iOS 图片压缩方法的示例代码

压缩图片尺寸

与之前类似，比较容易想到的方法是，通过循环逐渐减小图片尺寸，直到图片稍小于指定大小(maxLength)。具体代码省略。同样的问题是循环次数多，效率低，耗时长。可以用二分法来提高效率，具体代码省略。这里介绍另外一种方法，比二分法更好，压缩次数少，而且可以使图片压缩后刚好小于指定大小(不只是 < maxLength， > maxLength * 0.9)。

+ (UIImage *)compressImageSize:(UIImage *)image toByte:(NSUInteger)maxLength {
 UIImage *resultImage = image;
 NSData *data = UIImageJPEGRepresentation(resultImage, 1);
 NSUInteger lastDataLength = 0;
 while (data.length > maxLength && data.length != lastDataLength) {
  lastDataLength = data.length;
  CGFloat ratio = (CGFloat)maxLength / data.length;
  CGSize size = CGSizeMake((NSUInteger)(resultImage.size.width * sqrtf(ratio)), (NSUInteger)(resultImage.size.height * sqrtf(ratio))); // Use NSUInteger to prevent white blank
  UIGraphicsBeginImageContext(size);
  // Use image to draw (drawInRect:), image is larger but more compression time
  // Use result image to draw, image is smaller but less compression time
  [resultImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
  resultImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  data = UIImageJPEGRepresentation(resultImage, 1);
 }
 return resultImage;
}