Implement a FIDL client in Dart

Prerequisites

This tutorial assumes that you are familiar with writing and running a Fuchsia component and with implementing a FIDL server, which are both covered in the FIDL server tutorial. For the full set of FIDL tutorials, refer to the overview.

Overview

This tutorial implements a client for a FIDL protocol and runs it against the server created in the previous tutorial. The client in this tutorial is asynchronous. There is an alternate tutorial for synchronous clients.

If you want to write the code yourself, delete the following directories:

rm -r examples/fidl/dart/client/*

Create the component

Create a new component project at examples/fidl/dart/client:

Add a main() function to examples/fidl/dart/client/lib/main.dart:

import 'dart:async';

Future<void> main(List<String> args) async {
  print('hello world!');
}

Declare a target for the client in examples/fidl/dart/client/BUILD.gn:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/BUILD.gn" region_tag="imports" %}

# Declare a `dart_library` for the client executable.
dart_library("lib") {
  package_name = "echo_client"
  null_safe = true

  sources = [ "main.dart" ]
}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/BUILD.gn" region_tag="rest" %}

Add a component manifest in examples/fidl/dart/client/meta/client.cml:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/meta/client.cml" region_tag="example_snippet" %}

Once you have created your component, ensure that you can add it to the build configuration:
```
fx set core.x64 --with //examples/fidl/dart/client:echo-client
```
Build the Fuchsia image:
```
fx build
```

Edit GN dependencies

Add the following dependencies:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/BUILD.gn" region_tag="deps" %}

Then, import them in lib/main.dart:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/lib/main.dart" region_tag="imports" %}

These dependencies are explained in the server tutorial.

Connect to the server

The steps in this section explain how to add code to the main() function that connects the client to the server and makes requests to it.

Bind a client object

The FIDL bindings generate a class for each protocol that can be used to make requests to a server, called a proxy class. To connect to the server, the client needs to initialize a proxy class and then bind it to the server:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/lib/main.dart" region_tag="main" highlight="4,5,6,7" %}

Similar to the server code, the client uses ComponentContext to access the component's context. The difference is that the incoming property is used instead of the outgoing property, since the client is connecting to a protocol rather than offering one. Additionally, since no outgoing services are added, it uses the ComponentContext.createAndServe() convenience method.

The connectToService call does a number of things under the hood:

First, it creates a channel and binds one end to the EchoProxy. EchoProxy, similarly to EchoBinding, binds to a channel and listens for incoming messages (and sends messages back) on the channel. The channel end bound to the EchoProxy is a fidl.InterfaceHandle<Echo>, whereas the other end of the channel is a fidl.InterfaceRequest<Echo>.
It then makes a request to the component manager to connect to the Echo protocol. Specifically, it requests the other end of the channel (from the previous step) to be connected to the protocol located at the service name of the Echo protocol.

In the background, this request triggers the follow sequence of events:

The component framework routes this request to the server, where the requested service name matches the service offered by the server.
The connection request handler defined in the server code is invoked on the channel end (the fidl.InterfaceRequest<Echo>) that was provided by the client.
The handler code binds the server implementation to the channel, and starts handling any incoming messages on the channel. If the client started making requests before this point, these requests are buffered until the server is bound and starts reading from the channel. This is a process called request pipelining, which is covered in more depth in a separate tutorial.

Send requests to the server

The code makes two requests to the server:

An EchoString request
A SendString request

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/lib/main.dart" region_tag="main" highlight="9,10,11,12,13,14" %}

The call to EchoString returns a future, which resolves to the response returned by the server. The returned future will resolve to an error if there is either an error sending the request or receiving the response (e.g. when decoding the message, or if an epitaph was received).

The call to SendString returns a Future<void> since it is a fire and forget method.

The bindings reference describes how these proxy methods are generated.

Handle incoming events

The code then waits for a single OnString event from the server:

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/client/lib/main.dart" region_tag="main" highlight="15,16,17" %}

This is done by taking the event stream from the client object, then waiting for a single event from it.

Run the client

In order for the client and server to communicate using the Echo protocol, component framework must route the fuchsia.examples.Echo capability from the server to the client. For this tutorial, a [realm][glossary.realm] component is provided to declare the appropriate capabilities and routes.

Note: You can explore the full source for the realm component at //examples/fidl/echo-realm

Configure your build to include the provided package that includes the echo realm, server, and client:
```
fx set core.x64 --with examples/fidl/dart:echo-launcher-dart --with-base //src/dart \
  --args='core_realm_shards += [ "//src/dart:dart_runner_core_shard" ]'
```
NOTE: The flag --with-base //src/dart adds the required dart runner to the base packages; and the core_realm_shards argument updates the laboratory-env component environment (the environment provided to the ffx-laboratory realm, used in ffx component start) to include the required dart runner.
Build the Fuchsia image:
```
fx build
```

Run the echo_realm component. This creates the client and server component instances and routes the capabilities:

ffx component run /core/ffx-laboratory:echo_realm fuchsia-pkg://fuchsia.com/echo-dart-client#meta/echo_realm.cm

Start the echo_client instance:

ffx component start /core/ffx-laboratory:echo_realm/echo_client

The server component starts when the client attempts to connect to the Echo protocol. You should see output similar to the following in the device logs (ffx log):

[echo-server, main.dart(64)] INFO: Running Echo server
[echo-server, main.dart(33)] INFO: Received EchoString request: hello
[echo-server, main.dart(41)] INFO: Received SendString request: hi
[echo-client, main.dart(27)] INFO: Got response: hello
[echo-client, main.dart(33)] INFO: Got event: hi