Async Plugin Construction

[SpecificProtagonist]

Objective

• Fixes Plugin Dependencies #69
• alternative to Plugin dependency graph v2 #11228

Since the issue was made, Bevy's plugins have been split into three stages that are (typically) run before app update: Build, finish and cleanup. They also have a ready method to delay the switch from the build to the finish phase.

This allows plugins to delay up to twice while waiting on other plugins, but the specific dependency is implicit even when reading the code and the ordering of plugins matters. Solve this by making plugin building concurrent.

Also, I've just now seen #8607, which also makes plugins asynchronous. I'll need to take a closer look, but afaik it has the goal of running on bevy_tasks and doesn't address dependencies / plugin ordering (as it build plugins one at a time).

Solution

Adds Plugin::build_async, which runs concurrently for all plugins. The plugin can wait for the world to reach whatever state it needs to continue. While you can define a custom condition, there's also a helper for waiting on a resource; please suggest other ones.

This is intended to replace the old plugin lifecycle machinery, which can be deprecated (and eventually removed) in a later PR, which will make things much simpler. As noted in #1255, we'll want to defer app construction actions (like add_systems); with this PR, the collected actions can be run by turning them into an async plugin.

As a followup, we should be able to use a waker and achieve the goals of #8607.

Testing

(TODO: Add plenty of tests if we decide to do this)

Showcase

#[derive(Resource)]
struct A(i32);

struct MyPlugin;

impl Plugin for MyPlugin {
    async fn build_async(self: Box<Self>, mut ctx: PluginContext<'_>) {
        let a = ctx.resource::<A>().await.unwrap();
        println!("got A: {}", a.0);
    }
}

struct OtherPlugin;

impl Plugin for OtherPlugin {
    async fn build_async(self: Box<Self>, mut ctx: PluginContext<'_>) {
        ctx.app().world_mut().insert_resource(A(1));
    }
}

Migration Guide

(TODO depending on changes)

[JMS55]

Waiting on a specific plugin to finish would be nice since sometimes you have:

• Plugin A sometimes inserts a resource, sometimes doesn't
• Plugin B wants to wait for Plugin A to finish, and then check if the resource exists or not