Added HeadlessPlugins (#15203)

[Wcubed]

Added a HeadlessPlugins plugin group, that adds more default functionality (like logging) than the MinimumPlugins. Fixes #15203
Changed the headless example to use the new plugin group.

I am not entirely sure if the list of plugins is correct. Are there ones that should be added / removed?

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The TerminalCtrlCHandlerPlugin has interesting effects in the headless example: Installing it a second time it will give a log message about skipping installation, because it is already installed. Ctrl+C will terminate the application in that case. However, not installing it the second time (so only on the app that runs once) has the effect that the app that runs continuously cannot be stopped using Ctrl+C.
This implies that, even though the second app did not install the Ctrl+C handler, it did something because it was keeping the one from the first app alive.
Not sure if this is a problem or issue, or can be labeled a wierd quirk of having multiple Apps in one executable.

[Wcubed]

Update because of typo

[alice-i-cecile]

Including audio, input and dev tools in this set feels quite odd to me. The value for this plugin set is overwhelmingly on servers, which shouldn't benefit from any of those.

[Wcubed]

Good point, I have removed them from the list.

[cBournhonesque]

Accessibility might be less common than the rest, I wonder if it's worth including by default

[alice-i-cecile]

IMO this should also be excluded: it's very much tied to input handling and UI.

[alice-i-cecile]

@Wcubed once we cut AccessibilityPlugin from this set this LGTM and I'll merge.

[lewiszlw]

Suggested change

	/// when the `bevy_ci_testing`feature is enabled.
	/// when the `bevy_ci_testing` feature is enabled.

[Wcubed]

Good catch, fixed

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1689 if you'd like to help out.

[mockersf]

I'm not sure this is a good idea to have in Bevy. "Headless" meaning will change depending on the use case, and controlling it with a plugin rather than features means longer compile time and a lot of code that can't be used, events not registered, component not reflected, but still available when developing which will mean a less good developer experience.

[Luminoth]

Oh my gosh, why is this only behind a feature gate now? Is it still correct that to do a runtime headless you have to do this long mess of stuff?

        app.add_plugins((
            MinimalPlugins.set(bevy::app::ScheduleRunnerPlugin::run_loop(
                bevy::utils::Duration::from_secs_f64(1.0 / SERVER_TICK_RATE as f64),
            )),
            bevy::app::PanicHandlerPlugin,
            bevy::log::LogPlugin::default(),
            bevy::transform::TransformPlugin,
            bevy::hierarchy::HierarchyPlugin,
            bevy::diagnostic::DiagnosticsPlugin,
            bevy::asset::AssetPlugin::default(),
            bevy::scene::ScenePlugin,
            bevy::animation::AnimationPlugin,
            bevy::state::app::StatesPlugin,
        ));

I was looking forward to not having to maintain the entire set of "everything that isn't rendering" just to have a switch for this.

I guess this example is implying using DefaultPlugin with the ScheduleRunnerPlugin will get the same result?

[mockersf]

Is it still correct that to do a runtime headless you have to do this long mess of stuff?

No, headless is just

        app.add_plugins((
            MinimalPlugins.set(bevy::app::ScheduleRunnerPlugin::run_loop(
                bevy::utils::Duration::from_secs_f64(1.0 / SERVER_TICK_RATE as f64),
            )),
        ));

All the others are entirely dependent on what you're doing. In your list you've only selected some of the Bevy default plugins.

I guess this example is implying using DefaultPlugin with the ScheduleRunnerPlugin will get the same result?

And no primary window. You can also decide to disable the winit and window plugins

[Luminoth]

You're saying this is what I should do?

        app.add_plugins(
            DefaultPlugins
                .set(WindowPlugin {
                    primary_window: None,
                    ..default()
                })
                .disable::<bevy::winit::WinitPlugin>(),
        )
        .add_plugins(bevy::app::ScheduleRunnerPlugin::run_loop(
            bevy::utils::Duration::from_secs_f64(1.0 / SERVER_TICK_RATE as f64),
        ));