Bevy Minesweeper: Input Management

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We have a glorious board, but we can't interact with it, let's handle some input !

Bounds

To detect mouse input inside our board we will use common gamedev type called Bounds. It is strangely missing from bevy so we'll code a simple version for our plugin in board_plugin/src/bounds.rs

// bounds.rs
use bevy::prelude::Vec2;

#[derive(Debug, Copy, Clone)]
pub struct Bounds2 {
    pub position: Vec2,
    pub size: Vec2,
}

impl Bounds2 {
    pub fn in_bounds(&self, coords: Vec2) -> bool {
        coords.x >= self.position.x
            && coords.y >= self.position.y
            && coords.x <= self.position.x + self.size.x
            && coords.y <= self.position.y + self.size.y
    }
}

Ths structure defines a 2D rectangle and can check if coordinates are contained in its extents.

Connect the file to board_plugin/src/lib.rs:

// lib.rs
mod bounds;

The Board resource

The tile map we generate in our create_board startup system is lost after that system, we need to put it in a resource for it to last.
We also need to store our board Bounds for input detection.

Let's create a board.rs in our resources folder:

// mod.rs
// ..
pub use board_options::*;

mod board;

// board.rs
use crate::bounds::Bounds2;
use crate::{Coordinates, TileMap};
use bevy::prelude::*;

#[derive(Debug)]
pub struct Board {
    pub tile_map: TileMap,
    pub bounds: Bounds2,
    pub tile_size: f32,
}

impl Board {
    /// Translates a mouse position to board coordinates
    pub fn mouse_position(&self, window: &Window, position: Vec2) -> Option<Coordinates> {
        // Window to world space
        let window_size = Vec2::new(window.width(), window.height());
        let position = position - window_size / 2.;

        // Bounds check
        if !self.bounds.in_bounds(position) {
            return None;
        }
        // World space to board space
        let coordinates = position - self.bounds.position;
        Some(Coordinates {
            x: (coordinates.x / self.tile_size) as u16,
            y: (coordinates.y / self.tile_size) as u16,
        })
    }
}

Our Board resource stores a TileMap, the board Bounds and a tile_size which is the size of individual square tiles.

We provide a method converting mouse position to our own coordinate system. This computation seems strange because unlike our entities world space where the origin
is at the center of the screen (based on camera position), the window space origin is on the bottom left.

So we have to transform the mouse position so that it matches our world space, check the bounds and then convert the coordinates into a tile coordinate.

Now we defined our resource, we need to register it at the end of our create_board startup system

// lib.rs
use bounds::Bounds2;
use resources::Board;
use bevy::math::Vec3Swizzles;

// ..

// We add the main resource of the game, the board
        commands.insert_resource(Board {
            tile_map,
            bounds: Bounds2 {
                position: board_position.xy(),
                size: board_size,
            },
            tile_size,
        });
// ..

The Board is now available for any system.

Input system

We can now create or first regular system which will check every frame for a mouse click event.

Let's create a systems module in our board plugin with an input.rs file.

Small hierarchy recap:

├── Cargo.lock
├── Cargo.toml
├── assets
├── board_plugin
│    ├── Cargo.toml
│    └── src
│         ├── bounds.rs
│         ├── components
│         │    ├── bomb.rs
│         │    ├── bomb_neighbor.rs
│         │    ├── coordinates.rs
│         │    ├── mod.rs
│         │    ├── uncover.rs
│         ├── lib.rs
│         ├── resources
│         │    ├── board.rs
│         │    ├── board_options.rs
│         │    ├── mod.rs
│         │    ├── tile.rs
│         │    └── tile_map.rs
│         └── systems
│              ├── input.rs
│              └── mod.rs
├── src
│    └── main.rs

Don't forget to connect the systems module in lib.rs:

mod systems;

and the input module in systems/mod.rs:

pub mod input;

Let's define our input system !

// input.rs
use crate::Board;
use bevy::input::{mouse::MouseButtonInput, ElementState};
use bevy::log;
use bevy::prelude::*;

pub fn input_handling(
    windows: Res<Windows>,
    board: Res<Board>,
    mut button_evr: EventReader<MouseButtonInput>,
) {
    let window = windows.get_primary().unwrap();

    for event in button_evr.iter() {
        if let ElementState::Pressed = event.state {
            let position = window.cursor_position();
            if let Some(pos) = position {
                log::trace!("Mouse button pressed: {:?} at {}", event.button, pos);
                let tile_coordinates = board.mouse_position(window, pos);
                if let Some(coordinates) = tile_coordinates {
                    match event.button {
                        MouseButton::Left => {
                            log::info!("Trying to uncover tile on {}", coordinates);
                            // TODO: generate an event
                        }
                        MouseButton::Right => {
                            log::info!("Trying to mark tile on {}", coordinates);
                            // TODO: generate an event
                        }
                        _ => (),
                    }
                }
            }
        }
    }
}

This function is our input system, it takes three arguments:

• a Windows resource
• our own Board resource
• a MouseButtonInput event reader

We iterate throught the event reader to retrieve every event, keeping only Pressed events.
We retrieve the mouse position and use our Board to convert the mouse position into tile coordinates
and then we log the action (uncover or mark) according to the mouse button.

So if we press an other button we will still perform the conversion?

Yes, we could check the buttons first to optimize a bit but it would make the code less clear for a tutorial.

We can now register our system in our BoardPlugin::build() method:

// lib.rs
// ..
//    app.add_startup_system(Self::create_board)
        .add_system(systems::input::input_handling);
// ..

Running the app you can now use your left and right click buttons on the window and notice that:

• If you click on the board it logs the coordinates and the action
• If you click outside the board or with an other button, nothing happens

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Author: Félix de Maneville
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Published by Qongzi