Define Custom Source File Dependencies in SBT - plugins

Question

How can I cause SBT to recompile some file A whenever another non-scala file B changes?
I have defined a macro:
printMacro("path/to/file")
which creates a string literal from the file indicated by "path/to/file".
Whenever that file changes, the file that uses that macro needs to be recompiled to reflect those changes. I can use watchSources to monitor that file for changes and recompile the project when it does, but because of the incremental compiler, this recompile doesn't actually do anything.
I'll almost certainly need to write a plugin to accomplish this, but I cannot find which hooks into sbt will enable me to write such a plugin.
EDIT: Recompiling the whole project isn't desirable, since there might be multiple tracked files and the project itself might be very large.

Answer 1

How about this solution, which is based on FileFunction.cached.
Basically define a function, which takes:
cachedBaseDirectory - the place where it's going to keep cache metadata
inStyle - which determines how it checks for changes
action - which is invoked when the observed file is changed
The function returns another function, which takes a set of monitored files.
def cached(cacheBaseDirectory: File, inStyle: FilesInfo.Style)(action: => Unit): Set[File] => Unit = {
import Path._
lazy val inCache = Difference.inputs(cacheBaseDirectory / "in-cache", inStyle)
inputs => {
inCache(inputs) { inReport =>
if(!inReport.modified.isEmpty) action
}
}
}
This is how you can use it in build.sbt
val recompileWhenFileChanges = taskKey[Unit]("Recompiles the project when a file changes")
recompileWhenFileChanges := {
val base = baseDirectory.value
val mySpecialFile = baseDirectory.value / "path" / "to" / "file" / "test.txt"
val cache = cacheDirectory.value / "my_cache_dir"
val cachedFunction = cached(cache, FilesInfo.lastModified)(IO.delete((classDirectory in Compile).value))
cachedFunction(mySpecialFile.get.toSet)
}
compile in Compile := ((compile in Compile) dependsOn recompileWhenFileChanges).value
The task deletes classDirectory only if the file is changed. Deleting the classDirectory makes the project to recompile.
Last we make the original compile to depend on our newly created task.