iOS 大图缩小

- (UIImage *)downsampleImageAt:(NSURL *)imageURL to:(CGSize)pointSize scale:(CGFloat)scale {
    // 利用图像文件地址创建 image source
    NSDictionary *imageSourceOptions =
    @{
        (__bridge NSString *)kCGImageSourceShouldCache: @NO // 原始图像不要解码
    };
    CGImageSourceRef imageSource = CGImageSourceCreateWithURL((__bridge CFURLRef)imageURL, (__bridge CFDictionaryRef)imageSourceOptions);
    // 下采样
    CGFloat maxDimensionInPixels = MAX(pointSize.width, pointSize.height) * scale;
    NSDictionary *downsampleOptions =
    @{
        (__bridge NSString *)kCGImageSourceCreateThumbnailFromImageAlways: @YES,
        (__bridge NSString *)kCGImageSourceShouldCacheImmediately: @YES, // 缩小图像的同时进行解码
        (__bridge NSString *)kCGImageSourceCreateThumbnailWithTransform: @YES,
        (__bridge NSString *)kCGImageSourceThumbnailMaxPixelSize: @(maxDimensionInPixels)
    };
    CGImageRef downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, (__bridge CFDictionaryRef)downsampleOptions);
    UIImage *image = [[UIImage alloc] initWithCGImage:downsampledImage];
    CGImageRelease(downsampledImage);
    CFRelease(imageSource);
    return image;
}