Custom Mappers topic

You can create custom mappers to support custom types that are not part of the generated code. You can create a custom mapper in two ways:

  • Inherit from the low-level MapperBase and MapperElementBase interfaces, or
  • inherit from the high-level SimpleMapper interface.

For most use-cases it is recommended to use the SimpleMapper interface and only use the base interfaces when required for more advanced uses.

Simple Mapper

Create a class extending SimpleMapper<T> with T being the type that you want to decode to / encode from. Then implement the decode() and encode() methods.

A custom mapper for the Uri type would look like this:

class UriMapper extends SimpleMapper<Uri> {
  Uri decode(dynamic value) {
    return Uri.parse(value as String);

  dynamic encode(Uri self) {
    return self.toString();

In the encode() method you should return a primitive serializable type, like String, int, bool, etc. or a List or Map of those types. There are also additional methods you can override, like stringify, or equals. This will enable == checks and toString on classes using this type.


To use a custom mapper, add it to the MappableClass.includeCustomMappers annotation property of the target class:

@MappableClass(includeCustomMappers: [UriMapper])
class Domain {
  final Uri uri;

  const Domain(this.uri);

Alternatively you can add this manually to any mapper or as the default for all mappers by adding this somewhere in your initialization code (e.g. early inside main()):

// Add this only for the [Domain] model

// Add this as the default [Uri] mapper for all models.

Tip: Be aware that you can also unuse() (and replace) any mappers, both custom, generated, and for primitive types. This might come in handy if you want to switch between different custom mappers for the same type.

Generic Custom Types

When dealing with generic types with one or two type parameters, you can use the SimpleMapper1 or SimpleMapper2 variants. Here the decoding and encoding functions are generic functions for receiving the correct type parameters:

You also need to construct a typeFactory as shown below.

class GenericBox<T> {
  T content;

class CustomBoxMapper extends SimpleMapper1<GenericBox> {

  // use the type parameter [T] in the return type [GenericBox<T>]
  GenericBox<T> decode<T>(dynamic value) { 
    // use the type parameter [T] in your decoding logic
    T content = MapperContainer.defaults.fromValue<T>(value);
    return GenericBox<T>(content); 

  // use the type parameter [T] in the parameter type [GenericBox<T>]
  dynamic encode<T>(GenericBox<T> self) {
    // use the type parameter [T] in your encoding logic
    return MapperContainer.defaults.toValue<T>(self.content); 

  // In case of generic types, we also must specify a type factory. This is a special type of 
  // function used internally to construct generic instances of your type.
  // Specify any type arguments in alignment to the decode/encode functions.
  Function get typeFactory => <T>(f) => f<GenericBox<T>>();

Custom Iterables and Maps

For special Iterable and Map types, you can of course specify custom mappers as described in the previous section. However, we provide ready-to-use IterableMapper and MapMapper to make your life a little bit easier:

For both you have to provide

  1. a factory function, which converts a generic iterable / map to your special implementation,
  2. a type factory, similar to the one used in generic custom mappers.
  <T>(Iterable<T> i) => HashSet.of(i),
  <T>(f) => f<HashSet<T>>(),

  <K, V>(Map<K, V> m) => HashMap.of(m),
  <K, V>(f) => f<HashMap<K, V>>(),

HashSet<String> brands = MapperContainer.defaults.fromJson('["Toyota", "Audi", "Audi"]');
print(String); // {"Toyota", "Audi"}

Advanced Uses

For more advanced uses, extend from the MapperBase class instead of the SimpleMapper.

Here the setup requires a bit more boilerplate, but you get full control over your types:

// extend [MapperBase] and provide your type
class MyClassMapper extends MapperBase<MyClass> {
  // implement the [createElement] method and return your custom element instance
  MyClassMapperElement createElement(MapperContainer container) {
    return MyClassMapperElement._(this, container);
  // If your type is generic, specify a type factory. Else this can be skipped.
  Function get typeFactory => <T>(f) => f<MyClass<T>>();

// extend [MapperElementBase] and provide your type
class MyClassMapperElement extends MapperElementBase<MyClass> {
  MyClassMapperElement._(super.mapper, super.container);

  // all the following overrides are optional
  // if your type is generic, make this function also generic
  Function get decoder => (dynamic value) {
    // your decoding logic here
    throw UnimplementedError();
  // if your type is generic, make this function also generic
  Function get encoder => (MyClass self) {
    // your encoding logic here
    throw UnimplementedError();
  String stringify(MyClass self) {
    // your stringify logic here
    throw UnimplementedError();
  int hash(MyClass self) {
    // your hashcode logic here
    throw UnimplementedError();
  bool equals(MyClass self, MyClass other) {
    // your equals logic here
    throw UnimplementedError();

Next: Mapper Container


EnumMapper<T extends Enum> Custom Mappers
IterableMapper<I extends Iterable> Custom Mappers
MapMapper<M extends Map> Custom Mappers
MappableClass Introduction Models Configuration Polymorphism Mapping Hooks Custom Mappers
Used to annotate a class in order to generate mapping code
MapperBase<T> Custom Mappers
MapperElementBase<T> Custom Mappers
This class needs to be implemented by all mappers. It defaults to throwing unsupported exceptions for all methods.
SimpleMapper<T> Custom Mappers
Interface to define custom mappers.
SimpleMapper1<T> Custom Mappers
Interface to define custom mappers for generic types with 1 argument.
SimpleMapper2<T> Custom Mappers
Interface to define custom mappers for generic types with 2 arguments.