Programmatically create iOS 13 dynamic images

programmatic-dynamic-images.m

- (UIImage *)dynamicImage
{
    UITraitCollection *const baseTraitCollection = /* an existing trait collection */;
    UITraitCollection *const lightTraitCollection = [UITraitCollection traitCollectionWithTraitsFromCollections:@[baseTraitCollection, [UITraitCollection traitCollectionWithUserInterfaceStyle:UIUserInterfaceStyleLight]]];
    UITraitCollection *const purelyDarkTraitCollection = [UITraitCollection traitCollectionWithUserInterfaceStyle:UIUserInterfaceStyleDark];
    UITraitCollection *const darkTraitCollection = UITraitCollection *lightTraitCollection = [UITraitCollection traitCollectionWithTraitsFromCollections:@[baseTraitCollection, purelyDarkTraitCollection]];
    
    __block UIImage *lightImage;
    [lightTraitCollection performAsCurrentTraitCollection:^{
        lightImage = /* draw image */;
    }];
    __block UIImage *darkImage;
    [darkTraitCollection performAsCurrentTraitCollection:^{
        darkImage = /* draw image */;
    }];
    [lightImage.imageAsset registerImage:darkImage withTraitCollection:purelyDarkTraitCollection];
    
    return lightImage;
}