Swift nullability

[soffes]

It would be great if there FastImageCache was annotated for nullability so it was easier to use via Swift :)

[mallorypaine]

Any interest in submitting a PR?

[soffes]

If I have time to PR all of these recent issues I opened, I definitely will!

[mallorypaine]

Would love a PR to help FIC work better with Swift :-)

[Sega-Zero]

@mallorypaine that issue may be closed :)