Merging Builder

Build Status


Source code generation has become an integral software development tool when building and maintaining a large number of data models, data access object, widgets, etc.

The library merging_builder includes the classes MergingBuilder and StandaloneBuilder. Both builders use synthetic input which must be specified by choosing either $Lib$ or $Package$ as type parameter S (see figure below).

$Lib$ indicates that input and output files are located in the package directory lib or a subfolder thereof. For more information about synthetic input see: Writing an Aggregate Builder.

Class 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 for each annotated element 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.

Directed Graph Image

Class Standalone Builder

StandaloneBuilder reads one or several input files and writes standalone files to a custom location. 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.

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 (see point 7 in the next section) from the root directory of the package the build is applied to.


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

In the example provided with this library, 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. Include merging_builder and build as dependencies in the file pubspec.yaml of the package defining the builder. (In the example mentioned here, the generator also requires the packages analyzer and source_gen.)

  2. In the package defining the custom builder, create a custom generator that extends MergingGenerator. Users will have to implement the methods generateItemForAnnotatedElement and generateMergedContent. In the example shown below generateItemForAnnotatedElement reads a list of strings while generateMergedContent merges the data and generates output that is written to researchers.dart.

    Show details.
     import 'dart:async';
     import 'package:analyzer/dart/element/element.dart';
     import 'package:build/src/builder/build_step.dart';
     import 'package:merging_builder/merging_builder.dart';
     import 'package:merging_builder/src/annotations/add_names.dart';
     import 'package:source_gen/source_gen.dart';
     import 'package:quote_buffer/quote_buffer.dart';
     /// Reads numbers from annotated classes and emits the sum.
     class AddNamesGenerator extends MergingGenerator<List<String>, AddNames> {
       /// Portion of source code included at the top of the generated file.
       /// Should be specified as header when constructing the merging builder.
       static String get header {
         return '/// Added names.';
       /// Portion of source code included at the very bottom of the generated file.
       /// Should be specified as [footer] when constructing the merging builder.
       static String get footer {
         return '/// This is the footer.';
       List<String> generateStreamItemForAnnotatedElement(
         Element element,
         ConstantReader annotation,
         BuildStep buildStep,
       ) {
         final List<String> result = [];
         if (element is ClassElement) {
           final nameObjects =
           if (nameObjects != null) {
             for (final nameObj in nameObjects) {
             return result;
         return null;
       /// Returns merged content.
       FutureOr<String> generateMergedContent(Stream<List<String>> stream) async {
         final b = QuoteBuffer();
         int i = 0;
         final List<List<String>> allNames = [];
         // Iterate over stream:
         await for (final names in stream) {
           b.write('final name$i = [');
           b.writelnAllQ(names, separator2: ',');
         b.writeln('final List<List<String>> names = [');
         for (var names in allNames) {
           b.writeln('  [');
           b.writelnAllQ(names, separator2: ',');
           b.writeln('  ],');
         return b.toString();
  3. 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. The generator AddNamesGenerator extends MergingGenerator<List<String>, AddNames> (see step 2). Input sources may be specified using wildcard characters supported by Glob. The builder definition shown below honours the options input_files, output_file, header, footer, and sort_assets that can be set in the file build.yaml located in the package researcher (see step 5).

      import 'package:build/build.dart';
      import 'package:merging_builder/merging_builder.dart';
      import 'src/generators/add_names_generator.dart';
      import 'src/generators/assistant_generator.dart';
      /// Defines a merging builder.
      /// Honours the options: `input_files`, `output_file`, `header`, `footer`,
      /// and `sort_assets` that can be set in `build.yaml`.
      Builder addNamesBuilder(BuilderOptions options) {
        BuilderOptions defaultOptions = BuilderOptions({
          'input_files': 'lib/*.dart',
          'output_file': 'lib/output.dart',
          'header': AddNamesGenerator.header,
          'footer': AddNamesGenerator.footer,
          'sort_assets': true,
        // Apply user set options.
        options = defaultOptions.overrideWith(options);
        return MergingBuilder<List<String>, $Lib$>(
          generator: AddNamesGenerator(),
          inputFiles: options.config['input_files'],
          outputFile: options.config['output_file'],
          header: options.config['header'],
          footer: options.config['footer'],
          sortAssets: options.config['sort_assets'],
      /// Defines a standalone builder.
      Builder assistantBuilder(BuilderOptions options) {
        BuilderOptions defaultOptions = BuilderOptions({
          'input_files': 'lib/*.dart',
          'output_files': 'lib/output/assistant_(*).dart',
          'header': AssistantGenerator.header,
          'footer': AssistantGenerator.footer,
          'root': ''
        options = defaultOptions.overrideWith(options);
        return StandaloneBuilder<$Lib$>(
            generator: AssistantGenerator(),
            inputFiles: options.config['input_files'],
            outputFiles: options.config['output_files'],
            root: options.config['root']);
  4. In the package defining the builder, add the builder configuration for the builder add_names_builder (see below). 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. For more information consult the section: Writing a Builder using a synthetic input found in the documentation of the Dart package build.

         import: "package:researcher_builder/builder.dart"
         builder_factories: ["addNamesBuilder"]
         build_extensions: {"lib/$lib$": ["lib/output.dart"]}
         auto_apply: root_package
         build_to: source
         import: "package:researcher_builder/builder.dart"
         builder_factories: ["assistantBuilder"]
         build_extensions: {"lib/$lib$": ["*.dart"]}
         auto_apply: root_package
         build_to: source
  5. In the package using the custom builder, researcher, add add_names_builder to the list of known builders. The file build.yaml is shown below.

            # Configure the builder `pkg_name|builder_name`
              # Only run this builder on the specified input.
              enabled: true
              # generate_for:
              #   - lib/*.dart
                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.'
              enabled: true
                input_files: 'lib/input/*.dart'
                output_files: 'lib/output/assistant_(*).dart'
                root: ''
  6. In the package using the builder, researcher, add researcher_builder and build_runner as dev_dependencies in the file pubspec.yaml.

     name: researcher
         Example demonstrating how to use the library merging_builder.
         sdk: '>=2.8.1 <3.0.0'
         build_runner: ^1.10.0
           path: ../researcher_builder
  7. Initiate the build process by using the command:

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


For further information on how to use MergingBuilder see example.

Features and bugs

Please file feature requests and bugs at the issue tracker.


Library providing a merging builder which reads several input files and writes output to one file.