Mapping Hooks topic

Hooks provide a way to hook into the encoding and decoding process for a class or single field. When using hooks, you have the possibility to inspect and modify values before and after they are encoded or decoded.

To use hooks, create a custom class extending the MappingHook class, and set this as the hook argument of either the MappableClass or MappableField annotation like this:

class GameHook extends MappingHook {
  const GameHook();

class PlayerHook extends MappingHook {
  const PlayerHook();

@MappableClass(hook: GameHook())
class Game {
  @MappableField(hook: PlayerHook())
  Player player;


class Player {
  String id;

Now, whenever an instance of Game is encoded or decoded, the GameHook will be applied to the class and the PlayerHook will be applied to the player field. Inside your hook class, you have four methods that you can override:

  • Object? beforeDecode(Object? value)
  • Object? afterDecode(Object? value)
  • Object? beforeEncode(Object? value)
  • Object? afterEncode(Object? value)

Each method takes a dynamic value and returns an optionally modified value.

Tip: If the beforeDecode hook already returns an instance of the target type, the normal decoding logic is skipped. The same is true for the beforeEncode hook. This gives you the possibility to use different custom mappers on the same type, especially on a field-by-field level.

A simple use-case for this would be to modify the input json before an object is decoded. In the example below, the player field can either be a full json object, or a single string. The string would then be treated as the id of the player.

class PlayerHook extends MappingHook {
  const PlayerHook();

  Object? beforeDecode(Object? value) {
    if (value is String) {
      return {'id': value};
    return value;


void main() {
  // This works as usual
  Game game = GameMapper.fromJson('{"player": {"id": "Tom"}}');
  print(; // Tom;

  // Special case: 'player' is a string instead of an object
  Game game2 = GameMapper.fromJson('{"player": "John"}');
  print(; // John

Additionally, it is important to note that field- and class-hooks are inherited by any subclasses. When both the superclass and the subclass define class-hooks, both are applied in the following order: super.beforeDecode -> sub.beforeDecode -> decode -> sub.afterDecode -> super.afterDecode. For field-hooks, only the sub-class-hook is applied.

Chaining multiple MappingHooks

You can chain and apply multiple hooks using the ChainedHook class like this: @MappableClass(hook: ChainedHook([ MyHook1(), MyHook2(), ... ])).

The hooks are applied for encoding and decoding in nested order: first.beforeDecode -> second.beforeDecode -> ... -> decode -> ... -> second.afterDecode -> first.afterDecode.

Unmapped Properties

A frequently needed use-case for hooks is to catch additional, unmapped properties from json when decoding an object. Because of this, we provide a ready-to-use MappingHook called UnmappedPropertiesHook.

To use this hook, define a Map<String, dynamic> field in your class, and provide it's name to the UnmappedPropertiesHooks constructor. Be aware that you have to provide the matching json key of the field (after applying the case style, etc.) instead of the dart field name.

@MappableClass(hook: UnmappedPropertiesHook('unmappedProps'))
class Game with GameMappable{
  String id;
  Map<String, dynamic> unmappedProps;

  Game(, this.unmappedProps);

void main() {
  Game game = GameMapper.fromJson('{"id": 1, "type": "pacman", "score": 100}');
  print(; // 1
  print(game.unmappedProps); // {type: pacman, score: 100}

Next: Custom Mappers


ChainedHook Mapping Hooks
A MappingHook that will chain multiple other hooks.
EmptyToNullHook Mapping Hooks
A MappingHook that will change an empty string to null.
MappableClass Introduction Models Configuration Polymorphism Mapping Hooks Custom Mappers
Used to annotate a class in order to generate mapping code.
MappableField Models Records Configuration Mapping Hooks
Used to annotate a parameter or field to overwrite the mapped key.
MappingHook Mapping Hooks
Extend this class to define a custom MappingHook for a class or field.
UnescapeNewlinesHook Mapping Hooks
A MappingHook that will unescape all escaped newline characters in a string.
UnmappedPropertiesHook Mapping Hooks
A MappingHook to get all unmapped properties in a Map.