齒,可以先將其縮放後再使用.    

- (UIImage *)scaleToSize:(UIImage *)img size:(CGSize)size {
  UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);
  [img drawInRect:CGRectMake(0, 0, size.width, size.height)];
  UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return scaledImage;
}