Bevy dialogue system

When to use

Load this when lines, choices, or speaker ids advance. Rendering the line is bevy-ui-text.

ECS shape

• Components: a line id and a speaker id that you define.
• Resources: run-wide tables live in a resource (bevy-data-resources), not an autoload.
• Systems: mutate those components on Update or the fixed timestep. Use .chain() when damage must land before death.
• Messages: one-shot facts (hit, pickup, quest advance) go through bevy-events-messaging. This file does not add a new event type name.

Engine counterparts

Godot idea	Bevy idiom
Line table	A resource of ids and strings.
Current line	A component that stores the line index.
Portrait	An image handle via bevy-asset-scenes. Layout is bevy-ui-layout.

Scope

Dialogue data is a resource of lines plus a component that stores the current line index. Rich text rendering belongs on bevy-ui-text.

[dependencies]
bevy = "0.20"

#[derive(Clone)]
struct LineDef {
    speaker: &'static str,
    text: &'static str,
    next: Option<usize>,
}

#[derive(Resource)]
struct DialogueTable {
    lines: Vec<LineDef>,
}

#[derive(Component)]
struct LineMark {
    index: usize,
    open: bool,
}

fn advance_line(
    keys: Res<ButtonInput<KeyCode>>,
    table: Res<DialogueTable>,
    mut marks: Query<&mut LineMark>,
) {
    if !keys.just_pressed(KeyCode::Space) {
        return;
    }
    for mut mark in &mut marks {
        if !mark.open {
            continue;
        }
        let Some(line) = table.lines.get(mark.index) else {
            mark.open = false;
            continue;
        };
        match line.next {
            Some(next) => mark.index = next,
            None => mark.open = false,
        }
    }
}

fn spawn_conversation(mut commands: Commands) {
    commands.insert_resource(DialogueTable {
        lines: vec![
            LineDef {
                speaker: "guide",
                text: "The gate opens at dusk.",
                next: Some(1),
            },
            LineDef {
                speaker: "guide",
                text: "Bring the key.",
                next: None,
            },
        ],
    });
    commands.spawn(LineMark {
        index: 0,
        open: true,
    });
}

pub struct DialoguePlugin;

impl Plugin for DialoguePlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, spawn_conversation)
            .add_systems(Update, advance_line);
    }
}

Choices are data on the line. Hide a choice when its required flag is absent.

#[derive(Clone)]
struct ChoiceDef {
    label: &'static str,
    next: usize,
    requires_flag: Option<&'static str>,
}

#[derive(Resource)]
struct ChoiceList {
    choices: Vec<ChoiceDef>,
}

#[derive(Resource, Default)]
struct StoryFlags {
    names: Vec<&'static str>,
}

#[derive(Component)]
struct LineMark {
    index: usize,
}

fn shown_choices(flags: &StoryFlags, list: &ChoiceList) -> Vec<usize> {
    list.choices
        .iter()
        .enumerate()
        .filter_map(|(index, choice)| match choice.requires_flag {
            Some(flag) if !flags.names.iter().any(|name| *name == flag) => None,
            _ => Some(index),
        })
        .collect()
}

fn choose_first(
    keys: Res<ButtonInput<KeyCode>>,
    flags: Res<StoryFlags>,
    list: Res<ChoiceList>,
    mut marks: Query<&mut LineMark>,
) {
    if !keys.just_pressed(KeyCode::Digit1) {
        return;
    }
    let Some(choice_index) = shown_choices(&flags, &list).first().copied() else {
        return;
    };
    let next = list.choices[choice_index].next;
    for mut mark in &mut marks {
        mark.index = next;
    }
}

fn open_choice(mut commands: Commands) {
    commands.spawn(LineMark { index: 0 });
}

pub struct ChoicePlugin;

impl Plugin for ChoicePlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(StoryFlags {
            names: vec!["has_key"],
        })
        .insert_resource(ChoiceList {
            choices: vec![
                ChoiceDef {
                    label: "Open the gate",
                    next: 2,
                    requires_flag: Some("has_key"),
                },
                ChoiceDef {
                    label: "Leave",
                    next: 3,
                    requires_flag: None,
                },
            ],
        })
        .add_systems(Startup, open_choice)
        .add_systems(Update, choose_first);
    }
}

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 this skill	Load the skill that owns it	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 copy the line table onto every speaker. WHY: the table is one resource; the line index is the per-conversation component.

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

• No Godot node, resource, or signal types for this mechanic. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

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

• No invented APIs or unnamed third-party crates as the chosen solver. 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 site.

Official Documentation

• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_plugins.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-ecs-composition - Components and systems.
• bevy-ui-text - Draw the line.
• bevy-quest-system - Quest gates.
• bevy-data-resources - Tables and definitions.
• bevy-events-messaging - One-shot facts.

Complements

• bevy-state-machines - Mode flags.
• godot-dialogue-system - Equivalent Godot skill.

Downstream / consumers

• None yet.

Master

• bevy-master - Bevy router.