Implement a FIDL client


This tutorial builds on the FIDL server tutorial. For the full set of FIDL tutorials, refer to the 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/hlcpp/client/*

Create a stub component

Note: If necessary, refer back to the previous tutorial.

  1. Set up a hello world component in examples/fidl/hlcpp/client. You can name the component echo-client, and give the package a name of echo-hlcpp-client.

  2. Once you have created your component, ensure that the following works:

    fx set core.x64 --with //examples/fidl/hlcpp/client
  3. Build the Fuchsia image:

    fx build
  4. In a separate terminal, run:

    fx serve
  5. In a separate terminal, run:

    fx shell run fuchsia-pkg://

Edit GN dependencies

  1. Add the following dependencies:

    {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="deps" %}
  2. Then, include them in

    {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="includes" %}

    The reason for including these dependencies is explained in the server tutorial.

Edit component manifest

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

    {%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/client.cmx" %}

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.

Initialize the event loop

As in the server, the code first sets up an async loop so that the client can listen for incoming responses from the server without blocking.

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="main" highlight="2,28" %}

Initialize a proxy class

In the context of FIDL, proxy designates the code generated by the FIDL bindings that enables users to make remote procedure calls to the server. In HLCPP, the proxy takes the form of a class with methods corresponding to each FIDL protocol method.

The code then creates a proxy class for the Echo protocol, and connects it to the server.

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="main" highlight="4,5,6" %}
  • fuchsia::examples::EchoPtr is an alias for fidl::InterfaceRequest<fuchsia::examples::Echo> generated by the bindings.
  • Analogous to the fidl::Binding<fuchsia::examples::Echo> used in the server, fidl::InterfaceRequest<fuchsia::examples::Echo> is parameterized by a FIDL protocol and a channel it will proxy requests over the channel, and listen for incoming responses and events.
  • The code calls EchoPtr::NewRequest(), which creates a channel, binds the class to one end of the channel, and returns the other end of the channel.
  • The returned end of the channel is passed to sys::ServiceDirectory::Connect().
    • Analogous to the call to context->out()->AddPublicService() on the server side, Connect has an implicit second parameter here which is the protocol name ("fuchsia.examples.Echo"). This is where the input to the handler defined in the previous tutorial comes from: the client passes it in to Connect, which then passes it to the handler.

An important point to note here is that this code assumes that /svc already contains an instance of the Echo protocol. This is not the case by default because of the sandboxing provided by the component framework.

Set an error handler

Finally, the code sets an error handler for the proxy:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="main" highlight="8,9,10" %}

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/hlcpp/client/" region_tag="main" highlight="14,15,16,17,18,19,20" %}

For EchoString the code passes in a callback to handle the response. SendString does not require such a callback because the method does not have any response.

Set an event handler

The code then sets a handler for any incoming OnString events:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="main" highlight="21,22,23,24,25,26" %}

Terminate the event loop

The code waits to receive both a response to the EchoString method as well as an OnString event (which in the current implementation is sent after receiving a SendString request) before quitting from the loop. The code returns a successful exit code only if it receives both a response and an event:

{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/" region_tag="main" highlight="13,17,18,23,24,29" %}

Run the client

If you run the client directly, the error handler gets called 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.

  1. Configure your GN build as follows:

    fx set core.x64 --with //examples/fidl/hlcpp/server --with //examples/fidl/hlcpp/client --with //examples/fidl/test:echo-launcher
  2. Build the Fuchsia image:

    fx build
  3. 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-pkg:// fuchsia-pkg:// fuchsia.examples.Echo

You should see the client print output in the QEMU console (or using fx log).

[117659.968] 754089:754091> Running echo server
[117659.978] 754194:754196> Got event hi
[117659.978] 754194:754196> Got response hello