Generic Reader

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The premise of source code generation is that we can somehow specify (hopefully few) details and flesh out the rest of the classes, and methods during the build process. Dart's static analyzer provides access to libraries, classes, class fields, class methods, functions, variables, etc in the form of Elements.

Source code generation relies heavily on constants (instantiated by a constructor prefixed with the keyword const) since constants are known at compile time. Compile-time constant expressions are represented by a DartObject and can be accessed by using the method computeConstantValue() (available for elements representing a variable).

For built-in types, DartType has methods that allow reading the underlying constant object. It can be a more difficult task to read constant values of user defined (parameterized) data-types. The package generic_reader provides a systematic way of reading constants of arbitrary data-type.


To use the package generic_reader the following steps are required:

  1. Include generic_reader and source_gen as dependencies in your pubspec.yaml file.

  2. Create an instance of GenericReader (e.g. within a source code generator function):

    final reader = GenericReader(); // Note: [reader] is a singleton.
  3. Register a Decoder function for each data-type that is going to be read. If a decoder function is missing, an error will be thrown detailing what data-type needs to be registered with the reader.

    • The built-in types bool, double, int, String, Type, Symbol do not require a decoder function.

    • There is no need to define decoder functions for Dart enums as long as they are read using the method getEnum<T>().

      However, to read a Dart enum, e.g. MyEnum, using the method get<T>() it is required to register a decoder function for this type. This is due to fact that get<dynamic>(ConstantReader constantReader) tries to match the the static type of constantReader against a built-in or registered type.

      The file player_example.dart demonstrates how to read a constant of type List<dynamic> containing int, double, and enum values.

    • To register a decoder function for a Dart enumeration, e.g. MyEnum, use the method enumValue provided by TypeMethods, an extension on ConstantReader:

       reader.addDecoder<MyEnum>((cr) => cr.enumValue<MyEnum>());
  4. Retrieve the compile-time constant values using the methods get<T>(), getList<T>(), getSet<T>(), getMap<T>(), getEnum<T>().

  5. Process the retrieved compile-time constants and generate the required source code.

Decoder Functions

GenericReader provides a systematic method of retrieving constants of arbitrary data-types by allowing users to register Decoder functions (for lack of a better a name). Decoder functions can make use of other registered decoder functions enabling the retrieval of complex generic data-structures.

Decoders functions know how to decode a specific data-type and have the following signature:

typedef T Decoder<T>(ConstantReader constantReader);

The input argument is of type ConstantReader, a wrapper around DartObject, and the function returns an object of type T. It is presumed that the input argument constantReader represents an object of type T.

User defined types are often a composition of other types, as illustrated in the example below.

Click to show source-code.
 enum Title{Mr, Mrs, Dr}

 class Age{
   const Age(this.age);
   final int age;
   bool get isAdult => age > 21;

 class Name{
   const Name({this.firstName, this.lastName, this.middleName});
   final String firstName;
   final String lastName;
   final String middleName;

 class User{
   const User({,, this.age, this.title});
   final Name name;
   final Age age;
   final int id;
   final Title title;

In order to retrieve a constant value of type User one has to retrieve the constructor parameters of type int, Name, Title, and Age first.

The following shows how to define decoder functions for the types Age, Name, and User.

Note that each decoder knows the constructor parameter-names and parameter-types of the class it handles. For example, the decoder for User knows that age is of type Age and that the field-name is age.

import 'package:generic_reader/generic_reader.dart';
import 'package:source_gen/source_gen.dart' show ConstantReader;

// The reader instance. (It is a singleton).
final reader = GenericReader();

/// Decoder function for the data-type `Age`.
Age ageDecoder(constantReader) => Age(constantReader.peek('age').intValue);

Name nameDecoder(constantReader) {
  final firstName = constantReader.peek('firstName').stringValue;
  final lastName = constantReader.peek('lastName').stringValue;
  final middleName = constantReader.peek('middleName').stringValue;
  return Name(firstName: firstName, lastName: lastName, middleName: middleName);

User userDecoder(constantReader){
  final id = constantReader.peek('id').intValue;
  final age = reader.get<Age>(constantReader.peek('age'));
  final name = reader.get<Name>(constantReader.peek('name'));
  final tile = reader.getEnum<Title>(constantReader.peek('title'));
  return User(name: name, age: age, id: id, title: title);



// Retrieving a constant value of type User:
final User user = reader.get<User>(userCR);

Remark: The method peek returns an instance of ConstantReader representing the class field specified by the input String. It returns null if the field was not initialized or not present. Moreover, peek will recursively scan the super classes if the field could not be found in the current context.


Defining decoder functions for each data-type has its obvious limitiations when it comes to generic types.

In practice, however, generic classes are often designed in such a manner that only few type parameters are valid or likely to be useful. A demonstration on how to retrieve constant values with generic type is presented in example.

Last but not least, constants that need to be retrieved during the source-generation process are most likely annotations and simple data-types that convey information to source code generators.


For further information on how to use GenericReader to retrieve constants of arbitrary type see example.

Features and bugs

Please file feature requests and bugs at the issue tracker.


Library providing a customizable generic reader aimed at creating runtime constant objects from a static representation of a compile-time constant expression such as ConstantReader or DartObject.