Merging Builder

Introduction

Source code generation has become an important software development tool when building and maintaining a large number of data models, data access object, widgets, etc. The premise of source code generation is that we can specify (hopefully few) details and flesh out the rest of the classes, and methods during the build process.

The library merging_builder includes the following (synthetic input) builder classes:

• MergingBuilder reads several input files and writes merged output to one output file to a custom location.

• StandaloneBuilder reads one or several input files and writes standalone files to a custom location. In this context, standalone means the output files may be written to a custom folder and not only the extension but the name of the output file can be configured (as opposed to using part files).

Usage

Following the example of source_gen, it is common practice to separate builders and generators from the code using those builders.

In the example project provided, the package defining a new builder is called researcher_builder and the package using this builder is called researcher. To set up a build system the following steps are required:

1. Builder Package Setup

1. Get dependencies: Include merging_builder, build as dependencies in the file pubspec.yaml. In the example mentioned here, the generator also requires the packages analyzer and source_gen.

2. Create a generator: Create a custom generator that extends MergingGenerator. In example generateItemForAnnotatedElement reads a list of strings while generateMergedContent merges the data and generates output that is written to researchers.dart.

3. Create a builder: Create an instance of MergingBuilder. Following the example of source_gen, builders are typically placed in a file called: builder.dart located in the lib folder of the builder package.

   • Input sources may be specified using wildcard characters supported by Glob.
   • The builder definition includes the default values of the options input_files, output_file, header, footer, and sort_assets. These options can be overwritten by the user of the builder by specifying them explicitly in the file build.yaml located in the package researcher (see step 1 in section User Package Setup).

4. Add a builder configuration: The build extensions for MergingBuilder must be specified using the notation available for synthetic input. For example, "$lib$" indicates that the input files are located in the folder lib or a subfolder thereof.

   builders:
     add_names_builder:
       import: "package:researcher_builder/builder.dart"
       builder_factories: ["addNamesBuilder"]
       build_extensions: {"lib/$lib$": ["lib/output.dart"]}
       auto_apply: root_package
       build_to: source
     assistant_builder:
       import: "package:researcher_builder/builder.dart"
       builder_factories: ["assistantBuilder"]
       build_extensions: {"lib/$lib$": ["*.dart"]}
       auto_apply: root_package
       build_to: source

2. User Package Setup

The following steps are performed in the package that is using the custom builder:

1. Add dependencies: Add the custom builder package and build_runner as a dev_dependencies in the file pubspec.yaml.

2. Builder configuration: Add the builder to the list of known builders and configure the available options (thus overwriting the default values). A sample build.yaml file is shown below.

    targets:
      $default:
        builders:
          # Configure the builder `pkg_name|builder_name`
          researcher_builder|add_names_builder:
            # Only run this builder on the specified input.
            enabled: true
            # generate_for:
            #   - lib/*.dart
            options:
              input_files: 'lib/input/*.dart'
              output_file: 'lib/output/researchers.dart'
              sort_assets: false
              header: '// Header specified in build.yaml.'
              footer: '// Footer specified in build.yaml.'
          researcher_builder|assistant_builder:
            enabled: true
            options:
              input_files: 'lib/input/*.dart'
              output_files: 'lib/output/assistant_(*).dart'
              root: ''

3. Initiate the build process by using the command:

   # dart run build_runner build --delete-conflicting-outputs --verbose

Merging Builder

MergingBuilder reads several input files and writes merged output to one output file. The builder provides the option to sort the input files in reverse topological order. If the input file a.dart includes file b.dart then a.dart will be listed after b.dart. This option may be useful when generating code that needs to list variables or call functions in order of dependence. To enable topological sorting set the constructor parameter sortAsset: true. Note: If sorting of input assets is enabled, input files must not include each other directly or indirectly.

A conventional builder typically calls the generator method generate from within its build method to retrieve the generated source-code. MergingBuilder calls the MergingGenerator method generateStream. It allows the generator to pass a stream of data-type T to the builder, one stream item for each annotated element processed to the generator method generateStreamItemForAnnotatedElement.

The private builder method _combineStreams combines the streams received for each processed input file and calls the generator method generateMergedContent. As a result, this method has access to all stream items of type T generated f or each element annotated with an annotation of type A in each input file. It is the task of this method to generate the merged source-code output.

The figure below shows the flow of data between the builder and the generator. The data type is indicated by the starting point of the connectors. Dotted connectors represent a stream of data.

Standalone Builder

StandaloneBuilder reads input files and writes corresponding output files to a custom location.

The input file path (constructor parameter inputFiles) may include wild-card notation supported by Glob.

Output files are specified by using the custom symbol (*). For example, the output path output\assistant_(*).dart is interpreted such that (*) is replaced with the input file name (excluding the file extension). For more details, see the file example\researcher_builder\builder.dart.

Limitations: For builders extending StandaloneBuilder it is recommended to initiate the build command from the root directory of the package the build is applied to.

Examples

For further information on how to use MergingBuilder see example.

Features and bugs

Please file feature requests and bugs at the issue tracker.