Implement a Dart FIDL client

Prerequisites

This tutorial builds on 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][sync-client] for synchronous clients.

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

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

Create a stub component

Set up a hello world dart_app in examples/fidl/dart/client, with a name of echo-dart-client.
Once you have created your component, ensure that the following works:
```
fx set core.x64 --with //examples/fidl/dart/client
```
Note: If necessary, refer back to the previous tutorial.
Build the Fuchsia image:
```
fx build
```
In a separate terminal, run:
```
fx serve
```

In a separate terminal, run:

fx shell run fuchsia-pkg://fuchsia.com/echo-dart-client#meta/echo-dart-client.cmx

Edit GN dependencies

Add the following dependencies:

  deps = [
    "//examples/fidl/fuchsia.examples",
    "//sdk/dart/fidl",
    "//sdk/dart/fuchsia",
    "//sdk/dart/fuchsia_logger",
    "//sdk/dart/fuchsia_services",
  ]

Then, import them in lib/main.dart:

import 'dart:async';

import 'package:fidl_fuchsia_examples/fidl_async.dart' as fidl_echo;
import 'package:fuchsia/fuchsia.dart' show exit;
import 'package:fuchsia_logger/logger.dart';
import 'package:fuchsia_services/services.dart';

These dependencies are explained in the server tutorial.

Edit component manifest

Include the Echo protocol in the client component's sandbox by editing the component manifest in client.cmx.

{
    "program": {
        "data": "data/echo-dart-client"
    },
    "sandbox": {
        "services": [
            "fuchsia.examples.Echo",
            "fuchsia.logger.LogSink",
            "fuchsia.sys.Environment"
        ]
    }
}

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:

Future<void> main(List<String> args) async {
  // Create our component context and serve the outgoing directory.
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-client');

  // Bind. We bind EchoProxy, a generated proxy class, to the remote Echo
  // service.
  final client = fidl_echo.EchoProxy();
  context.svc.connectToService(client);

  // Invoke echoString with a value and print its response.
  final response = await client.echoString('hello');
  log.info('Got response: $response');

  // Invoke sendString, which does not have a response
  await client.sendString('hi');
  // Wait for one OnString event and print its value.
  final event = await client.onString.first;
  log.info('Got event: $event');

  // Allow log messages to get piped through to the syslogger before exiting
  // and killing this process
  await Future(() => exit(0));
}

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

Future<void> main(List<String> args) async {
  // Create our component context and serve the outgoing directory.
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-client');

  // Bind. We bind EchoProxy, a generated proxy class, to the remote Echo
  // service.
  final client = fidl_echo.EchoProxy();
  context.svc.connectToService(client);

  // Invoke echoString with a value and print its response.
  final response = await client.echoString('hello');
  log.info('Got response: $response');

  // Invoke sendString, which does not have a response
  await client.sendString('hi');
  // Wait for one OnString event and print its value.
  final event = await client.onString.first;
  log.info('Got event: $event');

  // Allow log messages to get piped through to the syslogger before exiting
  // and killing this process
  await Future(() => exit(0));
}

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:

Future<void> main(List<String> args) async {
  // Create our component context and serve the outgoing directory.
  final context = ComponentContext.createAndServe();
  setupLogger(name: 'echo-client');

  // Bind. We bind EchoProxy, a generated proxy class, to the remote Echo
  // service.
  final client = fidl_echo.EchoProxy();
  context.svc.connectToService(client);

  // Invoke echoString with a value and print its response.
  final response = await client.echoString('hello');
  log.info('Got response: $response');

  // Invoke sendString, which does not have a response
  await client.sendString('hi');
  // Wait for one OnString event and print its value.
  final event = await client.onString.first;
  log.info('Got event: $event');

  // Allow log messages to get piped through to the syslogger before exiting
  // and killing this process
  await Future(() => exit(0));
}

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

Run the client

If you run the client directly, it will not connect to the server correctly because the client does not automatically get the Echo protocol provided in its sandbox (in /svc). To get this to work, a launcher tool is provided that launches the server, creates a new Environment for the client that provides the server's protocol, then launches the client in it.

Configure your GN build as follows:

fx set core.x64 --with //examples/fidl/dart/server --with //examples/fidl/dart/client --with //examples/fidl/dart/launcher_bin

Build the Fuchsia image:
```
fx build
```

Run the launcher by passing it the client URL, the server URL, and the protocol that the server provides to the client:

fx shell run fuchsia-pkg://fuchsia.com/example-launcher-dart#meta/example-launcher-dart.cmx fuchsia-pkg://fuchsia.com/echo-dart-client#meta/echo-dart-client.cmx fuchsia-pkg://fuchsia.com/echo-dart-server#meta/echo-dart-server.cmx fuchsia.examples.Echo

You should see output similar to the following in the QEMU console (or using ffx log):

[105541.570] 489493:489495> Listening for incoming connections...
[105541.573] 489493:489495> Received EchoString request for string "hello"
[105541.574] 489493:489495> Response sent successfully
[105541.574] 489272:489274> response: "hello"
[105541.575] 489493:489495> Received SendString request for string "hi"
[105541.575] 489493:489495> Event sent successfully
[105541.575] 489272:489274> Received OnString event for string "hi"