Bevy Visual novel (thin router)

Genre router. Domain work lives in the Bevy skills below.

When to use

Load this for dialogue, portraits, and choice flags.

ECS shape

• Components hold the genre's per-entity flags as plain data. This file does not name new engine types.
• Resources hold run-wide counters (score, wave index, seed) via bevy-data-resources.
• Systems read those components on Update or the fixed timestep from bevy-master. Order them with .chain() when a later system must see the result.
• Schedule: gameplay mode changes go through bevy-state-machines. Do not invent a genre plugin.

Routing

• bevy-dialogue-system - Lines.
• bevy-ui-text - Body text.
• bevy-ui-layout - Choice buttons.
• bevy-save-load - Route flags.

Scope

Lines, the visible character index, and route flags are resources. Choice rows are entities. A full script parser stays data you load into those resources.

Examples

[package]
name = "visual-novel"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

One resource owns the script index and the flags later scenes read.

#[derive(Clone)]
struct Line {
    speaker: String,
    body: String,
}

#[derive(Resource)]
struct Script {
    lines: Vec<Line>,
    index: usize,
    flags: Vec<String>,
}

fn current_line(script: &Script) -> Option<&Line> {
    script.lines.get(script.index)
}

fn advance(script: &mut Script) {
    if script.index + 1 < script.lines.len() {
        script.index += 1;
    }
}

fn set_flag(script: &mut Script, flag: &str) {
    if !script.flags.iter().any(|f| f == flag) {
        script.flags.push(flag.to_string());
    }
}

Space advances. Number keys pick a choice and write its flag.

#[derive(Component)]
struct Choice {
    key: KeyCode,
    flag: String,
    next_index: usize,
}

#[derive(Clone)]
struct Line {
    speaker: String,
    body: String,
}

#[derive(Resource)]
struct Script {
    lines: Vec<Line>,
    index: usize,
    flags: Vec<String>,
}

fn advance(script: &mut Script) {
    if script.index + 1 < script.lines.len() {
        script.index += 1;
    }
}

fn set_flag(script: &mut Script, flag: &str) {
    if !script.flags.iter().any(|f| f == flag) {
        script.flags.push(flag.to_string());
    }
}

fn read_advance(keys: Res<ButtonInput<KeyCode>>, mut script: ResMut<Script>) {
    if keys.just_pressed(KeyCode::Space) {
        advance(&mut script);
    }
}

fn read_choice(
    keys: Res<ButtonInput<KeyCode>>,
    choices: Query<&Choice>,
    mut script: ResMut<Script>,
) {
    for choice in &choices {
        if keys.just_pressed(choice.key) {
            set_flag(&mut script, &choice.flag);
            script.index = choice.next_index.min(script.lines.len().saturating_sub(1));
        }
    }
}

pub struct NovelPlugin;

impl Plugin for NovelPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Script {
            lines: vec![Line {
                speaker: "Guide".to_string(),
                body: "The gate is shut.".to_string(),
            }],
            index: 0,
            flags: Vec::new(),
        })
        .add_systems(Update, (read_advance, read_choice).chain());
    }
}

Startup spawns two choices. The portrait name is a component on its own entity.

#[derive(Component)]
struct Portrait {
    name: String,
}

#[derive(Component)]
struct Choice {
    key: KeyCode,
    flag: String,
    next_index: usize,
}

#[derive(Resource)]
struct Script {
    lines: Vec<Line>,
    index: usize,
    flags: Vec<String>,
}

#[derive(Resource, Default)]
struct ShownLine {
    speaker: String,
    portrait: String,
    body: String,
}

#[derive(Clone)]
struct Line {
    speaker: String,
    body: String,
}

fn current_line(script: &Script) -> Option<&Line> {
    script.lines.get(script.index)
}

fn spawn_scene(mut commands: Commands) {
    commands.spawn(Portrait {
        name: "guide".to_string(),
    });
    commands.spawn(Choice {
        key: KeyCode::Digit1,
        flag: "opened_gate".to_string(),
        next_index: 1,
    });
    commands.spawn(Choice {
        key: KeyCode::Digit2,
        flag: "walked_away".to_string(),
        next_index: 2,
    });
}

fn show_portrait(
    portraits: Query<&Portrait>,
    script: Res<Script>,
    mut shown: ResMut<ShownLine>,
) {
    let Some(line) = current_line(&script) else {
        return;
    };
    let Some(portrait) = portraits.iter().next() else {
        return;
    };
    shown.speaker = line.speaker.clone();
    shown.portrait = portrait.name.clone();
    shown.body = line.body.clone();
}

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

Piece	Owner	Who may mutate	Schedule
Data on one entity	Component	One system via Query	Update or FixedMain
One value for the whole app	Resource	One system	Startup to insert, then the schedule that reads it
Spawn	Commands	The spawning system	Startup for the initial world; later schedules for runtime spawn
Order that must not race	.chain()	The chained systems only	Same schedule
Plugin boundary	Plugin	build registers systems and resources	Before the app runs

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

If you need	Do this	Do not
Per-entity data	Component + Query	A Godot node script
App-wide config or clock	Resource	Copy the value onto every entity
Stable simulation step	FixedMain and the Fixed clock	Tie simulation to raw frame Update
Startup spawn	Commands in Startup	Spawn from a render system
Cross-system notice that is not shared state	Message or observer slot in bevy-events-messaging	A new event type invented in this file
An API outside the documented set	Check the official docs first	A made-up crate or API

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.

• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.

• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No Godot types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• No invented genre APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

• Do not pick a physics or net crate here. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official pages stay on the unversioned latest branch.

Official Documentation

• Assets - Official asset catalog. No third-party crate was selected.
• Examples - Example tree on the latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.

Related Skills

Prerequisites

• bevy-master - ECS ownership and schedules.
• bevy-ecs-composition - Spawn and query shape.

Complements

• godot-genre-visual-novel - Visual novel on the Godot engine, same depth.
• bevy-dialogue-system - Lines.
• bevy-ui-text - Body text.
• bevy-ui-layout - Choice buttons.
• bevy-save-load - Route flags.

Downstream / consumers

• None. This router does not own a second implementation.

Master

• bevy-master - Bevy router.