Implement a Dart FIDL server

Prerequisites

This tutorial builds on the Dart FIDL packages tutorial. For the full set of FIDL tutorials, refer to the overview.

Overview

This tutorial shows you how to implement a FIDL protocol (fuchsia.examples.Echo) and run it on Fuchsia. This protocol has one method of each kind: a fire and forget method, a two-way method, and an event:

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

For more on FIDL methods and messaging models, refer to the FIDL concepts page.

This document covers how to complete the following tasks:

Implement a FIDL protocol.
Build and run a package on Fuchsia.
Serve a FIDL protocol.

The tutorial starts by creating a component that is served to a Fuchsia device and run. Then, it gradually adds functionality to get the server up and running.

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

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

Create and run a component

Create the component

To create a component:

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

void main(List<String> args) {
  print("Hello, world!");
}

Declare a target for the server in examples/fidl/dart/server/BUILD.gn:
```
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/server/BUILD.gn" %}
```
The dart_app template defines multiple parts:
- A Dart binary based on the specified sources and dependencies.
- A component that is set up to simply run the binary, which is described using the specified manifest file. path refers to the location of the file in the tree, and dest refers to the target location of the manifest within the component.
- A package containing the component. Packages are the unit of software distribution on Fuchsia.
For more details on packages, components, and how to build them, refer to Building components.
The dependencies will used later when implementing the FIDL server, and are not needed yet at this step.
Add a component manifest in examples/fidl/rust/server/server.cmx:
Note: The binary name in the manifest must match the name of the dart_app, which is used to define the Dart executable.
```
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/server/meta/server.cmx" %}
```

Run the component

Note: The instructions in this section are geared towards running the component on QEMU, as this is the simplest way to get started with running Fuchsia, but it is also possible to pick a different product configuration and run on actual hardware if you are familiar with running components on other product configurations.

Add the server to your configuration and build:

fx set core.x64 --with //examples/fidl/dart/server && fx build

Ensure fx serve is running in a separate tab and connected to an instance of Fuchsia (e.g. running in QEMU using fx qemu), then run the server:
Note: The component should be referenced by its [URL][glossary.component url], which is determined with the [fuchsia-pkg://][glossary.fuchsia-pkg URL] scheme. The package name in the URL matches the package_name field in the fuchsia_package declaration, and the manifest path in meta/ matches the target name of the fuchsia_component.
```
fx shell run fuchsia-pkg://fuchsia.com/echo-rust-dart#meta/echo-server.cmx
```

Implement the server

Add dependencies

Import the required dependencies in lib/main.dart:

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

Implement an Echo server

Add the following to lib/main.dart, above the main() function:

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

The implementation consists of the following elements:

The class inherits from the generated protocol class and overrides its abstract methods to define the protocol method handlers.
- The method for EchoString replies with the request value by returning it.
- The method for SendString returns void since this method does not have a response. Instead, the implementation sends an OnString event containing the request data.
The class contains an _onStringStreamController, which is used to implement the abstract onString method. The FIDL runtime will subscribe to the stream returned by this method, sending incoming events to the client. The server can therefore send an OnString event by sending an event on the stream.

You can verify that the implementation is correct by running:

fx build

Serve the protocol

To run a component that implements a FIDL protocol, you must make a request to the component manager to expose that FIDL protocol to other components. The component manager then routes any requests for the echo protocol to our server.

To fulfill these requests, the component manager requires the name of the protocol as well as a handler that it should call when it has any incoming requests to connect to a protocol matching the specified name.

The handler passed to it is a function that takes a channel (whose remote end is owned by the client), and binds it to an EchoBinding.

The EchoBinding is a class that takes a FIDL protocol implementation and a channel, and then listens on the channel for incoming requests. It will then decode the requests, dispatch them to the correct method on our server class, and write any response back to the client.

This complete process is described in further detail in the Life of a protocol open.

Initialize the binding

First, the code initializes the EchoBinding as mentioned above:

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

In order to run, a binding needs two things:

An implementation of a protocol.
A channel that the binding will listen for messages for that protocol on.

The binding binds itself to a channel and implementation when the server receives a request to connect to an Echo server.

Register the protocol request handler

Then, the code calls the component manager to expose the Echo FIDL protocol to other components:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/dart/server/lib/main.dart" region_tag="main" highlight="10,11,12,13,14,15,16,17" %}

It does so using the fuchsia_services package, which provides an API to access the startup context of the component. Specifically, each component receives a ComponentContext that the component can use to both access capabilties from other components and expose capabilities to other components. The call to sys.ComponentContext.create() obtains an instance of the component's context, and the outgoing property is used to expose the Echo protocol and later serveFromStartupInfo().

In order to add a service, the outgoing context needs to know:

The name of the service, so that clients are able to locate it using the correct path.
What to do with an incoming request to connect to the service.
- A connection request here is defined as a fidl.InterfaceRequest<Echo>. This is a type-safe wrapper around a channel.
  - InterfaceRequest indicates that this is the server end of a channel (i.e. a client is connected to the remote end of the channel)
  - The template parameter Echo means that the client expects that a server implementing the Echo protocol binds itself to this channel. The client analog of this (i.e. the type that is being used on the client side to represent the other end of this channel) is a fidl.InterfaceHandle<Echo>.

The name of the service is specified as the associated service name, and the handler is just a function that takes channel sent from the client and binds it to the EchoBinding.

Logging

The server uses the fuchsia_logger to log information. The logger needs to be initialized first using setupLogger(), then information can be logged using log.info or other methods corresponding to the various log levels.

Run the server

Build:

fx build

Then run the server:

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

You should see server hanging and the startup log using fx log. This is expected because an event loop to handle incoming requests is running. The next step will be to write a client for the server.