Missing/Required ingredients as a signal

[falci]

If you place an inserter between a sushi belt and a building, the inserter will know what to pick from the belt.
Could we have a way to wire this signal?

[GrumpyJoe]

Read hand content. Either pulse or duration.

If you meant what an inserter COULD pick up, I believe you can read assemblers in 2.0. Shoild be able to read ingredients

[Tertius]

In fff#394 we see we can read the current ingredients from assembling machines in 2.0. We cannot read the output or input inventory, but we're able to read the ingredients required by the current recipe. This way, we're able to automate requester chest requests.