UIGraphicsImageRenderer图片渲染优化

回顾一个经典面试题

加载网络图片，或者从相册读取图片的时候，如果ImageView的本身就是固定的200x200，加载800x800的图片会有什么问题？

答案：载入800x800的图片用到200x200的控件上是很浪费内存。需要消耗的内存大小800x800x4bit。（解决方案：在使用前把图片调整到需要的大小）

方法如下：

func imageWithImageScaledToSize(image:UIImage,newSize:CGSize) -> UIImage {
        UIGraphicsBeginImageContext(newSize)
        image.draw(in: CGRect.init(x: 0, y: 0, width: newSize.width, height: newSize.height))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!
    }
    
    func imageWithImageResizerFactor(image:UIImage,scale:CGFloat) -> UIImage {
        let originalSize = image.size
        let newSize = CGSize.init(width: originalSize.width*scale, height: originalSize.height*scale)
        UIGraphicsBeginImageContext(newSize)
        image.draw(in: CGRect.init(x: 0, y: 0, width: newSize.width, height: newSize.height))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!
    }

iOS10新增UIGraphicsImageRenderer来代替UIGraphicsBeginImageContext
UIGraphicsImageRenderer官方文档的解释：一个支持创建核心图像的渲染器。

        ggImageView = [[UIImageView alloc]initWithFrame:CGRectMake(0, 300, 400, 300)];
        [self.view addSubview:ggImageView];
        NSURL *imgUrl = [[NSBundle mainBundle] URLForResource:@"DSC_0645" withExtension:@"jpeg"];
        UIImage *ggImage = [self resiImage:imgUrl size:ggImageView.bounds.size];
        ggImageView.image = ggImage;
//常用的加载大图方法
 //        ggImageView.image = [UIImage imageWithContentsOfFile:imgUrl.path];



//绘制UIImage
- (UIImage*)resiImage:(NSURL*)url size:(CGSize)size{
    UIImage *testIm = [UIImage imageWithContentsOfFile:url.path];
    UIGraphicsImageRenderer *re = [[UIGraphicsImageRenderer alloc]initWithSize:size];
    return [re imageWithActions:^(UIGraphicsImageRendererContext * _Nonnull rendererContext) {
        [testIm drawInRect:CGRectMake(0, 0, size.width, size.height)];
    }];
}

加载10兆左右的大图速度确实提升很多。