iOS11以后才提供了相册识别条形码的功能。但是识别的时候，对清晰度和图片大小以及条形码在图片的位置都有严格的要求，所以经常失败。

解决思路：

1、压缩图片到合适的尺寸。
2、保证图片清晰。
3、为了保证条形码在中间位置，针对长图进行裁剪后的三次识别。

最终可以做到正常的条形码可以识别，长图的条形码在图顶部和尾部的也能识别。 但是拍摄模糊的就无能为力了。

/// 图片缩放到指定大小尺寸
- (UIImage *)scaleToSize:(UIImage *)img size:(CGSize)size {
    // 创建一个bitmap的context
    // 并把它设置成为当前正在使用的context
    UIGraphicsBeginImageContext(size);
    // 绘制改变大小的图片
    [img drawInRect:CGRectMake(0, 0, size.width, size.height)];
    // 从当前context中创建一个改变大小后的图片
    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    // 使当前的context出堆栈
    UIGraphicsEndImageContext();
    // 返回新的改变大小后的图片
    return scaledImage;
}

/*
  图片剪裁接口
 * 1.rect超过image.size则返回原图
 * 2.rect可以是size的任何一个子集，比如取中间半区域 CG
*/
- (UIImage *)imageByCropToRect:(CGRect)rect originImg:(UIImage *)originImg {
    rect.origin.x *= originImg.scale;
    rect.origin.y *= originImg.scale;
    rect.size.width *= originImg.scale;
    rect.size.height *= originImg.scale; // pt -> px (point -> pixel)
    if (rect.size.width <= 0 || rect.size.height <= 0) return nil;
    CGImageRef imageRef = CGImageCreateWithImageInRect(originImg.CGImage, rect);
    UIImage *image = [UIImage imageWithCGImage:imageRef scale:originImg.scale orientation:originImg.imageOrientation];
    CGImageRelease(imageRef);
    return image;
}

/// 获取缩放后的图片 -- 缩放到不太大的图片
- (UIImage *)getScaledImage:(UIImage *)originImg {
    UIImage *img = originImg;
    CGFloat maxSize = [UIScreen mainScreen].scale * 256.0; // 图片太小和太大都识别不了条形码
    if (img.size.width > img.size.height) {
        if (img.size.width < maxSize) {
            return originImg;
        }
        img = [self scaleToSize:originImg size:CGSizeMake(maxSize, maxSize * (originImg.size.height/originImg.size.width))];
    } else {
        if (img.size.height < maxSize) {
            return originImg;
        }
        img = [self scaleToSize:originImg size:CGSizeMake(maxSize * (originImg.size.width/originImg.size.height), maxSize)];
    }
    return img;
}

/// 获取上半部分图片
- (UIImage *)getScaledImageUp:(UIImage *)originImg {
    UIImage *img = originImg;
    CGFloat maxSize = [UIScreen mainScreen].bounds.size.height; // 图片太小和太大都识别不了条形码
    CGFloat width = originImg.size.width;
    CGFloat height = originImg.size.height;
    if (img.size.width > img.size.height) {
        width = maxSize/2.0;
        height = maxSize * (originImg.size.height/originImg.size.width);
    } else {
        height = maxSize/2.0;
        width = maxSize * (originImg.size.height/originImg.size.width);
    }
    img = [self imageByCropToRect:CGRectMake(0, 0, width, height) originImg:img];
    return img;
}

/// 获取下半部分图片
- (UIImage *)getScaledImageDown:(UIImage *)originImg {
    UIImage *img = originImg;
    CGFloat maxSize = [UIScreen mainScreen].bounds.size.height; // 图片太小和太大都识别不了条形码
    CGFloat width = originImg.size.width;
    CGFloat height = originImg.size.height;
    if (img.size.width > img.size.height) { // 宽图
        width = maxSize/2.0;
        height = maxSize * (originImg.size.height/originImg.size.width);
        img = [self imageByCropToRect:CGRectMake(width, 0, width, height) originImg:img];
    } else { // 高图
        height = maxSize/2.0;
        width = maxSize * (originImg.size.height/originImg.size.width);
        img = [self imageByCropToRect:CGRectMake(0, height, width, height) originImg:img];
    }
    return img;
}