Problems hooking task into asset phase

[roti]

I don't know if this is an issue with sbt-web, or an consequence of my lack of knowlege about sbt-web and sbt-plugin (the context is a Play 2.6 project). I apologize if the latter is the case, though I'd appreciate some help.

Essentially I'm trying to hook a task into the asset generation phase, as described here. It's sbt-webpack, which is defined as an InputTask (not a pipeline stage task).

It's dependent on the webjars, so to my knowlegde this should work:

val foo = TaskKey[Unit]("foo")
foo := { println("now running foo") }
foo in Assets := (foo in Assets).dependsOn(WebKeys.webModules in Assets).value

It does not. What I am looking for is a way to have foo run in any asset generation phase (i.e. assets, stage, dist, run).