Organizing components

<<../../_common/components/_organizing_intro.md>>

<<../../_common/components/_organizing_types.md>>

<<../../_common/components/_organizing_identifying.md>>

<<../../_common/components/_organizing_lifecycle.md>>

Exercise: Integrate components

In order for a component to be invoked, it must be present in the active component topology. For this exercise, you will add your component to the ffx-laboratory — a restricted collection used for development inside the product's core realm. Collections enable components to be dynamically created and destroyed at runtime.

<<../_common/_start_femu_with_packages.md>>

Add to the component topology

Create a new instance of the echo-args component using the following command:

ffx component create /core/ffx-laboratory:echo-args \
    fuchsia-pkg://fuchsia.com/echo-args#meta/echo_args.cm

This command accepts two parameters:

• /core/ffx-laboratory:echo-args: This is the component moniker, representing the path inside the component topology for the component instance.
• fuchsia-pkg://fuchsia.com/echo-args#meta/echo_args.cm: This is the component URL, indicating how Fuchsia should resolve the component from the package server.

A new component instance named echo-args now exists in the topology. Show the details of the new instance using the following command:

ffx component show echo-args

You should see the following output:

               Moniker: /core/ffx-laboratory:echo-args
                   URL: fuchsia-pkg://fuchsia.com/echo-args#meta/echo_args.cm
                  Type: CML dynamic component
       Component State: Unresolved
       Execution State: Stopped

Notice that the instance has been created, but the component URL has not been resolved. Resolution happens when the framework attempts to start the instance.

Start the component instance

Start the new echo-args component instance using the following command:

ffx component start /core/ffx-laboratory:echo-args

This command accepts one parameter:

• /core/ffx-laboratory:echo-args: This is the component moniker, representing the path inside the component topology for the component instance.

This causes the component instance to start, print a greeting to the log, then exit. Open a new terminal window and filter the device logs for messages from the example:

ffx log --filter echo

You should see the following output in the device logs:

[ffx-laboratory:echo-args][I] Hello, Alice, Bob, Spot!

Explore the instance

Show the details of the echo-args instance again using the following command:

ffx component show echo-args

You should now see the following output:

               Moniker: /core/ffx-laboratory:echo-args
                   URL: fuchsia-pkg://fuchsia.com/echo-args#meta/echo_args.cm
                  Type: CML dynamic component
       Component State: Resolved
 Incoming Capabilities: fuchsia.logger.LogSink
  Exposed Capabilities: diagnostics
       Execution State: Stopped

The component state has changed to Resolved and you can see more details about the component's capabilities.

Components have no ambient capabilities to access other parts of the system. Every capability a component requires must be explicitly routed to it through the component topology or provided by its environment.

The echo-args component requires the fuchsia.logger.LogSink capability to write to the system log. You were able to successfully view the log output because this capability is offered to components in the ffx-laboratory collection from the core realm:

{
    collections: [
        {
            name: "ffx-laboratory",
        },
    ],
    offer: [
        {
            protocol: [ "fuchsia.logger.LogSink" ],
            from: "parent",
            to: "#ffx-laboratory",
        },
    ],
}