Implement a 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 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.
Set up a hello world component in
examples/fidl/hlcpp/client
. You can name the componentecho-client
, and give the package a name ofecho-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
Edit GN dependencies
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.
Edit component manifest
Include the
Echo
protocol in the client component’s sandbox by editing the component manifest inclient.cmx
.{%includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="examples/fidl/hlcpp/client/client.cmx" %}
Connect to the server {#main}
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/main.cc" region_tag="main" highlight="2,28" %}
Initialize a proxy class {#proxy}
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 forfidl::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 toConnect
, which then passes it to the handler.
- Analogous to the call to
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/main.cc" 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/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.
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/main.cc" 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/main.cc" 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.
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