sbt/notes/1.0.3/watch.md

Fixes with compatibility implications

Improvements

• Add sbt.watch.mode system property to allow switching back to old polling behaviour for watch. See below for more details. #3597 by @stringbean

Bug fixes

Alternative watch mode

sbt 1.0.0 introduced a new mechanism for watching for source changes based on the NIO WatchService in Java 1.7. On some platforms (namely macOS) this has led to long delays before changes are picked up. An alternative WatchService for these platforms is planned for sbt 1.1.0 (#3527), in the meantime an option to select which watch service has been added.

The new sbt.watch.mode JVM flag has been added with the following supported values:

• polling: (default for macOS) poll the filesystem for changes (mechanism used in sbt 0.13).
• nio (default for other platforms): use the NIO based WatchService.

If you are experiencing long delays on a non-macOS machine then try adding -Dsbt.watch.mode=polling to your sbt options.