keyboard_just_input.md

Keyboard Just Input

The method pressed returns true whenever the corresponding key is pressed. If we want to catch the moment when the key is just pressed, we have to use the method just_pressed.

In the following example, the color of the circle is changed when the key space is pressed.

We declare two colors and store them in the resource MyColors.

#[derive(Resource, Default)]
struct MyColors {
    color1: Handle<ColorMaterial>,
    color2: Handle<ColorMaterial>,
}

We create a resource named MyCircleHighlighted and use its field 0 to indicate which color we are using.

#[derive(Resource, Default)]
struct MyCircleHighlighted(bool);

The two colors are initialized in the system setup and the dark one is bound to the circle.

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    mut my_colors: ResMut<MyColors>,
) {
    commands.spawn(Camera2dBundle::default());

    my_colors.color1 = materials.add(Color::hsl(210., 1., 0.4).into());
    my_colors.color2 = materials.add(Color::hsl(210., 1., 0.8).into());

    commands.spawn(ColorMesh2dBundle {
        mesh: meshes.add(Circle::new(50.).into()).into(),
        material: my_colors.color1.clone(),
        ..default()
    });
}

The color of the circle is changed when the key is just pressed.

fn handle_keys(
    keyboard_input: Res<Input<KeyCode>>,
    mut my_circle_highlighted: ResMut<MyCircleHighlighted>,
    mut circles: Query<&mut Handle<ColorMaterial>>,
    my_colors: ResMut<MyColors>,
) {
    let mut handle = circles.single_mut();

    if keyboard_input.just_pressed(KeyCode::Space) {
        my_circle_highlighted.0 = !my_circle_highlighted.0;
        if my_circle_highlighted.0 {
            *handle = my_colors.color2.clone();
        } else {
            *handle = my_colors.color1.clone();
        }
    }
}

The full code is as follows:

use bevy::{
    app::{App, Startup, Update},
    asset::{Assets, Handle},
    core_pipeline::core_2d::Camera2dBundle,
    ecs::system::{Commands, Query, Res, ResMut, Resource},
    input::{keyboard::KeyCode, Input},
    render::{
        color::Color,
        mesh::{shape::Circle, Mesh},
    },
    sprite::{ColorMaterial, ColorMesh2dBundle},
    utils::default,
    DefaultPlugins,
};

#[derive(Resource, Default)]
struct MyCircleHighlighted(bool);

#[derive(Resource, Default)]
struct MyColors {
    color1: Handle<ColorMaterial>,
    color2: Handle<ColorMaterial>,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<MyCircleHighlighted>()
        .init_resource::<MyColors>()
        .add_systems(Startup, setup)
        .add_systems(Update, handle_keys)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
    mut my_colors: ResMut<MyColors>,
) {
    commands.spawn(Camera2dBundle::default());

    my_colors.color1 = materials.add(Color::hsl(210., 1., 0.4).into());
    my_colors.color2 = materials.add(Color::hsl(210., 1., 0.8).into());

    commands.spawn(ColorMesh2dBundle {
        mesh: meshes.add(Circle::new(50.).into()).into(),
        material: my_colors.color1.clone(),
        ..default()
    });
}

fn handle_keys(
    keyboard_input: Res<Input<KeyCode>>,
    mut my_circle_highlighted: ResMut<MyCircleHighlighted>,
    mut circles: Query<&mut Handle<ColorMaterial>>,
    my_colors: ResMut<MyColors>,
) {
    let mut handle = circles.single_mut();

    if keyboard_input.just_pressed(KeyCode::Space) {
        my_circle_highlighted.0 = !my_circle_highlighted.0;
        if my_circle_highlighted.0 {
            *handle = my_colors.color2.clone();
        } else {
            *handle = my_colors.color1.clone();
        }
    }
}

When the app just starts:

When the key space just pressed at the first time:

The color change happens at the moment when the key is just pressed, and will not take effect when we continue to hold the key space down.

If we want to monitor the moment when a key is just released, we can use the method just_released. The usage is the same as just_pressed.

➡️ Next: Keyboard Events

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