Headless by features (#16401)

# Objective

- Fixes #16152 

## Solution

- Put `bevy_window` and `bevy_a11y` behind the `bevy_window` feature.
they were the only difference
- Add `ScheduleRunnerPlugin` to the `DefaultPlugins` when `bevy_window`
is disabled
- Remove `HeadlessPlugins`
- Update the `headless` example

Cargo.toml

@@ -125,6 +125,7 @@ default = [
   "bevy_text",
   "bevy_ui",
   "bevy_ui_picking_backend",
+  "bevy_window",
   "bevy_winit",
   "custom_cursor",
   "default_font",
@@ -218,6 +219,9 @@ bevy_ui = [
   "bevy_ui_picking_backend",
 ]
 
+# Windowing layer
+bevy_window = ["bevy_internal/bevy_window"]
+
 # winit window and input backend
 bevy_winit = ["bevy_internal/bevy_winit"]
 

crates/bevy_dev_tools/src/ci_testing/mod.rs

@@ -17,7 +17,7 @@ use core::time::Duration;
 /// (`ci_testing_config.ron` by default) and executes its specified actions. For a reference of the
 /// allowed configuration, see [`CiTestingConfig`].
 ///
-/// This plugin is included within `DefaultPlugins`, `HeadlessPlugins` and `MinimalPlugins`
+/// This plugin is included within `DefaultPlugins` and `MinimalPlugins`
 /// when the `bevy_ci_testing` feature is enabled.
 /// It is recommended to only used this plugin during testing (manual or
 /// automatic), and disable it during regular development and for production builds.

crates/bevy_internal/Cargo.toml

@@ -97,7 +97,7 @@ serialize = [
   "bevy_time/serialize",
   "bevy_transform/serialize",
   "bevy_ui?/serialize",
-  "bevy_window/serialize",
+  "bevy_window?/serialize",
   "bevy_winit?/serialize",
 ]
 multi_threaded = [
@@ -162,6 +162,7 @@ animation = ["bevy_animation", "bevy_gltf?/bevy_animation"]
 
 bevy_sprite = ["dep:bevy_sprite", "bevy_gizmos?/bevy_sprite"]
 bevy_pbr = ["dep:bevy_pbr", "bevy_gizmos?/bevy_pbr"]
+bevy_window = ["dep:bevy_window", "dep:bevy_a11y"]
 
 # Used to disable code that is unsupported when Bevy is dynamically linked
 dynamic_linking = ["bevy_diagnostic/dynamic_linking"]
@@ -248,7 +249,7 @@ ghost_nodes = ["bevy_ui/ghost_nodes"]
 
 [dependencies]
 # bevy
-bevy_a11y = { path = "../bevy_a11y", version = "0.15.0-dev" }
+bevy_a11y = { path = "../bevy_a11y", version = "0.15.0-dev", optional = true }
 bevy_app = { path = "../bevy_app", version = "0.15.0-dev" }
 bevy_core = { path = "../bevy_core", version = "0.15.0-dev" }
 bevy_derive = { path = "../bevy_derive", version = "0.15.0-dev" }
@@ -266,7 +267,7 @@ bevy_reflect = { path = "../bevy_reflect", version = "0.15.0-dev", features = [
 bevy_time = { path = "../bevy_time", version = "0.15.0-dev" }
 bevy_transform = { path = "../bevy_transform", version = "0.15.0-dev" }
 bevy_utils = { path = "../bevy_utils", version = "0.15.0-dev" }
-bevy_window = { path = "../bevy_window", version = "0.15.0-dev" }
+bevy_window = { path = "../bevy_window", version = "0.15.0-dev", optional = true }
 bevy_tasks = { path = "../bevy_tasks", version = "0.15.0-dev" }
 # bevy (optional)
 bevy_animation = { path = "../bevy_animation", optional = true, version = "0.15.0-dev" }

crates/bevy_internal/src/default_plugins.rs

@@ -13,7 +13,11 @@ plugin_group! {
         bevy_hierarchy:::HierarchyPlugin,
         bevy_diagnostic:::DiagnosticsPlugin,
         bevy_input:::InputPlugin,
+        #[custom(cfg(not(feature = "bevy_window")))]
+        bevy_app:::ScheduleRunnerPlugin,
+        #[cfg(feature = "bevy_window")]
         bevy_window:::WindowPlugin,
+        #[cfg(feature = "bevy_window")]
         bevy_a11y:::AccessibilityPlugin,
         #[custom(cfg(not(target_arch = "wasm32")))]
         bevy_app:::TerminalCtrlCHandlerPlugin,
@@ -72,55 +76,6 @@ plugin_group! {
     ///
     /// [`DefaultPlugins`] contains all the plugins typically required to build
     /// a *Bevy* application which includes a *window* and presentation components.
-    /// For *headless* cases – without a *window* or presentation, see [`HeadlessPlugins`].
-    /// For the absolute minimum number of plugins needed to run a Bevy application, see [`MinimalPlugins`].
-}
-
-plugin_group! {
-    /// This plugin group will add all the default plugins for a headless (no *window* or rendering) *Bevy* application:
-    pub struct HeadlessPlugins {
-        bevy_app:::PanicHandlerPlugin,
-        bevy_log:::LogPlugin,
-        bevy_core:::TaskPoolPlugin,
-        bevy_core:::TypeRegistrationPlugin,
-        bevy_core:::FrameCountPlugin,
-        bevy_time:::TimePlugin,
-        bevy_transform:::TransformPlugin,
-        bevy_hierarchy:::HierarchyPlugin,
-        bevy_diagnostic:::DiagnosticsPlugin,
-        bevy_app:::ScheduleRunnerPlugin,
-        #[custom(cfg(not(target_arch = "wasm32")))]
-        bevy_app:::TerminalCtrlCHandlerPlugin,
-        #[cfg(feature = "bevy_asset")]
-        bevy_asset:::AssetPlugin,
-        #[cfg(feature = "bevy_scene")]
-        bevy_scene:::ScenePlugin,
-        #[cfg(feature = "bevy_animation")]
-        bevy_animation:::AnimationPlugin,
-        #[cfg(feature = "bevy_state")]
-        bevy_state::app:::StatesPlugin,
-        #[cfg(feature = "bevy_ci_testing")]
-        bevy_dev_tools::ci_testing:::CiTestingPlugin,
-        #[doc(hidden)]
-        :IgnoreAmbiguitiesPlugin,
-    }
-    /// This group of plugins is intended for use for *headless* programs, for example: dedicated game servers.
-    /// See the [*Bevy* *headless* example](https://github.com/bevyengine/bevy/blob/main/examples/app/headless.rs)
-    ///
-    /// [`HeadlessPlugins`] obeys *Cargo* *feature* flags. Users may exert control over this plugin group
-    /// by disabling `default-features` in their `Cargo.toml` and enabling only those features
-    /// that they wish to use.
-    ///
-    /// [`HeadlessPlugins`] contains all the plugins typically required to build
-    /// a *Bevy* application. In contrast with [`DefaultPlugins`], it leaves out *window* and presentation components.
-    /// This allows applications built using this plugin group to run on devices that do not have a screen or rendering
-    /// capabilities.
-    /// It includes a [schedule runner (`ScheduleRunnerPlugin`)](crate::app::ScheduleRunnerPlugin)
-    /// to provide functionality that would otherwise be driven by a windowed application's
-    /// *event loop* or *message loop*.
-    ///
-    /// Windowed applications that wish to use a reduced set of plugins should consider the
-    /// [`DefaultPlugins`] plugin group which can be controlled with *Cargo* *feature* flags.
     /// For the absolute minimum number of plugins needed to run a Bevy application, see [`MinimalPlugins`].
 }
 
@@ -162,8 +117,6 @@ plugin_group! {
     }
     /// This plugin group represents the absolute minimum, bare-bones, bevy application.
     /// Use this if you want to have absolute control over the plugins used.
-    /// If you are looking to make a *headless* application - without a *window* or rendering,
-    /// it is usually best to use [`HeadlessPlugins`].
     ///
     /// It includes a [schedule runner (`ScheduleRunnerPlugin`)](crate::app::ScheduleRunnerPlugin)
     /// to provide functionality that would otherwise be driven by a windowed application's

crates/bevy_internal/src/lib.rs

@@ -13,6 +13,7 @@ pub mod prelude;
 mod default_plugins;
 pub use default_plugins::*;
 
+#[cfg(feature = "bevy_window")]
 pub use bevy_a11y as a11y;
 #[cfg(feature = "bevy_animation")]
 pub use bevy_animation as animation;
@@ -66,6 +67,7 @@ pub use bevy_transform as transform;
 #[cfg(feature = "bevy_ui")]
 pub use bevy_ui as ui;
 pub use bevy_utils as utils;
+#[cfg(feature = "bevy_window")]
 pub use bevy_window as window;
 #[cfg(feature = "bevy_winit")]
 pub use bevy_winit as winit;

crates/bevy_internal/src/prelude.rs

@@ -2,10 +2,13 @@
 pub use crate::{
     app::prelude::*, core::prelude::*, ecs::prelude::*, hierarchy::prelude::*, input::prelude::*,
     log::prelude::*, math::prelude::*, reflect::prelude::*, time::prelude::*,
-    transform::prelude::*, utils::prelude::*, window::prelude::*, DefaultPlugins, HeadlessPlugins,
-    MinimalPlugins,
+    transform::prelude::*, utils::prelude::*, DefaultPlugins, MinimalPlugins,
 };
 
+#[doc(hidden)]
+#[cfg(feature = "bevy_window")]
+pub use crate::window::prelude::*;
+
 #[doc(hidden)]
 #[cfg(feature = "bevy_image")]
 pub use crate::image::prelude::*;

docs/cargo_features.md

@@ -34,6 +34,7 @@ The default feature set enables most of the expected features of a game engine,
 |bevy_text|Provides text functionality|
 |bevy_ui|A custom ECS-driven UI framework|
 |bevy_ui_picking_backend|Provides an implementation for picking ui|
+|bevy_window|Windowing layer|
 |bevy_winit|winit window and input backend|
 |custom_cursor|Enable winit custom cursor support|
 |default_font|Include a default font, containing only ASCII characters, at the cost of a 20kB binary size increase|

examples/app/headless.rs

@@ -1,26 +1,37 @@
-//! This example only enables a minimal set of plugins required for bevy to run.
-//! You can also completely remove rendering / windowing Plugin code from bevy
-//! by making your import look like this in your Cargo.toml.
+//! This example shows how to configure the `ScheduleRunnerPlugin` to run your
+//! application without windowing. You can completely remove rendering / windowing
+//! Plugin code from bevy by making your import look like this in your Cargo.toml.
 //!
 //! ```toml
 //! [dependencies]
 //! bevy = { version = "*", default-features = false }
 //! # replace "*" with the most recent version of bevy
 //! ```
-
+//!
+//! And then enabling the features you need.
+//! See the full list: <https://docs.rs/bevy/latest/bevy/#cargo-features>
 use bevy::{app::ScheduleRunnerPlugin, log::LogPlugin, prelude::*, utils::Duration};
 
 fn main() {
+    if cfg!(feature = "bevy_window") {
+        println!("This example is running with the bevy_window feature enabled and will not run headless.");
+        println!("Disable the default features and rerun the example to run headless.");
+        println!("To do so, run:");
+        println!();
+        println!("    cargo run --example headless --no-default-features");
+        return;
+    }
+
     // This app runs once
     App::new()
-        .add_plugins(HeadlessPlugins.set(ScheduleRunnerPlugin::run_once()))
+        .add_plugins(DefaultPlugins.set(ScheduleRunnerPlugin::run_once()))
         .add_systems(Update, hello_world_system)
         .run();
 
     // This app loops forever at 60 fps
     App::new()
         .add_plugins(
-            HeadlessPlugins
+            DefaultPlugins
                 .set(ScheduleRunnerPlugin::run_loop(Duration::from_secs_f64(
                     1.0 / 60.0,
                 )))