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Customizing builds

Customizing the build behavior of a package is done by creating a build.yaml file, which describes your configuration.

Dividing a package into Build targets

When a Builder should be applied to a subset of files in a package the package can be broken up into multiple 'targets'. Targets are configured in the targets section of the build.yaml. The key for each target makes up the name for that target. Targets can be referred to in '$definingPackageName:$targetname'. When the target name matches the package name it can also be referred to as just the package name. One target in every package must use the package name so that consumers will use it by default. In the build.yaml file this target can be defined with the key $default or with the name of the package.

Each target may also contain the following keys:

• sources: List of Strings or Map, Optional. The set of files within the package which make up this target. Files are specified using glob syntax. If a List of Strings is used they are considered the 'include' globs. If a Map is used can only have the keys include and exclude. Any file which matches any glob in include and no globs in exclude is considered a source of the target. When include is omitted every file is considered a match.
• dependencies: List of Strings, Optional. The targets that this target depends on. Strings in the format '$packageName:$targetName' to depend on a target within a package or $packageName to depend on a package's default target. By default this is all of the package names this package depends on (from the pubspec.yaml).
• builders: Map, Optional. See "configuring builders" below.

Configuring Builders applied to your package

Each target can specify a builders key which configures the builders which are applied to that target. The value is a Map from builder to configuration for that builder. The key is in the format '$packageName|$builderName'. The configuration may have the following keys:

• enabled: Boolean, Optional: Whether to apply the builder to this target. Omit this key if you want the default behavior based on the builder's auto_apply configuration. Builders which are manually applied (auto_apply: none) are only ever used when there is a target specifying the builder with enabled: True.
• generate_for: List of String or Map, Optional:. The subset of files within the target's sources which should have this Builder applied. See sources configuration above for how to configure this.
• options: Map, Optional: A free-form map which will be passed to the Builder as a BuilderOptions when it is constructed. Usage varies depending on the particular builder.

Defining Builders to apply to dependents (similar to transformers)

If users of your package need to apply some code generation to their package, then you can define Builders and have those applied to packages with a dependency on yours.

The key for a Builder will be normalized so that consumers of the builder can refer to it in '$definingPackageName|$builderName' format. If the builder name matches the package name it can also be referred to with just the package name.

Exposed Builders are configured in the builders section of the build.yaml. This is a map of builder names to configuration. Each builder config may contain the following keys:

• target: The name of the target which defines contains the Builder class definition.
• import: Required. The import uri that should be used to import the library containing the Builder class. This should always be a package: uri.
• builder_factories: A List<String> which contains the names of the top-level methods in the imported library which are a function fitting the typedef Builder factoryName(List<String> args).
• build_extensions: Required. A map from input extension to the list of output extensions that may be created for that input. This must match the merged buildExtensions maps from each Builder in builder_factories.
• auto_apply: Optional. The packages which should have this builder automatically to applied. Defaults to 'none' The possibilities are:
  • "none": Never apply this Builder unless it is manually configured
  • "dependents": Apply this Builder to the package with a direct dependency on the package exposing the builder.
  • "all_packages": Apply this Builder to all packages in the transitive dependency graph.
  • "root_package"; Apply this Builder only to the top-level package.
• required_inputs: Optional, list of extensions. If a Builder must see every input with one or more file extensions they can be specified here and it will be guaranteed to run after any Builder which might produce an output of that type. For instance a compiler must run after any Builder which can produce .dart outputs or those libraries can't be compiled. This option should be rare. Defaults to an empty list.
• is_optional: Optional, boolean. Specifies whether a Builder can be run lazily, such that it won't execute until one of it's outputs is requested by a later Builder. This option should be rare. Defaults to False.
• build_to: Optional. The location that generated assets should be output to. The possibilities are:
  • "source": Outputs go to the source tree next to their primary inputs.
  • "cache": Outputs go to a hidden build cache and won't be published. The default is "cache". If a Builder specifies that it outputs to "source" it will never run on any package other than the root - but does not necessarily need to use the "root_package" value for "auto_apply". If it would otherwise run on a non-root package it will be filtered out.
• defaults: Optional: Default values to apply when a user does not specify the corresponding key in their builders section. May contain the following keys:
  • generate_for: A list of globs that this Builder should run on as a subset of the corresponding target, or a map with include and exclude lists of globs.

Example builders config:

targets:
  # The target containing the builder sources.
  _my_builder: # By convention, this is private
    sources:
      - "lib/src/builder/**/*.dart"
      - "lib/builder.dart"
    dependencies:
      - "build"
      - "source_gen"
builders:
  # The actual builder config.
  my_builder:
    target: ":_my_builder"
    import: "package:my_package/builder.dart"
    builder_factories: ["myBuilder"]
    build_extensions: {".dart": [".my_package.dart"]}
    auto_apply: dependents

Publishing build.yaml files

build.yaml configuration should be published to pub with the package and checked in to source control. Whenever a package is published with a build.yaml it should mark a dependency on build_config to ensure that the package consuming the config has a compatible version. Breaking version changes which do not impact the configuration file format will be clearly marked in the changelog.