Add bevy_shader_hot_reloading feature for easier renderer development

[superdump]

Description

Having to recompile bevy and re-run test apps in order to iterate on shaders for bevy renderer features is slower than necessary. The goal of this change is to add a non-default feature that allows bevy renderer feature developers to edit core bevy renderer shaders and have them hot reload. This will enormously speed up development due to reducing iteration time. More changes may be needed to not replace shader assets if they in any way invalid, but this should be a good start.

Solution

NOTE: Builds on top of #3643 and watch for changes needs to be enabled using the resource as in that PR for this PR to work.

This was way more complicated than I expected:

• Hot-reloading only notifies the handle which was created from AssetServer::load which is constructed from the AssetPath. This is convenient as the filesystem watcher for hot-reloading naturally and unsurprisingly produces events based on paths to files that changed.
  • To solve this I chose to add what I have called 'aliases' for handles.
  • Add aliases: HashMap<HandleId, HashSet<HandleId>> to Assets<T> to hold all handles that are aliases for the strong handle for an asset as obtained from AssetServer::load(). An alias handle is a handle made via other means, such as pub const PBR_SHADER_HANDLE: HandleUntyped = HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 4805239651767701046);.
  • Add alias_to_handle: HashMap<HandleId, HandleId> to Assets<T>
  • Add:
    • add_alias<H: Into<HandleId>, A: Into<HandleId>>(&mut self, handle: H, alias: A),
    • remove_alias<H: Into<HandleId>, A: Into<HandleId>>(&mut self, handle: H, alias: A),
    • and get_handle_from_alias<A: Into<HandleId>>(&self, alias: A) -> Option<&HandleId>
    • ...to Assets<T> to be able to add/remove aliases, and for a given alias handle, get the strong handle. Assets<T>::get() uses the latter to attempt to get the asset using an alias handle if it exists. This allows materials to use PBR_SHADER_HANDLE and due to setting that as an alias handle for the strong handle, Assets<Shader>::get() 'just works'.
• When using Assets<Shader>::set_untracked() with the const HandleUntyped (e.g. PBR_SHADER_HANDLE), the Shader is constructed and immediately chained with Shader::with_import_path() to define the shader import. When using AssetServer::load(), the Shader does not exist until loaded.
  • If a shader import path is necessary, use a system to check for when the shader asset is loaded, mutably get it and set the import path. I think this could be possibly improved somehow but it's good enough as a first shot perhaps.
• Making use of these changes:
  • Move all the shader files into the assets/shaders/ directory in subdirectories named after their respective crates, e.g. assets/shaders/bevy_pbr/pbr.wgsl.
  • Add the non-default bevy_shader_hot_reloading feature
  • Where shaders are used in bevy rendering crates:
    • Move the current Assets<Shader>::set_untracked() code into a block for when the feature is not set.
    • Add a new block for when the feature is set, and use a pattern of:

pub const SHADER_HANDLE: HandleUntyped =
    HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 1234567890);
pub const IMPORT_HANDLE: HandleUntyped =
    HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 0987654321);
const IMPORT_PATH: &str = "my_crate::import_name";

...

let asset_server = app.world.get_resource::<AssetServer>().unwrap();
let shader_handle = asset_server.load("path/to/shader.wgsl");
let import_handle = asset_server.load("path/to/import.wgsl");

let mut shaders = app.world.get_resource_mut::<Assets<Shader>>().unwrap();
shaders.add_alias(&shader_handle, SHADER_HANDLE);
shaders.add_alias(&import_handle, IMPORT_HANDLE);

// NOTE: We need to store the strong handles created from the asset paths
let mut hot_reload_shaders = app.world.get_resource_mut::<HotReloadShaders>().unwrap();
hot_reload_shaders.keep_shader_alive(shader_handle);
hot_reload_shaders.add_hot_reload_shader(import_handle, IMPORT_PATH.to_string());

[superdump]

bevy_core_shader_hot_reloading.mp4

[johanhelsing]

I only get a gray window with when running the sprite example on this branch.

I also checked on #3643 and there sprite shows up as it should.

[superdump]

I only get a gray window with when running the sprite example on this branch.

Fixed.

[superdump]

Just noting that I did this for convenience. It saves adding this as a resource in example just to set it to true when hacking/testing stuff.

[superdump]

I also updated the usage example in the PR description for shaders with entrypoints and for imports.

[mockersf]

the new feature should be mentioned in https://github.com/bevyengine/bevy/blob/main/docs/cargo_features.md

[mockersf]

Should this PR be closed now that #3966 has been merged?

[superdump]

Implemented differently by #3966