Event Bus

---
name: event-bus
description: Use when implementing decoupled communication between nodes — global EventBus autoload with typed signals
---

# Event Bus in Godot 4.3+

A global signal hub that lets unrelated nodes communicate without holding references to each other. All examples target Godot 4.3+ with no deprecated APIs.

> **Related skills:** **component-system** for direct signal communication between components, **csharp-signals** for C#-specific signal patterns, **dependency-injection** for alternative decoupling approaches.

---

## 1. What is an Event Bus

An EventBus is a singleton autoload that acts as a central registry for signals. Instead of nodes connecting directly to each other, every node connects to (or emits on) the shared EventBus. This removes the need for one node to hold a reference to another.

```
Without EventBus             With EventBus
──────────────               ──────────────────────────
NodeA ──signal──► NodeB      NodeA ──emit──► EventBus ──signal──► NodeB
                                                       ──signal──► NodeC
                                                       ──signal──► NodeD
```

**Flow diagram**

```
┌─────────┐   emit(player_died)   ┌───────────┐   player_died   ┌──────────┐
│  NodeA  │ ────────────────────► │ EventBus  │ ───────────────► │  NodeB   │
│(Player) │                       │(Autoload) │                  │   (UI)   │
└─────────┘                       └───────────┘ ───────────────► └──────────┘
                                                  player_died    ┌──────────┐
                                                                 │  NodeC   │
                                                                 │(AudioMgr)│
                                                                 └──────────┘
```

NodeA emits the signal. NodeB and NodeC each connected to EventBus independently. Neither knows the other exists.

---

## 2. When to Use vs Direct Signals

| Scenario                                   | Recommended approach          |
|--------------------------------------------|-------------------------------|
| Parent notifying its own child             | Direct signal or method call  |
| Child notifying its parent                 | Direct signal (bubble up)     |
| Two nodes with the same parent             | Direct signal via parent      |
| Completely unrelated nodes in the tree     | Event bus                     |
| UI reacting to gameplay state changes      | Event bus                     |
| Audio manager reacting to game events      | Event bus                     |
| Data manager / save system reacting        | Event bus                     |
| Tight, performance-sensitive inner loop    | Direct method call            |

**Rule of thumb:** if you would otherwise need `get_node("../../SomeDistantNode")` or a hard-coded NodePath, the event bus is a better fit.

---

## 3. Basic EventBus

Create `res://autoloads/event_bus.gd` (or `EventBus.cs`), then register it in **Project → Project Settings → Autoload** with the name `EventBus`.

### GDScript (`autoloads/event_bus.gd`)

```gdscript
extends Node

## Emitted when the player character has died.
signal player_died

## Emitted whenever the score changes.
signal score_changed(new_score: int)

## Emitted when a level finishes successfully.
signal level_completed(level_id: int)

## Emitted when the player picks up a collectible.
signal item_collected(item_name: String)

## Emitted when the player's health changes.
signal health_changed(current: int, maximum: int)
```

### C# (`Autoloads/EventBus.cs`)

```csharp
using Godot;

/// <summary>
/// Global signal hub. Register as an autoload named "EventBus".
/// </summary>
public partial class EventBus : Node
{
    /// <summary>Emitted when the player character has died.</summary>
    [Signal] public delegate void PlayerDiedEventHandler();

    /// <summary>Emitted whenever the score changes.</summary>
    [Signal] public delegate void ScoreChangedEventHandler(int newScore);

    /// <summary>Emitted when a level finishes successfully.</summary>
    [Signal] public delegate void LevelCompletedEventHandler(int levelId);

    /// <summary>Emitted when the player picks up a collectible.</summary>
    [Signal] public delegate void ItemCollectedEventHandler(string itemName);

    /// <summary>Emitted when the player's health changes.</summary>
    [Signal] public delegate void HealthChangedEventHandler(int current, int maximum);
}
```

---

## 4. Connecting to Events

Consumers connect in `_ready()`. In C#, always disconnect in `_ExitTree()` to avoid dangling delegates and memory leaks.

### GDScript

```gdscript
extends CanvasLayer

# GDScript connections are reference-counted and cleaned up automatically
# when the node is freed, but explicit disconnection is still good practice
# for long-lived nodes that reconnect frequently.

func _ready() -> void:
    EventBus.player_died.connect(_on_player_died)
    EventBus.score_changed.connect(_on_score_changed)
    EventBus.health_changed.connect(_on_health_changed)


func _exit_tree() -> void:
    EventBus.player_died.disconnect(_on_player_died)
    EventBus.score_changed.disconnect(_on_score_changed)
    EventBus.health_changed.disconnect(_on_health_changed)


func _on_player_died() -> void:
    $DeathScreen.show()


func _on_score_changed(new_score: int) -> void:
    $ScoreLabel.text = "Score: %d" % new_score


func _on_health_changed(current: int, maximum: int) -> void:
    $HealthBar.value = float(current) / float(maximum) * 100.0
```

### C#

```csharp
using Godot;

public partial class HudLayer : CanvasLayer
{
    private EventBus _eventBus;

    public override void _Ready()
    {
        _eventBus = GetNode<EventBus>("/root/EventBus");

        // Connect using strongly-typed delegate handlers
        _eventBus.PlayerDied      += OnPlayerDied;
        _eventBus.ScoreChanged    += OnScoreChanged;
        _eventBus.HealthChanged   += OnHealthChanged;
    }

    // IMPORTANT: Always disconnect in _ExitTree() in C#.
    // C# delegates are not automatically cleaned up when a node is freed.
    // Failing to disconnect causes the EventBus to hold a reference to the
    // freed node, leading to memory leaks and InvalidOperationExceptions.
    public override void _ExitTree()
    {
        _eventBus.PlayerDied    -= OnPlayerDied;
        _eventBus.ScoreChanged  -= OnScoreChanged;
        _eventBus.HealthChanged -= OnHealthChanged;
    }

    private void OnPlayerDied()
    {
        GetNode<Control>("DeathScreen").Show();
    }

    private void OnScoreChanged(int newScore)
    {
        GetNode<Label>("ScoreLabel").Text = $"Score: {newScore}";
    }

    private void OnHealthChanged(int current, int maximum)
    {
        GetNode<ProgressBar>("HealthBar").Value = (double)current / maximum * 100.0;
    }
}
```

---

## 5. Emitting Events

Producers call `EventBus.<signal_name>.emit(...)` (GDScript) or `EmitSignal(SignalName.*)` (C#). The producer does not know which nodes are listening.

### GDScript

```gdscript
extends CharacterBody2D

@export var max_health: int = 100
var current_health: int = max_health
var score: int = 0


func take_damage(amount: int) -> void:
    current_health = clampi(current_health - amount, 0, max_health)
    EventBus.health_changed.emit(current_health, max_health)

    if current_health == 0:
        EventBus.player_died.emit()


func add_score(points: int) -> void:
    score += points
    EventBus.score_changed.emit(score)


func collect_item(item_name: String) -> void:
    EventBus.item_collected.emit(item_name)


func complete_level(level_id: int) -> void:
    EventBus.level_completed.emit(level_id)
```

### C#

```csharp
using Godot;

public partial class Player : CharacterBody2D
{
    [Export] public int MaxHealth { get; set; } = 100;

    private int _currentHealth;
    private int _score;
    private EventBus _eventBus;

    public override void _Ready()
    {
        _currentHealth = MaxHealth;
        _eventBus = GetNode<EventBus>("/root/EventBus");
    }

    public void TakeDamage(int amount)
    {
        _currentHealth = Mathf.Clamp(_currentHealth - amount, 0, MaxHealth);
        _eventBus.EmitSignal(EventBus.SignalName.HealthChanged, _currentHealth, MaxHealth);

        if (_currentHealth == 0)
            _eventBus.EmitSignal(EventBus.SignalName.PlayerDied);
    }

    public void AddScore(int points)
    {
        _score += points;
        _eventBus.EmitSignal(EventBus.SignalName.ScoreChanged, _score);
    }

    public void CollectItem(string itemName)
    {
        _eventBus.EmitSignal(EventBus.SignalName.ItemCollected, itemName);
    }

    public void CompleteLevel(int levelId)
    {
        _eventBus.EmitSignal(EventBus.SignalName.LevelCompleted, levelId);
    }
}
```

---

## 6. Typed Signal Parameters

For signals that need to pass multiple related values, prefer a dedicated `Resource` (strongly typed, Inspector-friendly) over a plain `Dictionary` (flexible but untyped).

### Option A — Resource payload (recommended for structured data)

```gdscript
# combat_event_data.gd
class_name CombatEventData
extends Resource

@export var attacker_id: int = 0
@export var target_id: int = 0
@export var damage_amount: int = 0
@export var damage_type: String = "physical"
@export var is_critical: bool = false
```

```gdscript
# In event_bus.gd — add the signal:
signal combat_hit(data: CombatEventData)
```

```gdscript
# Producer
var data := CombatEventData.new()
data.attacker_id   = get_instance_id()
data.target_id     = target.get_instance_id()
data.damage_amount = 25
data.damage_type   = "fire"
data.is_critical   = true
EventBus.combat_hit.emit(data)
```

```gdscript
# Consumer
func _on_combat_hit(data: CombatEventData) -> void:
    if data.is_critical:
        _show_critical_text(data.target_id, data.damage_amount)
```

```csharp
// CombatEventData.cs — Resource-based payload (Inspector-friendly, fully typed)
using Godot;

public partial class CombatEventData : Resource
{
    [Export] public int AttackerId   { get; set; }
    [Export] public int TargetId     { get; set; }
    [Export] public int DamageAmount { get; set; }
    [Export] public string DamageType { get; set; } = "physical";
    [Export] public bool IsCritical  { get; set; }
}

// In EventBus.cs — add the signal:
// [Signal] public delegate void CombatHitEventHandler(CombatEventData data);

// Producer
public void FireCombatHit(Node target, int damageAmount, string damageType, bool isCritical)
{
    var data = new CombatEventData
    {
        AttackerId   = (int)GetInstanceId(),
        TargetId     = (int)target.GetInstanceId(),
        DamageAmount = damageAmount,
        DamageType   = damageType,
        IsCritical   = isCritical,
    };
    _eventBus.EmitSignal(EventBus.SignalName.CombatHit, data);
}

// Consumer
private void OnCombatHit(CombatEventData data)
{
    if (data.IsCritical)
        ShowCriticalText(data.TargetId, data.DamageAmount);
}
```

### Option B — Dictionary payload (acceptable for prototyping)

```gdscript
# In event_bus.gd:
signal combat_hit(data: Dictionary)

# Producer
EventBus.combat_hit.emit({
    "attacker_id":   get_instance_id(),
    "target_id":     target.get_instance_id(),
    "damage_amount": 25,
    "is_critical":   true,
})

# Consumer — note: no compile-time safety
func _on_combat_hit(data: Dictionary) -> void:
    if data.get("is_critical", false):
        _show_critical_text(data["target_id"], data["damage_amount"])
```

The C# equivalent uses `Godot.Collections.Dictionary` in place of the Resource type and `data["key"].AsInt32()` / `.AsString()` to read values — no compile-time safety.

**Prefer Resources** for structured, reusable payloads with more than 2–3 fields. **Use Dictionary** only when prototyping or the shape changes frequently.

---

## 7. Anti-patterns

### Using the event bus for everything (over-decoupling)

```gdscript
# BAD — a parent querying its own child through the event bus
# is unnecessarily indirect and hard to follow.
func _ready() -> void:
    EventBus.request_player_position.connect(_on_request_player_position)

func _on_request_player_position() -> void:
    EventBus.player_position_response.emit(global_position)

# GOOD — a parent can access its child directly.
var player_pos: Vector2 = $Player.global_position
```

### Side effects in handlers that emit further signals

```gdscript
# BAD — handler emits another signal, which triggers another handler,
# which emits another signal. Tracing the flow requires reading all handlers.
func _on_player_died() -> void:
    _save_high_score()          # side effect
    EventBus.high_score_saved.emit()  # triggers yet another chain

# GOOD — each handler does one thing; orchestration lives in one place.
func _on_player_died() -> void:
    _show_death_screen()

# A dedicated GameManager handles multi-step reactions:
func _on_player_died() -> void:
    _save_high_score()
    get_tree().reload_current_scene()
```

### Circular event chains

```gdscript
# BAD — PlayerHealth connects to health_changed and re-emits it.
func _on_health_changed(current: int, maximum: int) -> void:
    _current = current
    EventBus.health_changed.emit(_current, maximum)  # infinite loop

# GOOD — update internal state only; let the original emitter own the signal.
func _on_health_changed(current: int, maximum: int) -> void:
    _current = current
    _update_display()
```

### Connecting without disconnecting in C#

```csharp
// BAD — node is freed but EventBus still holds a reference to the delegate.
// The next emission raises an InvalidOperationException or silently leaks memory.
public override void _Ready()
{
    GetNode<EventBus>("/root/EventBus").PlayerDied += OnPlayerDied;
    // No _ExitTree() override — memory leak.
}

// GOOD — always pair Connect with Disconnect in C#.
public override void _ExitTree()
{
    GetNode<EventBus>("/root/EventBus").PlayerDied -= OnPlayerDied;
}
```

---

## 8. Testing

Use [GUT](https://github.com/bitwes/Gut) to verify both producer-side emission (`watch_signals(event_bus)` then `assert_signal_emitted_with_parameters(...)`) and consumer-side reactions (emit on the bus, then assert on the consumer's state). Always test against the real autoload EventBus retrieved via `get_tree().root.get_node("EventBus")`, not a fresh instance.

See [references/testing.md](references/testing.md) for full producer-side and consumer-side test files plus a GUT-helper reference table.

---

## 9. Checklist

- [ ] `EventBus` autoload is registered in **Project → Project Settings → Autoload**
- [ ] All signals use typed parameters (`signal foo(bar: int)`) — no untyped signals
- [ ] Every consumer connects in `_ready()` and disconnects in `_exit_tree()` (mandatory in C#)
- [ ] Producers emit through `EventBus`, not by calling consumer methods directly
- [ ] No node holds a direct reference to another unrelated node just to emit or receive signals
- [ ] Complex payloads use a `Resource` subclass, not a raw `Dictionary`
- [ ] No handler re-emits the same signal it just received (prevents infinite loops)
- [ ] No event bus signal used where a direct parent-child call or signal is simpler
- [ ] GUT tests cover both emission (producer) and reception (consumer) for critical signals
- [ ] C# handlers verified to disconnect in `_ExitTree()` before merging