Dart bindings


Given the library declaration:

library fuchsia.examples;

The bindings code for this library is generated into a fidl_fuchsia_examples_async dart library. The fidl_ prefix and _async suffix are hardcoded by the FIDL toolchain.

This code can then be imported using:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="import" %}


All constants are generated as a const. For example, the following constants:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="consts" %}

Are generated as:

const int BOARD_SIZE = 9;
const String NAME = "Tic-Tac-Toe";

The correspondence between FIDL primitive types and Dart types is outlined in built-in types.


This section describes how the FIDL toolchain converts FIDL types to native types in Dart. These types can appear as members in an aggregate type, as parameters to a protocol method, or as the type contained in an event or method response Future.

Nullable types do not have different generated types than their non-nullable counterparts in Dart.

Built-in types

The FIDL types are converted to Dart types based on the following table:

FIDL TypeDart Type
int8, int16, int32, int64, uint8, uint16, uint32, uint64int
float32, float64double
array<int8>:N, vector<int8>:NInt8List
array<int16>:N, vector<int16>:NInt16List
array<int32>:N, vector<int32>:NInt32List
array<int64>:N, vector<int64>:NInt64List
array<uint8>:N, vector<uint8>:NUint8List
array<uint16>:N, vector<uint16>:NUint16List
array<uint32>:N, vector<uint32>:NUint32List
array<uint64>:N, vector<uint64>:NUint64List
array<float32>:N, vector<float32>:NFloat32List
array<float64>:N, vector<float64>:NFloat64List
array<T>:N, vector<T>:NList<T>
handle:S, handlezircon.Handle

Response and event parameters

Method response and event types (see Protocols) are represented using Future<T>, where T is a type containing all of the response/event parameters. This section describes how the FIDL toolchain generates this inner type T.

  • Empty responses and events use void.
  • Responses and events with a single parameter T just use T as the response or event type.
  • Responses and events with multiple parameters use a generated wrapper class which follows the naming scheme [Protocol]$[Method]$Response. For example, an event OnOpponentMove for protocol TicTacToe that has multiple parameters would use generated class TicTacToe$OnOpponentMove$Response. This class provides a single method: the constructor, which has positional arguments corresponding to the response or event parameters.

Note that methods that do not have a response will have a response type of Future<void>, which is the same type used by methods with an empty response. In the former case, the Future can be expected to resolve immediately after sending the request, whereas in the latter case, the Future is only resolved after receiving the empty response from the server.

Type definitions


Given the bits definition:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="bits" %}

The FIDL toolchain generates a FileMode class with static const variables for each bits member, as well as for a FileMode with no flag set ($none) or every flag set ($mask):

  • static const FileMode read
  • static const FileMode write
  • static const FileMode execute
  • static const FileMode $none
  • static const FileMode $mask

FileMode provides the following methods:

  • int get $value: Getter for the underlying int value.
  • String toString(): Returns a readable representation of the FileMode.
  • FileMode operator |(FileMode other): Bitwise or operator.
  • FileMode operator &(FileMode other): Bitwise and operator.
  • bool operator(dynamic other): Equality operator.
  • int getUnknownBits(): Returns only the set bits that are unknown. Always returns 0 for strict bits.
  • bool hasUnknownBits(): Returns whether this value contains any unknown bits. Always returns false for strict bits.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="bits" adjust_indentation="auto" %}


Given the enum definition:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="enums" %}

The FIDL toolchain generates a LocationType class with static const variables for each enum member:

  • static const LocationType museum
  • static const LocationType airport
  • static const LocationType restaurant

As well as the following variables:

  • static const Map<String, LocationType> $valuesMap: A mapping of the string representation of the member ('museum', 'airport', or 'restaurant') to its corresponding enum value (LocationType.museum, LocationType.airport, or LocationType.restaurant)
  • static const List<LocationType> $values: A list of all of the enum values.

If LocationType is flexible, it will have an unknown placeholder member as well:

  • static const LocationType $unknown

If the enum has a member tagged with the [Unknown] attribute, the placeholder variable will have the same value as the tagged unknown member.

LocationType provides the following methods:

  • static LocationType $valueOf(String name): Look up a string name in the $valuesMap.
  • String toString(): Returns a readable representation of the LocationType.
  • bool isUnknown(): Returns whether this enum is unknown. Always returns false for strict enums.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="enums" adjust_indentation="auto" %}


Given the struct declaration:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="structs" %}

The FIDL toolchain generates a Color class with the following methods:

  • const Color({@required id, name}): The constructor for Color takes named arguments corresponding to the struct's fields. Fields that are not nullable and do not have a default value specified are marked as @required.
  • int get id: Getter for the id field.
  • String get name: Getter for the name field.
  • Color.clone(Color, {int id, String name}): Clone constructor that will clone an existing Color, possibly overriding specific field values based on the provided named arguments.
  • List<Object> get $fields: Returns a list of fields in declaration order.
  • String toString(): Returns a readable string of the Color
  • bool operator==(dynamic other): Equality operator that performs a deep comparison when compared to another instance of a Color.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="structs" adjust_indentation="auto" %}


Given the union definition:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="unions" %}

FIDL generates an enum representing the tags of the union:

enum JsonValueTag {

As well as a JsonValue class with the following methods:

  • const JsonValue.withIntValue(int) and const JsonValue.withStringValue(String): Constructors for each variant.
  • JsonValueTag get $tag: Getter for the tag corresponding to this the variant of this union.
  • int? get intValue and String? get stringValue: Getter for the underlying value. If the instance's variant does not match the getter method, null is returned.
  • String toString(): Returns a readable string of the JsonValue.
  • int get $ordinal: Getter for the underlying ordinal value.
  • Object get $data: Getter for the underlying union data.
  • bool operator ==(dynamic other): Equality operator that performs deep comparison when compared to another JsonValue of the same variant.
  • fidl.UnknownRawData? get $unknownData: Returns the bytes and handles of the unknown data if this union contains an unknown variant, or null otherwise. Always returns null for strict unions.

If JsonValue is flexible, it will have the following additional methods:

  • const JsonValue.with$UnknownData(int ordinal, fidl.UnknownRawData data): Constructor for a value with an unknown variant set. This should only be used for testing, e.g. to check that code handles unknown unions correctly.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="unions" adjust_indentation="auto" %}

Flexible unions and unknown variants

Flexible unions have an extra variant in the generated tag class:

enum JsonValueTag {

When a FIDL message containing a union with an unknown variant is decoded into JsonValue, JsonValue.$tag returns JsonValueTag.$unknown, and JsonValue.$ordinal returns the unknown ordinal.

Encoding a union with an unknown variant writes the unknown data and the original ordinal back onto the wire.

Strict unions fail when decoding an unknown variant. Flexible unions that are value types fail when decoding an unknown variant with handles.


Given the table definition:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/fuchsia.examples/types.test.fidl" region_tag="tables" %}

The FIDL toolchain generates a User class that defines the following methods:

  • const User({$unknownData, age, name}): Constructor for User. Contains an optional parameter for each field as well as a map containing any unknown fields, as a Map<int, fidl.UnknownRawData>. Specifying a value for the unknown fields should only be done for testing, e.g. to test that a table with unknown fields is handled correctly.
  • int get age: Getter for the age field.
  • String get name: Getter for the name field.
  • Map<int, dynamic> get $fields: Returns a map of ordinals to field values.
  • Map<int, fidl.UnknownRawData>? get $unknownData: Returns a map of ordinals to unknown field values (i.e. bytes and handles). The list of handles is returned in [traversal order][traversal], and is guaranteed to be empty if the table is a value type.
  • bool operator ==(dynamic other): Equality operator that performs deep comparison when compared to another User.

Example usage:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/fidl_packages/test/types_test.dart" region_tag="tables" adjust_indentation="auto" %}


Given the protocol:

protocol TicTacToe {
    StartGame(bool start_first);
    MakeMove(uint8 row, uint8 col) -> (bool success, GameState? new_state);
    -> OnOpponentMove(GameState new_state);

Note: The MakeMove method above returns a bool representing success, and a nullable response value. This is considered un-idiomatic, you should use an error type instead.

FIDL generates an abstract TicTacToe class, which defines the interface of the service used by clients to proxy calls to the server, and for the server for implementing the protocol.

TicTacToe contains a static const String $serviceName, which is defined depending on the presence of the [Transitional] attribute.

TicTacToe has the following abstract methods, representing the protocol methods:

  • async.Future<void> startGame(bool start_first): Abstract method for a fire and forget protocol method. It takes as arguments the request parameters and returns a future of void.
  • async.Future<TicTacToe$MakeMove$Response> makeMove(int row, int col): Abstract method for a two way protocol method. It takes as arguments the request parameters and returns a future of the response type.
  • async.Stream<GameState> get onOpponentMove: Getter for a Stream of onOpponentMove events.


The FIDL toolchain generates a TicTacToeProxy class that extends fidl.AsyncProxy<TicTacToe>, and provides an implementation for the abstract TicTacToe class that encodes and sends the request to the server end of the channel.

Example client code could thus look like the following:

final tictactoe = fidl_tictactoe.TicTacToeProxy();
// ...bind the proxy, omitted from this example
final state = await tictactoe.makeMove(0, 0);

Examples on how to set up and bind a proxy class to a channel are covered in the Dart tutorial.


Implementing a server for a FIDL protocol involves providing a concrete implementation of TicTacToe abstract class.

The bindings provide a TicTacToeBinding class that can bind to a TicTacToe instance and a channel, and listens to incoming messages on the channel, dispatches them to the server implementation, and sends messages back through the channel. This class implements


Examples on how to set up and bind a server implementation are covered in the Dart tutorial.



The TicTacToeProxy class automatically implements the onOpponentMove getter. Clients obtain an async.Stream of onOpponentMove events sent from the server using this getter.


Servers send events by implementing the onOpponentMove getter on the abstract TicTacToe class. A TicTacToeBinding (see tutorial) that is bound to an instance of TicTacToe that has implemented the onOpponentMove getter will listen for events on the returned async.Stream, forwarding them to the client.


Given the method with an error type:

protocol TicTacToe {
    MakeMove(uint8 row, uint8 col) -> (GameState new_state) error MoveError;

The method signature for MakeMove on the generated abstract TicTacToe class is:

async.Future<GameState> makeMove(int row, int col)

The encapsulated Future corresponds to the generated response type for the success case, and the error case is represented by having the server implementation or the proxy class throw a fidl.MethodException.

Note: Unlike the previous example, the response type is just a Future<GameState> instead of a TicTacToe$MakeMove$Response class. This is because the method went from having two parameters to one parameter, following the response and event type rules.

Using this feature, an example implementation of MakeMove on the server side could look like:

async.Future<GameState> makeMove(int row, int col) {
  if (row > 9 || col > 9) {
    return async.Future.error(fidl.MethodException(MoveError.OutOfBounds));
  return async.Future.value(doSomething(row, col));

The TicTacToeBinding class will catch fidl.MethodExceptions and encode it as an error.

An example of using this on the client side would be:

myproxy.makeMove(1, 2).then((gameState) { ... })
                      .catchError((moveError) { ... });

Protocol composition

FIDL does not have a concept of inheritance, and generates full code as described above for all composed protocols. In other words, the code generated for:

protocol A {

protocol B {
    compose A;

Provides the same API as the code generated for:

protocol A {

protocol B {

The generated code is identical except for the method ordinals.

Protocol and method attributes


For protocol methods annotated with the [Transitional] attribute, the FIDL toolchain generates a default implementation on the abstract class so that server implementations will continue to compile without having to override the new method.


The generated class for a protocol annotated with the [Discoverable] attribute has a non-null $serviceName field.

Test scaffolding

The FIDL toolchain generates a fidl_test.dart file that contains convenience code for testing FIDL server implementations. This file contains a class for each protocol that provides stub implementations for each of the class’s methods, making it possible to implement only the methods that are used during testing.

Given the example protocol above, The FIDL toolchain generates a TicTacToe$TestBase class that extends the TicTacToe abstract class. All methods are implemented by returning async.Future.error(UnimplementedError()), and all events are implemented by returning a Stream with a single UnimplementedError event.