bevy_transform/
plugins.rs

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use bevy_app::{App, Plugin, PostStartup, PostUpdate};
use bevy_ecs::schedule::{IntoSystemConfigs, IntoSystemSetConfigs, SystemSet};
use bevy_hierarchy::ValidParentCheckPlugin;

use crate::{
    prelude::{GlobalTransform, Transform},
    systems::{propagate_transforms, sync_simple_transforms},
};

/// Set enum for the systems relating to transform propagation
#[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
pub enum TransformSystem {
    /// Propagates changes in transform to children's [`GlobalTransform`]
    TransformPropagate,
}

/// The base plugin for handling [`Transform`] components
#[derive(Default)]
pub struct TransformPlugin;

impl Plugin for TransformPlugin {
    fn build(&self, app: &mut App) {
        // A set for `propagate_transforms` to mark it as ambiguous with `sync_simple_transforms`.
        // Used instead of the `SystemTypeSet` as that would not allow multiple instances of the system.
        #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
        struct PropagateTransformsSet;

        app.register_type::<Transform>()
            .register_type::<GlobalTransform>()
            .add_plugins(ValidParentCheckPlugin::<GlobalTransform>::default())
            .configure_sets(
                PostStartup,
                PropagateTransformsSet.in_set(TransformSystem::TransformPropagate),
            )
            // add transform systems to startup so the first update is "correct"
            .add_systems(
                PostStartup,
                (
                    sync_simple_transforms
                        .in_set(TransformSystem::TransformPropagate)
                        // FIXME: https://github.com/bevyengine/bevy/issues/4381
                        // These systems cannot access the same entities,
                        // due to subtle query filtering that is not yet correctly computed in the ambiguity detector
                        .ambiguous_with(PropagateTransformsSet),
                    propagate_transforms.in_set(PropagateTransformsSet),
                ),
            )
            .configure_sets(
                PostUpdate,
                PropagateTransformsSet.in_set(TransformSystem::TransformPropagate),
            )
            .add_systems(
                PostUpdate,
                (
                    sync_simple_transforms
                        .in_set(TransformSystem::TransformPropagate)
                        .ambiguous_with(PropagateTransformsSet),
                    propagate_transforms.in_set(PropagateTransformsSet),
                ),
            );
    }
}