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Getting started with build_runner

To use build_runner, you need a 2.x version of the Dart SDK.

If you have issues using build_runner, see the Troubleshooting section, below.

Using build_runner as a development server

  1. Edit your package's pubspec.yaml file, adding dev dependencies on build_runner and build_web_compilers:

    dart pub add dev:build_runner dev:build_web_compilers

  2. Get package dependencies:

    dart pub get

  3. Start the server:

    dart run build_runner serve

While the serve command runs, every change you save triggers a rebuild.

The first build is the slowest. After that, assets are cached on disk and incremental builds are faster.

Creating an output directory

Build with --output <directory name> to write files into a merged output directory with file paths that match internally referenced URIs. This can be used with the build, watch, and serve commands. This directory can also be used with a different server if the serve command is insufficient.

To output only part of the package, for example to output only the web directory, use --output web:<directory name>.

Using other build_runner commands

In addition to serve you can use:

  • build: Runs a single build and exits. This is most useful if your build also generates output to your source directory. With --output <dirname> this also creates a merged output directory with all sources and generated assets.

  • watch: Like build but reruns after file changes. With --output <dirname> the merged output directory will be kept up to date with changes. This can be used to keep the outputs updated for use with another filed-based development server.

  • test: Creates an output directory and runs dart run test within it. This command requires a dev dependency on build_test.

Switching to dart2js

By default build_web_compilers uses dartdevc. To switch to dart2js, pass --release to dart run build_runner build (or serve). Pass args to dart2js by creating a build.yaml file.

targets:
  $default:
    builders:
      build_web_compilers:entrypoint:
        options:
          dart2js_args:
          - --minify
          - --fast-startup

Troubleshooting

Diagnosing build times

See https://github.com/dart-lang/build/blob/master/docs/measuring_performance.md.

build_runner has no versions that match...

  1. Make sure you're using a 2.x SDK.

    dart --version

  2. Check the versions of the packages that your app depends on. They should all be compatible with a 2.x SDK.

Too many open files

If you see a FileSystemException, saying the directory listing failed due to too many open files, you might need to increase the OS limits.

For details, see #857.