IOS 按比例裁剪图片

//NavView.m

  #define IMAGE_MAX_SIZE_WIDTH 640  

  #define IMAGE_MAX_SIZE_GEIGHT 1136

+(UIImage *)fitSmallImage:(UIImage *)image
{
    if (nil == image)
    {
        return nil;
    }
    if (image.size.width<IMAGE_MAX_SIZE_WIDTH && image.size.height<IMAGE_MAX_SIZE_GEIGHT)
    {
        return image;
    }
    CGSize size = [self fitsize:image.size];
    UIGraphicsBeginImageContext(size);
    CGRect rect = CGRectMake(0, 0, size.width, size.height);
    [image drawInRect:rect];
    UIImage *newing = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newing;
}

+ (CGSize)fitsize:(CGSize)thisSize
{
    if(thisSize.width == 0 && thisSize.height ==0)
        return CGSizeMake(0, 0);
    CGFloat wscale = thisSize.width/IMAGE_MAX_SIZE_WIDTH;
    CGFloat hscale = thisSize.height/IMAGE_MAX_SIZE_GEIGHT;
    CGFloat scale = (wscale>hscale)?wscale:hscale;
    CGSize newSize = CGSizeMake(thisSize.width/scale, thisSize.height/scale);
    return newSize;
}
使用eg:
把要操作的图片设为image(uiimage)
image=[NavView fitSmallImage:image];