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/*
Note: If necessary, refer back to the previous tutorial.
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
.
Once you have created your component, ensure that the following works:
fx set core.x64 --with //examples/fidl/hlcpp/client
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-hlcpp-client#meta/echo-client.cmx
Add the following dependencies:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/BUILD.gn" region_tag="deps" %}
Then, include them in main.cc
:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="includes" %}
The reason for including these dependencies is explained in the server tutorial.
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" %}
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.
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/main.cc" region_tag="main" highlight="2,28" %}
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/main.cc" region_tag="main" highlight="4,5,6" %}
fuchsia::examples::EchoPtr
is an alias for fidl::InterfaceRequest<fuchsia::examples::Echo>
generated by the bindings.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.EchoPtr::NewRequest()
, which creates a channel, binds the class to one end of the channel, and returns the other end of the channel.sys::ServiceDirectory::Connect()
.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.
Finally, the code sets an error handler for the proxy:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="main" highlight="8,9,10" %}
The code makes two requests to the server:
EchoString
requestSendString
request{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" 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.
The code then sets a handler for any incoming OnString
events:
{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/main.cc" region_tag="main" highlight="21,22,23,24,25,26" %}
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/main.cc" region_tag="main" highlight="13,17,18,23,24,29" %}
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.
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
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/echo-launcher#meta/launcher.cmx fuchsia-pkg://fuchsia.com/echo-hlcpp-client#meta/echo-client.cmx fuchsia-pkg://fuchsia.com/echo-hlcpp-server#meta/echo-server.cmx 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