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Where are the generated files?

There are 3 places you might see files getting generated. On every build files might go to cache (.dart_tool/build/generated/*), and source (the working tree of your package). These are determined by the build_to configuration of the Builders you are using. When a Builder specifies build_to: source you will see them next to your other files but should not edit them by hand, and they are generally meant to be published with your package. If you see a warning about "conflicting outputs" they refer to source outputs, because when the build tool is starting with no prior information it can't tell if it might be overwriting some file you wrote by hand.

Separately the --output option can specify a directory to create a merged view of all the files in the build system. This copies hand written source files, along with source and cache outputs and puts them all together in the same directory structure.

How can I debug my release mode web app (dart2js)?

By default, the dart2js compiler is only enabled in --release mode, which does not include source maps or the original .dart files. If you need to debug an error which only happens in dart2js, you will want to change your debug mode compiler to dart2js. You can either do this using the --define command line option:

--define "build_web_compilers|entrypoint=compiler=dart2js"

Or by editing your build.yaml file:

          compiler: dart2js

How can I include additional sources in my build?

By default, the build_runner package only includes some specifically whitelisted directories, derived from the package layout conventions.

If you have some additional files which you would like to be included as part of the build, you will need a custom build.yaml file. You will want to modify the sources field on the $default target:

      - my_custom_sources/**
      - lib/**
      - web/**
      # Note that it is important to include these in the default target.
      - pubspec.*

Why do Builders need unique outputs?

build_runner relies on determining a static build graph before starting a build - it needs to know every file that may be written and which Builder would write it. If multiple Builders are configured to (possibly) output the same file you can:

  • Add a generate_for configuration for one or both Builders so they do not both operate on the same primary input.
  • Disable one of the Builders if it is unneeded.
  • Contact the author of the Builder and ask that a more unique output extension is chosen.
  • Contact the author of the Builder and ask that a more unique input extension is chose, for example only generating for files that end in _something.dart rather than all files that end in .dart.

How can I use my own development server to serve generated files?

There are 2 options for using a different server during development:

  1. Run build_runner serve web:<port> and proxy the requests to it from your other server. This has the benefit of delaying requests while a build is ongoing so you don't get an inconsistent set of assets.

  2. Run build_runner watch --output web:build and use the created build/ directory to serve files from. This will include a build/packages directory that has these files in it.

How can I fix AssetNotFoundExceptions for swap files?

Some editors create swap files during saves, and while build_runner uses some heuristics to try and ignore these, it isn't perfect and we can't hardcode knowledge about all editors.

One option is to disable this feature in your editor. Another option is you can explicitly ignore files with a given extension by configuring the exclude option for your targets sources in build.yaml:

        # Example that excludes intellij's swap files
        - **/*___jb_tmp___

Why are some logs "(cached)"?

build_runner will only run actions that have changes in their inputs. When an action fails, and a subsequent build has exactly the same inputs for that action it will not be rerun - the previous error messages, however, will get reprinted to avoid confusion if a build fails with no printed errors. To force the action to run again make an edit to any file that is an input to that action, or throw away all cached values with pub run build_runner clean before starting the next build.

How can I resolve "Skipped compiling" warnings?

These generally come up in the context of a multi-platform package (generally due to a mixture of vm and web tests), and look something like this:

[WARNING] build_vm_compilers|entrypoint on example|test/my_test.dart:

Skipping compiling example|test/my_test.dart for the vm because some of its
transitive libraries have sdk dependencies that not supported on this platform:


While we are smart enough not to attempt to compile your web tests for the vm, it is slow for us to figure that out so we print this warning to encourage you to set up the proper configuration so we will never even attempt to compile these tests.

You can set up this configuration in your build.yaml file, using the generate_for option on the builder. It helps a lot if you separate your web and vm tests into separate directories, but you don't have to.

For example, your build.yaml might look like this:

        - test/multiplatform/**_test.dart
        - test/web/**_test.dart
        - web/**.dart
        - test/multiplatform/**_test.dart
        - test/vm/**_test.dart
        - bin/**.dart
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