从ALAssetRepresentation生成自定义缩略图

ALAsset *asset = [self.assetGroup asset];
ALAssetRepresentation *representation = [asset defaultRepresentation];
NSData *imageData = [representation defaultPixelBuffer];

CGImageRef imageRef = CGImageCreate(
    CGImageGetWidth(imageData),
    CGImageGetHeight(imageData),
    CGImageGetBitsPerComponent(imageData),
    CGImageGetBytesPerRow(imageData),
    CGColorSpaceCreateWithName(kCGColorSpaceDisplayP3),
    kCGImageAlphaNoneSkipLast
);

CGContextRef context = CGBitmapContextCreate(nil,
    CGImageGetWidth(imageRef),
    CGImageGetHeight(imageRef),
    8,
    CGImageGetBytesPerRow(imageRef),
    CGColorSpaceCreateWithName(kCGColorSpaceDisplayP3),
    kCGImageAlphaNoneSkipLast);

CGImageRef drawnImageRef = CGBitmapContextCreateImage(context);
CGContextRelease(context);

UIImage *drawnImage = [UIImage imageWithCGImage:drawnImageRef];
CGImageRelease(drawnImageRef);

return drawnImage;