generic_reader 0.0.3

GenericReader provides a systematic method of retrieving constants of arbitrary data-type from a static representation of a Dart compile-time constant.

example/README.md

Generic Reader - Example

Retrieving Constants with Parametrized Type

The file player_example.dart demonstrates how to use generic_reader to read the value of a constant with parametrized type from a static representation of a compile-time constant expression. The program also shows how to register Decoder functions for the types Column and SqliteType.

The constant values that are going to be read are the fields of the const class Player:

Click to show player.dart.

import 'package:generic_reader/src/test_types/column.dart';
import 'package:generic_reader/src/test_types/sponsor.dart';
import 'package:generic_reader/src/test_types/sqlite_type.dart';
import 'package:generic_reader/src/test_types/unregistered_test_type.dart';

/// Class modelling a player.
class Player {
  const Player();

  /// Column name
  final columnName = 'Player';

  /// Column storing player id.
  final id = const Column<Integer>();

  /// Column storing first name of player.
  final firstName = const Column<Text>(
    defaultValue: Text('Thomas'),
  );

  /// List of sponsors
  final List<Sponsor> sponsors = const [
    Sponsor('Johnson\'s'),
    Sponsor('Smith Brothers'),
  ];

  /// Test unregistered type.
  final unregistered = const UnRegisteredTestType();

  /// Test [Set<int>].
  final Set<int> primeNumbers = const {1, 3, 5, 7, 11, 13};

In the simple example below, the function initializeLibraryReaderForDirectory provided by source_gen_test is used to load the source code and initialize objects of type LibraryReader.

In a standard setting this task is delegated to a builder that reads a builder configuration and loads the relevant assets.

Click to show player_example.dart.

import 'package:ansicolor/ansicolor.dart';
import 'package:example/src/column.dart';
import 'package:example/src/sponsor.dart';
import 'package:example/src/sqlite_type.dart';
import 'package:generic_reader/generic_reader.dart';
import 'package:source_gen/source_gen.dart' show ConstantReader;
import 'package:source_gen_test/src/init_library_reader.dart';

/// To run this program navigate to the folder: /example
/// in your local copy the package [generic_reader] and
/// use the command:
///
/// # dart bin/player_example.dart

/// Demonstrates how to use [GenericReader] to read constants
/// with parametrized type from a static representation
/// of a compile-time constant expression
/// represented by a [ConstantReader].
Future<void> main() async {
  /// Reading libraries.
  final playerLib = await initializeLibraryReaderForDirectory(
    'lib/src',
    'player.dart',
  );

  // ConstantReader representing field 'columnName'.
  final columnNameCR =
      ConstantReader(playerLib.classes.first.fields[0].computeConstantValue());

  // ConstantReade representing field 'firstName'.
  final firstNameCR =
      ConstantReader(playerLib.classes.first.fields[2].computeConstantValue());

  final sponsorsCR =
      ConstantReader(playerLib.classes.first.fields[3].computeConstantValue());

  // Get singleton instance of the reader.
  final reader = GenericReader();

  // Add a decoder function for constants of type [SqliteType].
  reader.addDecoder<SqliteType>((cr) {
    final value = cr.peek('value');
    if (value.isInt) return Integer(value.intValue);
    if (value.isBool) return Boolean(value.boolValue);
    if (value.isString) return Text(value.stringValue);
    if (value.isDouble) return Real(value.doubleValue);
    return null;
  });

  // Adding a decoder for constants of type [Column].
  reader.addDecoder<Column>((cr) {
    final defaultValueCR = cr.peek('defaultValue');
    final defaultValue = reader.get<SqliteType>(defaultValueCR);
    final nameCR = cr.peek('name');
    final name = reader.get<String>(nameCR);

    Column<T> columnFactory<T extends SqliteType>() {
      return Column<T>(
        defaultValue: defaultValue,
        name: name,
      );
    }

    if (reader.isA<Text>(defaultValueCR)) return columnFactory<Text>();
    if (reader.isA<Integer>(defaultValueCR)) return columnFactory<Integer>();
    if (reader.isA<Boolean>(defaultValueCR)) return columnFactory<Boolean>();
    if (reader.isA<Real>(defaultValueCR)) return columnFactory<Real>();
    return null;
  });

  AnsiPen green = AnsiPen()..green(bold: true);

  // Retrieve an instance of [String].
  final columnName = reader.get<String>(columnNameCR);
  print(green('Retrieving a [String]'));
  print('columnName = \'$columnName\'');
  print('');
  // Prints:
  // Retrieving a [String]
  // columnName = 'Player'

  // Retrieve an instance of [Column<Text>].
  final columnFirstName = reader.get<Column>(firstNameCR);
  print(green('Retrieving a [Column<Text>]:'));
  print(columnFirstName);
  // Prints:
  // Retrieving a [Column<Text>]:
  // Column<Text>(
  //   defaultValue: Text('Thomas')
  // )

  // Adding a decoder function for type [Sponsor].
  reader.addDecoder<Sponsor>((cr) => Sponsor(cr.peek('name').stringValue));

  final sponsors = reader.getList<Sponsor>(sponsorsCR);

  print('');
  print(green('Retrieving a [List<Sponsor>]:'));
  print(sponsors);
  // Prints:
  // Retrieving a [List<Sponsor>]:
  // [Sponsor: Johnson's, Sponsor: Smith Brothers]
}

Taking advantage of the fact that SqliteType is the super-type of Integer, Boolean, Text, and Real, the decoder function of Column can be shortened to:

// Adding a decoder for constants of type [Column].
  reader.addDecoder<Column>((cr) {
    final defaultValueCR = cr.peek('defaultValue');
    final defaultValue = reader.get<SqliteType>(defaultValueCR);
    final nameCR = cr.peek('name');
    final name = reader.get<String>(nameCR);

    return Column(defaultValue: defaultValue, name: name,);
  });

The only difference is that the resulting variable columnFirstName will then be of type Column<SqliteType>.

Retrieving Constants with Arbitrary Type

The example in the section above demonstrates how to retrieve constants with known parametrized type. The program presented below shows how to proceed if the constant has an arbitrary type parameter.

For this purpose consider the following generic class that wraps a value of type T:

/// Wraps a variable of type [T].
class Wrapper<T> {
  const Wrapper(this.value);

  /// Value of type [T].
  final T value;

  @override
  String toString() => 'Wrapper<$T>(value: $value)';
}

The type argument T can assume any data-type and it is impractical to handle all available types manually in the decoder function of Wrapper.

Instead, one can use the method get with the type dynamic. This signals to the reader to match the static type of the ConstantReader input to a registered data-type. If a match is found get<dynamic>(constantReader) returns a constant with the appropriate value, otherwise a ReaderError is thrown.

The program below retrieves the constant wrappedVariable defined in wrapper_test.dart. Note the use of the method get<dynamic>() when defining the Decoder function for the data-type Wrapper.

Click to show wrapper_example.dart.

import 'package:ansicolor/ansicolor.dart';
import 'package:example/src/sqlite_type.dart';
import 'package:example/src/wrapper.dart';
import 'package:generic_reader/generic_reader.dart';
import 'package:source_gen/source_gen.dart' show ConstantReader;
import 'package:source_gen_test/src/init_library_reader.dart';

/// To run this program navigate to the folder: /example
/// in your local copy the package [generic_reader] and
/// use the command:
///
/// # dart bin/wrapper_example.dart

/// Demonstrates how use [GenericReader] to read constants
/// with parametrized type from a static representation
/// of a compile-time constant expression
/// represented by a [ConstantReader].
Future<void> main() async {
  /// Reading libraries.
  final wrapperTestLib = await initializeLibraryReaderForDirectory(
    'lib/src',
    'wrapper_test.dart',
  );

  final wrappedCR = ConstantReader(
      wrapperTestLib.classes.first.fields[0].computeConstantValue());

  // Get singleton instance of the reader.
  final reader = GenericReader();

  AnsiPen green = AnsiPen()..green(bold: true);

  // Adding a decoder function for type [Wrapper].
  reader.addDecoder<Wrapper>((cr) {
    final valueCR = cr.peek('value');
    final value = reader.get<dynamic>(valueCR);
    return Wrapper(value);
  });

  final wrapped = reader.get<Wrapper>(wrappedCR);
  print(green('Retrieving a [Wrapper<dynamic>]:'));
  print(wrapped);
  // Prints:
  // Retrieving a [Wrapper<dynamic>]:
  // Wrapper<dynamic>(value: 29)
}

Features and bugs

Please file feature requests and bugs at the issue tracker.