//! A shader that uses the GLSL shading language.

use bevy::{
    pbr::{MaterialPipeline, MaterialPipelineKey},
    prelude::*,
    reflect::TypeUuid,
    render::{
        mesh::MeshVertexBufferLayout,
        render_resource::{
            AsBindGroup, RenderPipelineDescriptor, ShaderRef, SpecializedMeshPipelineError,
        },
    },
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugin(MaterialPlugin::<CustomMaterial>::default())
        .add_startup_system(setup)
        .run();
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<CustomMaterial>>,
) {
    // cube
    commands.spawn().insert_bundle(MaterialMeshBundle {
        mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        material: materials.add(CustomMaterial {
            color: Color::GREEN,
        }),
        ..default()
    });

    // camera
    commands.spawn_bundle(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

#[derive(AsBindGroup, Clone, TypeUuid)]
#[uuid = "4ee9c363-1124-4113-890e-199d81b00281"]
pub struct CustomMaterial {
    #[uniform(0)]
    color: Color,
}

impl Material for CustomMaterial {
    fn vertex_shader() -> ShaderRef {
        "shaders/custom_material.vert".into()
    }

    fn fragment_shader() -> ShaderRef {
        "shaders/custom_material.frag".into()
    }

    // Bevy assumes by default that vertex shaders use the "vertex" entry point
    // and fragment shaders use the "fragment" entry point (for WGSL shaders).
    // GLSL uses "main" as the entry point, so we must override the defaults here
    fn specialize(
        _pipeline: &MaterialPipeline<Self>,
        descriptor: &mut RenderPipelineDescriptor,
        _layout: &MeshVertexBufferLayout,
        _key: MaterialPipelineKey<Self>,
    ) -> Result<(), SpecializedMeshPipelineError> {
        descriptor.vertex.entry_point = "main".into();
        descriptor.fragment.as_mut().unwrap().entry_point = "main".into();
        Ok(())
    }
}