Run components in a test

This guide demonstrates how to run a test component.

Note: This guide is specific to components v2.

A component instance is started in Fuchsia when other components request a capability from it. For instance, a test component is started by a test manager, which is also a component, on behalf of a request to run a test.

The guide uses the hello_world_bin_test component in the basic example package. When you run this package’s hello-world-tests test suite, the test manager starts the hello_world_bin_test component. As a result, the component’s test binary runs on a Fuchsia device.

To run this test component, the steps are:

Build a Fuchsia image.
Start the emulator.
Run the test suite.

Prerequisites

Verify the following requirements:

Build a Fuchsia image

Configure and build your Fuchsia image to include the test component:

To include a specific component, run the fx set command with the --with option:
```
fx set core.x64 --with //examples/components/basic:hello-world-tests
```
//examples/components/basic is the directory of the example package and hello-world-tests is the name of the build target defined in the package's BUILD.gn file.
Build your Fuchsia image:
```
fx build
```
When the fx build command completes, your new Fuchsia image now includes the hello_world_bin_test component that can be fetched and launched on-demand.

Start the emulator

Start the emulator with your Fuchsia image and run a package repository server:

Note: The steps in this section assume that you don't have any terminals currently running FEMU or the fx serve command.

Configure an IPv6 network for the emulator (you only need to do this once):

sudo ip tuntap add dev qemu mode tap user $USER && sudo ifconfig qemu up

In a new terminal, start the emulator:
```
fx emu -N
```
Set the emulator to be your device:
```
fx set-device
```
If you have multiple devices, select step-atom-yard-juicy (the emulator’s default device name), for example:
In another new terminal, start a package repository server:
```
fx serve
```
Keep the fx serve command running as a package server for your device.

Run the test suite

Run the hello-world-tests test suite:

fx test hello-world-tests

This command prints the following output:

$ fx test hello-world-tests

...

[0/1] 00:00 🤔  /home/fuchsia/.jiri_root/bin/fx shell run-test-suite fuchsia-pkg://fuchsia.com/hello-world-tests#meta/hello_world_bin_test.cm
 >> Runtime has exceeded 2 seconds (adjust this value with the -s|--slow flag)
Running test 'fuchsia-pkg://fuchsia.com/hello-world-tests#meta/hello_world_bin_test.cm'

[RUNNING]   tests::assert_0_is_0
[PASSED]    tests::assert_0_is_0
1 out of 1 tests passed...
fuchsia-pkg://fuchsia.com/hello-world-tests#meta/hello_world_bin_test.cm completed with result: PASSED

[1/1] 00:05 ✅  /home/fuchsia/.jiri_root/bin/fx shell run-test-suite fuchsia-pkg://fuchsia.com/hello-world-tests#meta/hello_world_bin_test.cm

🎉  Ran 1 tests with 0 failures (use the -v flag to see each test) 🎉

The output shows that the hello_world_bin_test component is fetched from the package repository server and the component instance runs the test binary on the Fuchsia device (the emulator). See hello_world.rs for the source code of this test binary.