bevy/release-content/migration-guides/render_startup.md

title	pull_requests
Many render resources now initialized in `RenderStartup`	19841 19926 19885 19886 19897 19898 19901 20002 20147

Many render resources are no longer present during Plugin::finish. Instead they are initialized during RenderStartup (which occurs once the app starts running). If you only access the resource during the Render schedule, then there should be no change. However, if you need one of these render resources to initialize your own resource, you will need to convert your resource initialization into a system.

The following are the (public) resources that are now initialized in RenderStartup.

• CasPipeline
• FxaaPipeline
• SmaaPipelines
• TaaPipeline
• AutoExposurePipeline
• MotionBlurPipeline
• SkyboxPrepassPipeline
• BlitPipeline
• DepthOfFieldGlobalBindGroupLayout
• DepthPyramidDummyTexture
• OitBuffers
• PostProcessingPipeline
• TonemappingPipeline
• AtmosphereSamplers
• BoxShadowPipeline
• GradientPipeline
• UiPipeline
• UiMaterialPipeline<M>
• UiTextureSlicePipeline
• VolumetricFogPipeline
• DeferredLightingLayout
• RenderLightmaps
• PrepassPipeline
• PrepassViewBindGroup
• Wireframe3dPipeline
• MaterialPipeline
• Wireframe2dPipeline
• Material2dPipeline
• SpritePipeline
• Mesh2dPipeline
• BatchedInstanceBuffer<Mesh2dUniform>

The vast majority of cases for initializing render resources look like so (in Bevy 0.16):

impl Plugin for MyRenderingPlugin {
    fn build(&self, app: &mut App) {
        // Do nothing??
    }

    fn finish(&self, app: &mut App) {
        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app.init_resource::<MyRenderResource>();
        render_app.add_systems(Render, my_render_system);
    }
}

pub struct MyRenderResource {
    ...
}

impl FromWorld for MyRenderResource {
    fn from_world(world: &mut World) -> Self {
        let render_device = world.resource::<RenderDevice>();
        let render_adapter = world.resource::<RenderAdapter>();
        let asset_server = world.resource::<AssetServer>();

        MyRenderResource {
            ...
        }
    }
}

The two main things to focus on are:

1. The resource implements the FromWorld trait which collects all its dependent resources (most commonly, RenderDevice), and then creates an instance of the resource.
2. The plugin adds its systems and resources in Plugin::finish.

First, we need to rewrite our FromWorld implementation as a system. This generally means converting calls to World::resource into system params, and then using Commands to insert the resource. In the above case, that would look like:

// Just a regular old system!!
fn init_my_resource(
    mut commands: Commands,
    render_device: Res<RenderDevice>,
    render_adapter: Res<RenderAdapter>,
    asset_server: Res<AssetServer>,
) {
    commands.insert_resource(MyRenderResource {
        ...
    });
}

Each case will be slightly different. Two notes to be wary of:

1. Functions that accept &RenderDevice for example may no longer compile after switching to Res<RenderDevice>. This can be resolved by passing &render_device instead of render_device.
2. If you are using load_embedded_asset(world, "my_asset.png"), you may need to first add asset_server as a system param, then change this to load_embedded_asset(asset_server.as_ref(), "my_asset.png").

Now that we have our initialization system, we just need to add the system to RenderStartup:

impl Plugin for MyRenderingPlugin {
    fn build(&self, app: &mut App) {
        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app
            .add_systems(RenderStartup, init_my_resource)
            .add_systems(Render, my_render_system);
    }

    // No more finish!!
}

In addition, if your resource requires one of the affected systems above, you will need to use system ordering to ensure your resource initializes after the other system. For example, if your system uses Res<UiPipeline>, you will need to add an ordering like:

render_app.add_systems(RenderStartup, init_my_resource.after(init_ui_pipeline));