merging_builder 0.0.2

Generic Dart builder and generator that reads several input files and writes merged output to a stand-alone file. Has support for specifying a header and footer.

Merging Builder

Introduction

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 provides a Dart builder that reads several input files and writes merged output to one output file. The builder has support for specifying a header and footer to be placed at the top and bottom of the output file.

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

The private builder method _combineStreams combines the streams receives for each processed file asset 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 orange connectors. Dotted connectors represent a stream of data.

Usage

Following the guidelines 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 mergedContent. In the example shown below generateItemForAnnotatedElement reads a list of strings while mergedContent 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.';
     }
   
     @override
     List<String> generateStreamItemForAnnotatedElement(
       Element element,
       ConstantReader annotation,
       BuildStep buildStep,
     ) {
       final List<String> result = [];
       if (element is ClassElement) {
         final nameObjects =
             element.getField('names')?.computeConstantValue()?.toListValue();
         if (nameObjects != null) {
           for (final nameObj in nameObjects) {
             result.add(nameObj.toStringValue());
           }
           return result;
         }
       }
       return null;
     }
   
     /// Returns merged content.
     @override
     FutureOr<String> mergedContent(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('];');
         ++i;
         allNames.add(names);
       }
   
       b.writeln('');
       b.writeln('final List<List<String>> names = [');
       for (var names in allNames) {
         b.writeln('  [');
         b.writelnAllQ(names, separator2: ',');
         b.writeln('  ],');
       }
       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 can be specified using wildecard characters supported by Glob.

    import 'package:build/build.dart';
    import 'package:merging_builder/merging_builder.dart';
   
    import 'generators/add_names_generator.dart';
   
    Builder addNamesBuilder(BuilderOptions options) =>
      MergingBuilder<List<String>>(
        generator: AddNamesGenerator(),
        inputFiles: 'lib/input/*.dart',
        outputFile: 'lib/researchers.dart',
        header: AddNamesGenerator.header,
        footer: AddNamesGenerator.footer,
        sortAssets: false,
      );
   

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.

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

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.

    targets:
      $default:
        builders:
          # Configure the builder `pkg_name|builder_name`
          researcher_builder|add_names_builder:
            enabled: true
            # Only run this builder on the specified input.
            # generate_for:
            #   - lib/*.dart
   

6. In the package using the builder, researcher, add researcher_builder and build_runner as dev_dependencies in the file pubspec.yaml.

   name: researcher
     description:
       Example demonstrating how to use the library merging_builder.
   
     environment:
       sdk: '>=2.8.1 <3.0.0'
   
     dev_dependencies:
       build_runner: ^1.10.0
       researcher_builder:
         path: ../researcher_builder
   

7. Initiate the build process by using the command:

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

Examples

For further information on how to use MergingBuilder see example.

Features and bugs

Please file feature requests and bugs at the issue tracker.