Watch for project files also

Question

I use sbt in the following fashion: I run ~ test:compile in sbt and then work in IDE, watching occasionaly if the project still compiles, because the IDE's presentation compiler tends to be buggy. When I git pull some code, there might be changes in the project/ files, so I want to have reload. Is there a way, how to watch both source files and project files, so when there is change in project files, I actually get the update?

Answer 1

As jsuereth explained this isn't a task SBT can handle in 1 instance. What's required is a reboot of SBT to abort the watching process and reload it's own configuration.

The following Scala script does exactly this, it uses Java NIO WatchService and Scala Process to monitor a path and restart a process. The code should be fairly simple to understand:

#!/usr/bin/env scala

import java.nio.file._
import scala.collection.JavaConversions._
import scala.sys.process._

val file = Paths.get(args(0))
val cmd = args(1)
val watcher = FileSystems.getDefault.newWatchService

file.register(
  watcher, 
  StandardWatchEventKinds.ENTRY_CREATE,
  StandardWatchEventKinds.ENTRY_MODIFY,
  StandardWatchEventKinds.ENTRY_DELETE
)

def exec = cmd run true

@scala.annotation.tailrec
def watch(proc: Process): Unit = {
  val key = watcher.take
  val events = key.pollEvents

  val newProc = 
    if (!events.isEmpty) {
      proc.destroy()
      exec
    } else proc

  if (key.reset) watch(newProc)
  else println("aborted")
}

watch(exec)

Usage in your sbt dir would be:

watchr.scala project/ "sbt ~ test:compile"

If anything is unclear don't hesitate to ask, of course any scripting language could be used to implement this behavior.

Answer 2

You actually can't use ~ <task> and have it rebuild the project itself right now, because ~ <task> needs to read the build definition itself to determine:

1. What source files to watch
2. How to run the task.

What you're doing is altering the config whe project/ changes. This requires a full reload or reboot of sbt to pull in the new configuration.

So, as of sbt 0.13, this isn't possible. You can have it so it will rebuild your source code when project/ changes, but without rebuilding the build definition, not much help.

You could create a new sbt prompt, or task, that when run could check to see if source files in project/ are updated and issue a warning/error so you know to reboot. That's probably the best option right now.