SBT classpath dependency with ScalaJS crosscompile project

I have 2 different separate projects A and B, both of which use the ScalaJS default way of wiring cross compile projects (see here: https://www.scala-js.org/doc/project/cross-build.html).

Now I want to add a SBT Classpath Dependency from B to A in a manner similar to this:

projectA.dependsOn(projectB)

where the js part of project B can use the js and shared parts of project A and the jvm part can access jvm and shared parts of project A.

As I am using upickle for serializing my data, I cannot just use a libraryDependeny with a publishLocalof project A (as upickle requires compile time information about project A in my case).

How can I solve this?

Solution

For CrossProject's in the same build, you can simply use .dependsOn:

lazy val a = crossProject
lazy val b = crossProject.dependsOn(a)

// snip aJS, aJVM, bJS, bJVM

Currently, there is no way to import a CrossProject from a URI / File like a RootProject. However, a CrossProject is nothing more than a two normal sbt projects. You can import these individually:

// a.sbt

lazy val a = crossProject
lazy val aJS = a.js
lazy val aJVM = a.jvm

// b.sbt

lazy val b = crossProject.
  jsConfigure(_.dependsOn(aJS)).
  jvmConfigure(_.dependsOn(aJVM))

lazy val aJS = ProjectRef(file("projectA"), "aJS")
lazy val aJVM = ProjectRef(file("projectA"), "aJVM")

Note that there doesn't seem a reason we cannot create a CrossProjectRef that abstracts this away from you. So if you need this often, feel free to open an issue so we can look at it in more detail.