Rendering system

It's time for our first system, the rendering system. This system will be responsible for drawing all our entities on the screen.

Rendering system setup

First we'll define the RenderingSystem struct, it will need access to the ggez context in order to actually render.


#![allow(unused)]
fn main() {
pub struct RenderingSystem<'a> {
    context: &'a mut Context,
}
}

We've got some new syntax here; 'a is called a lifetime annotation. It's needed because the compiler can't see how long the reference in RenderingSystem is valid, meaning that we have to specify the lifetime annotation.

MORE: Read more about lifetimes here.

Now let's implement the System trait for our Rendering system. This doesn't do anything yet, we're just setting up the scaffolding. The definition of SystemData means that we will have access to the storage of position and renderable components, and the fact that it's read storage means we only get immutable access, which is exactly what we need.


#![allow(unused)]
fn main() {
// System implementation
impl<'a> System<'a> for RenderingSystem<'a> {
    // Data
    type SystemData = (ReadStorage<'a, Position>, ReadStorage<'a, Renderable>);

    fn run(&mut self, data: Self::SystemData) {
        let (positions, renderables) = data;
        // implementation here
    }
}
}

Finally let's run the rendering system in our draw loop. This means that every time the game updates we will render the latest state of all our entities.


#![allow(unused)]
fn main() {
impl event::EventHandler for Game {
    fn update(&mut self, _context: &mut Context) -> GameResult {
        Ok(())
    }

    fn draw(&mut self, context: &mut Context) -> GameResult {
        // Render game entities
        {
            let mut rs = RenderingSystem { context };
            rs.run_now(&self.world);
        }

        Ok(())
    }
}
}

Running the game now should compile, but it will probably not do anything yet, since we haven't filled in any of the implementation of the rendering system and also we haven't created any entities.

Rendering system implementation

Here is the implementation of the rendering system. It does a few things:

  • clear the screen (ensuring we don't keep any of the state rendered on the previous frame)
  • get all entities with a renderable component and sort them by z (we do this in order to ensure we can render things on top of each other, for example the player should be above the floor, otherwise we wouldn't be able to see them)
  • iterate through sorted entities and render each of them as an image
  • finally, present to the screen

#![allow(unused)]
fn main() {
    fn run(&mut self, data: Self::SystemData) {
        let (positions, renderables) = data;

        // Clearing the screen (this gives us the backround colour)
        graphics::clear(self.context, graphics::Color::new(0.95, 0.95, 0.95, 1.0));

        // Get all the renderables with their positions and sort by the position z
        // This will allow us to have entities layered visually.
        let mut rendering_data = (&positions, &renderables).join().collect::<Vec<_>>();
        rendering_data.sort_by_key(|&k| k.0.z);

        // Iterate through all pairs of positions & renderables, load the image
        // and draw it at the specified position.
        for (position, renderable) in rendering_data.iter() {
            // Load the image
            let image = Image::new(self.context, renderable.path.clone()).expect("expected image");
            let x = position.x as f32 * TILE_WIDTH;
            let y = position.y as f32 * TILE_WIDTH;

            // draw
            let draw_params = DrawParam::new().dest(na::Point2::new(x, y));
            graphics::draw(self.context, &image, draw_params).expect("expected render");
        }

        // Finally, present the context, this will actually display everything
        // on the screen.
        graphics::present(self.context).expect("expected to present");
    }
}

Add some test entities

Let's create some test entities to make sure things are working correctly.


#![allow(unused)]
fn main() {
pub fn initialize_level(world: &mut World) {
    create_player(
        world,
        Position {
            x: 0,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
    create_wall(
        world,
        Position {
            x: 1,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
    create_box(
        world,
        Position {
            x: 2,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
}
}

Finally, let's put everything together and run. You should see something like this! This is super exciting, now we have a proper rendering system and we can actually see something on the screen for the first time. Next up, we're going to work on the gameplay so it can actually feel like a game!

Screenshot

Final code below.

NOTE: Note that this is a very basic implementation of rendering and as the number of entities grow the performance will not be good enough. A more advanced implementation of rendering which uses batch rendering can be found in Chapter 3 - Batch Rendering.

use ggez;
use ggez::graphics;
use ggez::graphics::DrawParam;
use ggez::graphics::Image;
use ggez::nalgebra as na;
use ggez::{conf, event, Context, GameResult};
use specs::{
    join::Join, Builder, Component, ReadStorage, RunNow, System, VecStorage, World, WorldExt,
};

use std::path;

const TILE_WIDTH: f32 = 32.0;

// Components
#[derive(Debug, Component, Clone, Copy)]
#[storage(VecStorage)]
pub struct Position {
    x: u8,
    y: u8,
    z: u8,
}

#[derive(Component)]
#[storage(VecStorage)]
pub struct Renderable {
    path: String,
}

#[derive(Component)]
#[storage(VecStorage)]
pub struct Wall {}

#[derive(Component)]
#[storage(VecStorage)]
pub struct Player {}

#[derive(Component)]
#[storage(VecStorage)]
pub struct Box {}

#[derive(Component)]
#[storage(VecStorage)]
pub struct BoxSpot {}

// Systems
pub struct RenderingSystem<'a> {
    context: &'a mut Context,
}

// System implementation
impl<'a> System<'a> for RenderingSystem<'a> {
    // Data
    type SystemData = (ReadStorage<'a, Position>, ReadStorage<'a, Renderable>);

    fn run(&mut self, data: Self::SystemData) {
        let (positions, renderables) = data;

        // Clearing the screen (this gives us the backround colour)
        graphics::clear(self.context, graphics::Color::new(0.95, 0.95, 0.95, 1.0));

        // Get all the renderables with their positions and sort by the position z
        // This will allow us to have entities layered visually.
        let mut rendering_data = (&positions, &renderables).join().collect::<Vec<_>>();
        rendering_data.sort_by_key(|&k| k.0.z);

        // Iterate through all pairs of positions & renderables, load the image
        // and draw it at the specified position.
        for (position, renderable) in rendering_data.iter() {
            // Load the image
            let image = Image::new(self.context, renderable.path.clone()).expect("expected image");
            let x = position.x as f32 * TILE_WIDTH;
            let y = position.y as f32 * TILE_WIDTH;

            // draw
            let draw_params = DrawParam::new().dest(na::Point2::new(x, y));
            graphics::draw(self.context, &image, draw_params).expect("expected render");
        }

        // Finally, present the context, this will actually display everything
        // on the screen.
        graphics::present(self.context).expect("expected to present");
    }
}

// This struct will hold all our game state
// For now there is nothing to be held, but we'll add
// things shortly.
struct Game {
    world: World,
}

// This is the main event loop. ggez tells us to implement
// two things:
// - updating
// - rendering
impl event::EventHandler for Game {
    fn update(&mut self, _context: &mut Context) -> GameResult {
        Ok(())
    }

    fn draw(&mut self, context: &mut Context) -> GameResult {
        // Render game entities
        {
            let mut rs = RenderingSystem { context };
            rs.run_now(&self.world);
        }

        Ok(())
    }
}

// Register components with the world
pub fn register_components(world: &mut World) {
    world.register::<Position>();
    world.register::<Renderable>();
    world.register::<Player>();
    world.register::<Wall>();
    world.register::<Box>();
    world.register::<BoxSpot>();
}

// Create a wall entity
pub fn create_wall(world: &mut World, position: Position) {
    world
        .create_entity()
        .with(Position { z: 10, ..position })
        .with(Renderable {
            path: "/images/wall.png".to_string(),
        })
        .with(Wall {})
        .build();
}

pub fn create_floor(world: &mut World, position: Position) {
    world
        .create_entity()
        .with(Position { z: 5, ..position })
        .with(Renderable {
            path: "/images/floor.png".to_string(),
        })
        .build();
}

pub fn create_box(world: &mut World, position: Position) {
    world
        .create_entity()
        .with(Position { z: 10, ..position })
        .with(Renderable {
            path: "/images/box.png".to_string(),
        })
        .with(Box {})
        .build();
}

pub fn create_box_spot(world: &mut World, position: Position) {
    world
        .create_entity()
        .with(Position { z: 9, ..position })
        .with(Renderable {
            path: "/images/box_spot.png".to_string(),
        })
        .with(BoxSpot {})
        .build();
}

pub fn create_player(world: &mut World, position: Position) {
    world
        .create_entity()
        .with(Position { z: 10, ..position })
        .with(Renderable {
            path: "/images/player.png".to_string(),
        })
        .with(Player {})
        .build();
}

// Initialize the level
pub fn initialize_level(world: &mut World) {
    create_player(
        world,
        Position {
            x: 0,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
    create_wall(
        world,
        Position {
            x: 1,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
    create_box(
        world,
        Position {
            x: 2,
            y: 0,
            z: 0, // we will get the z from the factory functions
        },
    );
}

pub fn main() -> GameResult {
    let mut world = World::new();
    register_components(&mut world);
    initialize_level(&mut world);

    // Create a game context and event loop
    let context_builder = ggez::ContextBuilder::new("rust_sokoban", "sokoban")
        .window_setup(conf::WindowSetup::default().title("Rust Sokoban!"))
        .window_mode(conf::WindowMode::default().dimensions(800.0, 600.0))
        .add_resource_path(path::PathBuf::from("./resources"));

    let (context, event_loop) = &mut context_builder.build()?;

    // Create the game state
    let game = &mut Game { world };
    // Run the main event loop
    event::run(context, event_loop, game)
}

CODELINK: You can see the full code in this example here.