Add bevy_camera_controllers crate and move freecam implementation into it (#20215)

# Objective

Moving the camera around is really important to validate that our scenes
are rendered correctly.

Bevy engine devs currently do this via a fairly cursed "library example"
which includes a basic freecam controller (which is technically distinct
from a flycam apparently). This has three problems:

1. Oh god why are we taking pseudo-dependencies on other examples.
2. Something like this would be useful for debugging for users.
3. Something like this would be useful for the editor and other tooling.

## Solution

Create a new `bevy_camera_controllers` crate, and move the existing
freecam implementation into it, nearly verbatim.

This cleans up some ugly tech debt in how we do this, makes it easier to
add a camera controller to other examples, and also gives us a scaffold
for future camera controllers.

This PR has been marked `X-Blessed`, as I went over this plan with @cart
in a [meeting on
2025-06-23](https://discord.com/channels/691052431525675048/692572690833473578/1386847828923519038),
and we both agreed that this was the right first step.

## Testing

I've tested the examples that rely on this camera controller: they work
just like before.

## Notes for reviewers

- I don't intend to land this in Bevy 0.17: the existing example code is
not high enough quality for me to be happy shipping this
- we're also soft feature-frozen; I was just in the mood to do this
today
- this PR has some minimal cleanup: just enough naming and docs work to
make this code not an active liability. Any more will risk derailing
this PR and making it harder to review
- I've opted to expose top-level feature flags for the various camera
controllers. This is a bit noisy, but it was the only sensible way I
could see to both have "no camera controllers by default" and "still
easy to use these controllers in examples"
- this will not be the only camera controller in this crate, or the Bevy
Editor
- this is not the place to discuss "what should the default camera
controller for the Bevy Editor be"
- if you think this crate, or this particular move is the wrong strategy
for some other reason though, please do speak up :)
- my personal (unblessed) hope is that we continue to add camera
controllers to this crate as needed for various first-party tooling. I
will probably largely push-back on adding camera controllers that we do
not actively use to this crate, since I don't think they make great
libraries in general (much better to copy-paste)

## TODO

- [x] add release note. I'm waiting until after Bevy 0.17 ships to do
this
- [x] update the examples/README.md file. I was lazy and didn't want to
constantly resolve merge conflicts on that while this PR sits
- [x] change to singular crate name

## Future work

- split apart the giant camera controller component
- make keybinding better, either with or without a real input manager
solution
- split apart the system to make it more modular
- make the system handle fixed timesteps better
- use the camera controller in more examples for debugging
- add more camera controllers!

---------

Co-authored-by: atlv <email@atlasdostal.com>
Co-authored-by: Jan Hohenheim <jan@hohenheim.ch>

Author: 3 people

Cargo.toml

@@ -291,6 +291,12 @@ bevy_gizmos = ["bevy_internal/bevy_gizmos"]
 # Provides a collection of developer tools
 bevy_dev_tools = ["bevy_internal/bevy_dev_tools"]
 
+# Provides a collection of prebuilt camera controllers
+bevy_camera_controller = ["bevy_internal/bevy_camera_controller"]
+
+# Enables the free cam from bevy_camera_controller
+free_cam = ["bevy_internal/free_cam"]
+
 # Enable the Bevy Remote Protocol
 bevy_remote = ["bevy_internal/bevy_remote"]
 
@@ -1160,6 +1166,7 @@ wasm = true
 name = "decal"
 path = "examples/3d/decal.rs"
 doc-scrape-examples = true
+required-features = ["free_cam"]
 
 [package.metadata.example.decal]
 name = "Decal"
@@ -1271,6 +1278,7 @@ wasm = true
 name = "shadow_biases"
 path = "examples/3d/shadow_biases.rs"
 doc-scrape-examples = true
+required-features = ["free_cam"]
 
 [package.metadata.example.shadow_biases]
 name = "Shadow Biases"
@@ -1293,6 +1301,7 @@ wasm = true
 name = "skybox"
 path = "examples/3d/skybox.rs"
 doc-scrape-examples = true
+required-features = ["free_cam"]
 
 [package.metadata.example.skybox]
 name = "Skybox"
@@ -3196,6 +3205,7 @@ wasm = false
 name = "scene_viewer"
 path = "examples/tools/scene_viewer/main.rs"
 doc-scrape-examples = true
+required-features = ["free_cam"]
 
 [package.metadata.example.scene_viewer]
 name = "Scene Viewer"
@@ -3971,6 +3981,7 @@ wasm = true
 name = "3d_gizmos"
 path = "examples/gizmos/3d_gizmos.rs"
 doc-scrape-examples = true
+required-features = ["free_cam"]
 
 [package.metadata.example.3d_gizmos]
 name = "3D Gizmos"
@@ -4603,18 +4614,6 @@ description = "Demonstration of Occlusion Culling"
 category = "3D Rendering"
 wasm = false
 
-[[example]]
-name = "camera_controller"
-path = "examples/helpers/camera_controller.rs"
-doc-scrape-examples = true
-crate-type = ["lib"]
-
-[package.metadata.example.camera_controller]
-name = "Camera Controller"
-description = "Example Free-Cam Styled Camera Controller"
-category = "Helpers"
-wasm = true
-
 [[example]]
 name = "widgets"
 path = "examples/helpers/widgets.rs"

crates/bevy_camera_controller/Cargo.toml

@@ -0,0 +1,39 @@
+[package]
+name = "bevy_camera_controller"
+version = "0.18.0-dev"
+edition = "2024"
+description = "Premade camera controllers for Bevy"
+homepage = "https://bevy.org"
+repository = "https://github.com/bevyengine/bevy"
+license = "MIT OR Apache-2.0"
+keywords = ["bevy", "input", "camera", "control"]
+rust-version = "1.85.0"
+
+[dependencies]
+bevy_app = { path = "../bevy_app", version = "0.18.0-dev", default-features = false }
+bevy_camera = { path = "../bevy_camera", version = "0.18.0-dev", default-features = false }
+bevy_ecs = { path = "../bevy_ecs", version = "0.18.0-dev", default-features = false }
+bevy_input = { path = "../bevy_input", version = "0.18.0-dev", default-features = false }
+bevy_log = { path = "../bevy_log", version = "0.18.0-dev", default-features = false }
+bevy_math = { path = "../bevy_math", version = "0.18.0-dev", default-features = false, features = [
+  "curve",
+] }
+bevy_reflect = { path = "../bevy_reflect", version = "0.18.0-dev", default-features = false, optional = true }
+bevy_time = { path = "../bevy_time", version = "0.18.0-dev", default-features = false }
+bevy_transform = { path = "../bevy_transform", version = "0.18.0-dev", default-features = false }
+bevy_window = { path = "../bevy_window", version = "0.18.0-dev", default-features = false }
+
+[features]
+default = ["bevy_reflect"]
+bevy_reflect = ["dep:bevy_reflect"]
+libm = ["bevy_math/libm"]
+
+# Camera controllers
+free_cam = []
+
+[lints]
+workspace = true
+
+[package.metadata.docs.rs]
+rustdoc-args = ["-Zunstable-options", "--generate-link-to-definition"]
+all-features = true

crates/bevy_camera_controller/LICENSE-APACHE

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crates/bevy_camera_controller/LICENSE-MIT

@@ -0,0 +1,19 @@
+MIT License
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