Bevy Dialogue System Agent Skill

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.