better invalidation of interface version.properties file

project/Sbt.scala

@@ -1,4 +1,3 @@
-// TODO(high): proper incremental xsbt.version.properties generation
 // TODO(low): proper generated API sources caching: doesn't detect output directory change
 
 	import sbt._
@@ -192,7 +191,7 @@ object Sbt extends Build
 		exportJars := true,
 		componentID := Some("xsbti"),
 		watchSources <++= apiDefinitions,
-		resourceGenerators in Compile <+= (version, resourceManaged, streams) map generateVersionFile,
+		resourceGenerators in Compile <+= (version, resourceManaged, streams, compile in Compile) map generateVersionFile,
 		apiDefinitions <<= baseDirectory map { base => (base / "definition") :: (base / "other") :: (base / "type") :: Nil },
 		sourceGenerators in Compile <+= (cacheDirectory, apiDefinitions, fullClasspath in Compile in datatypeSub, sourceManaged in Compile, mainClass in datatypeSub in Compile, runner, streams) map generateAPICached
 	)

project/Util.scala

@@ -55,15 +55,20 @@ object Util
 		toError(run.run(mainClass, cp.files, args, s.log))
 		(out ** "*.java").get
 	}
-	def generateVersionFile(version: String, dir: File, s: TaskStreams): Seq[File] =
+	def lastCompilationTime(analysis: sbt.inc.Analysis): Long =
+	{
+		val times = analysis.apis.internal.values.map(_.compilation.startTime)
+		if(times.isEmpty) 0L else times.max
+	}
+	def generateVersionFile(version: String, dir: File, s: TaskStreams, analysis: sbt.inc.Analysis): Seq[File] =
 	{
 		import java.util.{Date, TimeZone}
 		val formatter = new java.text.SimpleDateFormat("yyyyMMdd'T'HHmmss")
 		formatter.setTimeZone(TimeZone.getTimeZone("GMT"))
 		val timestamp = formatter.format(new Date)
 		val content = "version=" + version + "\ntimestamp=" + timestamp
 		val f = dir / "xsbt.version.properties"
-		if(!f.exists) { // TODO: properly handle this
+		if(!f.exists || f.lastModified < lastCompilationTime(analysis)) {
 			s.log.info("Writing version information to " + f + " :\n" + content)
 			IO.write(f, content)
 		}