pw_watch is similar to file system watchers found in the web development space. These watchers trigger a web server reload on source change, increasing iteration. In the embedded space, file system watchers are less prevalent but no less useful! The Pigweed watcher module makes it easy to instantly compile, flash, and run tests upon save.



pw_watch currently only works with Pigweed’s GN and CMake builds.

Module Usage#

The simplest way to get started with pw_watch is to launch it from a shell using the Pigweed environment as pw watch. By default, pw_watch watches for repository changes and triggers the default Ninja build target at out/. To override this behavior, provide the -C argument to pw watch.

# Use ./out/ as the build directory and build the default target
pw watch

# Use ./out/ as the build directory and build the stm32f429i target
pw watch python.lint stm32f429i

# Build pw_run_tests.modules in the out/cmake directory
pw watch -C out/cmake pw_run_tests.modules

# Build the default target in out/ and pw_apps in out/cmake
pw watch -C out -C out/cmake pw_apps

# Build python.tests in out/ and build pw_apps in out/cmake
pw watch python.tests -C out/cmake pw_apps

# Build the default target, but only run up to 8 jobs in parallel.
pw watch -j8

# Build the default target and keep building past the first error as far as
# possible.
pw watch --keep-going

# Build the default target and start a docs server on
pw watch --serve-docs

# Build the default target and start a docs server on
pw watch --serve-docs --serve-docs-port=5555

# Build with a full screen terminal user interface similar to pw_console.
pw watch --fullscreen

pw watch only rebuilds when a file that is not ignored by Git changes. Adding exclusions to a .gitignore causes watch to ignore them, even if the files were forcibly added to a repo. By default, only files matching certain extensions are applied, even if they’re tracked by Git. The --patterns and --ignore_patterns arguments can be used to include or exclude specific patterns. These patterns do not override Git’s ignoring logic.

The --exclude_list argument can be used to exclude directories from being watched. This decreases the number of files monitored with inotify in Linux.

By default, pw watch automatically restarts an ongoing build when files change. This can be disabled with the --no-restart option. While running pw watch, you may also press enter to immediately restart a build.

See pw watch -h for the full list of command line arguments.

Unit Test Integration#

Thanks to GN’s understanding of the full dependency tree, only the tests affected by a file change are run when pw_watch triggers a build. By default, host builds using pw_watch will run unit tests. To run unit tests on a device as part of pw_watch, refer to your device’s target documentation.