Event capabilities

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Event capabilities allow components to subscribe to specific events that occur during the various stages of the component lifecycle. This includes information about component state changes as well as capability routing.

For a complete list of supported events and their descriptions, see the fuchsia.sys2.EventType reference documentation.

Providing event capabilities

Event capabilities always originate from component manager. They are provided to components as a framework capability. Individual components cannot declare events in their manifest capabilities.

Each event capability covers a specific scope, determined by the realm of the component where the capability was initially offered. The scope of the capability does not change, even when the capability is routed to another component.

Routing event capabilities

Components can offer and expose event capabilities that they receive from the framework to other components.

For more details on how the framework routes component capabilities, see capability routing.

Exposing

Exposing an event capability gives the component's parent access to that capability:

{
    expose: [
        {
            event: "started",
            from: "framework",
        },
    ]
}

The from: "framework" directive means that the protocol capability is provided by component manager.

Offering

Offering an event capability gives a child component access to that capability:

{
    offer: [
        {
            event: "started",
            from: "parent",
            to: [ "#child-a", "#child_b" ],
        },
        {
            event: "stopped",
            from: "framework",
            to: "#child-c",
        }
    ]
}

Events can be offered from two sources:

• framework: The event capability provided by component manager. The scope of this event will be the offering component's realm.
• parent: An event capability that was offered to the parent by another component. The scope of the offered capability matches the component that originally routed the capability from framework.

Consuming event capabilities

To consume an event capability, the component must request the fuchsia.sys2.EventSource protocol and the set of fuchsia.sys2.EventType for subscription.

To request the capability, add a use declaration for it:

{
    use: [
        { protocol: "fuchsia.sys2.EventSource" },
        { 
            event: [
                "started",
                "stopped",
            ],
            from: "framework",
        },
    ]
}

Events can be used from two sources:

• framework: The event capability provided by component manager. The scope of this event will be the consuming component's realm.
• parent: An event capability that was offered to the parent by another component. The scope of the offered capability matches the component that originally routed the capability from framework.

Caution: The default source for capabilities is parent unless otherwise specified.

Static event streams

Event subscriptions can be set up statically in the component manifest using static event streams. Static event streams are similar to event streams created through the fuchsia.sys2.EventSource/Subscribe FIDL method but they are set up by the framework during the resolution of a component's manifest.

The following is an example of the syntax of a static event stream declaration:

use: [
    {
        event: "started",
        from: "parent",
    },
    {
        event_stream: "MyEventStream",
        subscriptions: [
            {
                event: "started",
            }
        ],
    },
]

To connect to a static event stream, call the fuchsia.sys2.EventSource/TakeStaticEventStream FIDL method and provide the name of the event stream defined in the manifest. In the above example, the name is MyEventStream.

Static event streams are limited to the events listed in the subscriptions section of the manifest. Attempting to subscribe to a different event produces a validation error. In the above example, the only events available to the stream are started events.

Event routing example

Consider the following example of a component topology:

Notice the following key aspects of this example:

• core/archivist: Receives started events for the entire topology through an events capability routed from root.

  // root.cml
  {
      offer: [
          {
              protocol: "fuchsia.sys2.EventSource",
              from: "parent",
              to: "#core",
          },
          {
              event: "started",
              from: "parent",
              to: "#core",
          },
      ]
  }
  
  // core.cml
  {
      offer: [
          {
              protocol: "fuchsia.sys2.EventSource",
              from: "parent",
              to: [ "#archivist", "#test_manager" ],
          },
          {
              event: "started",
              from: "parent",
              to: [ "#archivist", "#test_manager" ],
          },
      ]
  }
  
  // archivist.cml
  {
      use: [
          { protocol: "fuchsia.sys2.EventSource" },
          { event: "started" },
      ]
  }
  

• core/test_manager/archivist: Receives started events for all test components through an events capability routed from test_manager.

  // test_manager.cml
  {
      offer: [
          {
              protocol: "fuchsia.sys2.EventSource",
              from: "parent",
              to: [ "#tests", "#archivist" ],
          },
          {
              event: "started",
              from: "framework",
              to: "#archivist",
          },
      ]
  }
  
  // archivist.cml
  {
        use: [
            { protocol: "fuchsia.sys2.EventSource" },
            { event: "started" },
      ]
  }
  

• core/test_manager/tests:test-12345: Receives started events for foo and bar through the events capability provided by framework.

  // test-12345.cml
  {
      use: [
          { protocol: "fuchsia.sys2.EventSource" },
          {
              event: "started",
              from: "framework",
          },
      ]
  }