iOS 压缩图片分辨率

+ (UIImage *)imageWithMaxSide:(CGFloat)length sourceImage:(UIImage *)image
{
    CGFloat scale = [[UIScreen mainScreen] scale];
    CGSize imgSize = SizeReduce(image.size, length);
    UIImage *img = nil;

    UIGraphicsBeginImageContextWithOptions(imgSize, YES, scale);  // 创建一个 bitmap context

    [image drawInRect:CGRectMake(0, 0, imgSize.width, imgSize.height)
            blendMode:kCGBlendModeNormal alpha:1.0];              // 将图片绘制到当前的 context 上

    img = UIGraphicsGetImageFromCurrentImageContext();            // 从当前 context 中获取刚绘制的图片 
    UIGraphicsEndImageContext();

    return img;
}


static inline
CGSize SizeReduce(CGSize size, CGFloat limit)   // 按比例减少尺寸
{
    CGFloat max = MAX(size.width, size.height);
    if (max < limit) {
        return size;
    }

    CGSize imgSize;
    CGFloat ratio = size.height / size.width;

    if (size.width > size.height) {
        imgSize = CGSizeMake(limit, limit*ratio);
    } else {
        imgSize = CGSizeMake(limit/ratio, limit);
    }

    return imgSize;
}