iOS图片加载优化---下采样

最近在优化广告SDK，有需求把下载的大图展示出来，不能太影响性能。

参考官方文档Image and Graphics Best Practices

- (UIImage *)downThumbNailImgUrl:(NSURL *)imgURL
                    toPointSize:(CGSize)pointSize
                          scale:(CGFloat)scale {

    CFStringRef key[1];
    key[0] = kCGImageSourceShouldCache;
    CFTypeRef value[1];
    value[0] = (CFTypeRef)kCFBooleanFalse;

    CFDictionaryRef imageSourceOption = CFDictionaryCreate(NULL,
                                                           (const void **) key,
                                                           (const void **) value,
                                                           1,
                                                           &kCFTypeDictionaryKeyCallBacks,
                                                           &kCFTypeDictionaryValueCallBacks);
    CGImageSourceRef imageSourceRef = CGImageSourceCreateWithURL((__bridge CFURLRef)imgURL, imageSourceOption);

    CFMutableDictionaryRef mutOption = CFDictionaryCreateMutable(NULL,
                                                                 4,
                                                                 &kCFTypeDictionaryKeyCallBacks,
                                                                 &kCFTypeDictionaryValueCallBacks);


    CGFloat maxDimension = MAX(pointSize.width, pointSize.height) * scale;
    NSNumber *maxDimensionNum = [NSNumber numberWithFloat:maxDimension];

    // · kCGImageSourceCreateThumbnailFromImageAlways
    //这个选项控制是否生成缩略图（没有设为true的话 kCGImageSourceThumbnailMaxPixelSize 以及 CGImageSourceCreateThumbnailAtIndex不会起作用）默认为false，所以需要设置为true
    CFDictionaryAddValue(mutOption, kCGImageSourceCreateThumbnailFromImageAlways, kCFBooleanTrue);//

    // · kCGImageSourceShouldCacheImmediately
    // 是否在创建图片时就进行解码（当然要这么做，避免在渲染时解码占用cpu）并缓存，
    /* Specifies whether image decoding and caching should happen at image creation time.
    * The value of this key must be a CFBooleanRef. The default value is kCFBooleanFalse (image decoding will
    * happen at rendering time). //默认为不缓存，在图片渲染时进行图片解码
    */
    CFDictionaryAddValue(mutOption, kCGImageSourceShouldCacheImmediately, kCFBooleanTrue);

    // · kCGImageSourceCreateThumbnailWithTransform
    //指定是否应根据完整图像的方向和像素纵横比旋转和缩放缩略图
    /* Specifies whether the thumbnail should be rotated and scaled according
     * to the orientation and pixel aspect ratio of the full image.（默认为false
     */
    CFDictionaryAddValue(mutOption, kCGImageSourceCreateThumbnailWithTransform, kCFBooleanTrue);


    // · kCGImageSourceThumbnailMaxPixelSize
    /* Specifies the maximum width and height in pixels of a thumbnail.  If
     * this this key is not specified, the width and height of a thumbnail is
     * not limited and thumbnails may be as big as the image itself.  If
     * present, this value of this key must be a CFNumberRef. */

    CFDictionaryAddValue(mutOption, kCGImageSourceThumbnailMaxPixelSize, (__bridge CFNumberRef)maxDimensionNum);

    CFDictionaryRef dowsamplingOption = CFDictionaryCreateCopy(NULL, mutOption);

    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(imageSourceRef, 0, dowsamplingOption);
    
    return [UIImage imageWithCGImage:imageRef];
}