Dependency Injection

Dependency Injection in Godot 4.3+

Patterns for wiring dependencies between systems so nodes stay loosely coupled, swappable, and testable. All examples target Godot 4.3+ with no deprecated APIs.

Related skills: godot-testing for test-friendly architecture, event-bus for signal-based decoupling, godot-project-setup for autoload registration.

1. The Problem

Tight coupling makes code hard to test, extend, and swap. The most common form in Godot is reaching directly into a global autoload from everywhere in the codebase.

# BAD — tight coupling via direct autoload access scattered everywhere

# player.gd
func take_damage(amount: int) -> void:
    health -= amount
    AudioManager.play_sfx("hurt")          # hard dependency on AudioManager
    UIManager.update_health_bar(health)    # hard dependency on UIManager
    if health <= 0:
        GameState.record_death()           # hard dependency on GameState

# enemy.gd
func attack() -> void:
    AudioManager.play_sfx("attack")        # same AudioManager dependency again

// BAD — tight coupling via direct autoload / global access scattered everywhere

// Player.cs
public partial class Player : CharacterBody3D
{
    private int _health = 100;

    public void TakeDamage(int amount)
    {
        _health -= amount;
        GetNode<AudioManager>("/root/AudioManager").PlaySfx("hurt");        // hard dependency
        GetNode<UIManager>("/root/UIManager").UpdateHealthBar(_health);     // hard dependency
        if (_health <= 0)
            GetNode<GameState>("/root/GameState").RecordDeath();            // hard dependency
    }
}

// Enemy.cs
public partial class Enemy : CharacterBody3D
{
    public void Attack()
    {
        GetNode<AudioManager>("/root/AudioManager").PlaySfx("attack");      // same dependency again
    }
}

Problems with this approach:

• Every node that calls AudioManager directly is coupled to its concrete implementation.
• Swapping AudioManager for a different implementation requires changing every caller.
• Unit-testing Player in isolation is impossible — AudioManager, UIManager, and GameState must all exist and be valid.
• Autoload initialization order bugs silently break behaviour when scenes load.
• Hidden dependencies make it hard to see what a class actually needs to function.

2. Approach Comparison

3. Autoloads as Singletons

Register a script in Project Settings → Autoload for global access (AudioManager.play_sfx(...), GameState.score = 100). Best for cross-cutting concerns: audio, save state, event bus, settings. Resist autoloading domain-specific systems (those should be scene-injected).

See references/autoloads.md for the full AudioManager example (SFX + crossfade music) in GDScript + C#.

4. @export Node Injection

Expose collaborator nodes as @export var health_component: HealthComponent, then wire in the Inspector or via parent scene. Lifecycle: @export properties are assigned BEFORE _ready().

See references/export-injection.md for full @export patterns (GDScript + C#) and lifecycle notes.

5. Service Locator Pattern

A central registry autoload mapping String keys to service instances. Services register themselves at _ready(), deregister at _exit_tree(), consumers call ServiceLocator.get(name). Useful when you want flexible runtime swap of implementations (testing, mods, A/B variants).

See references/service-locator.md for the full Service Locator (GDScript + C#) with typed helper methods.

6. Scene Injection

Parent scene loads its children, then in _ready() walks the tree assigning dependencies (enemy.player = $Player). Children declare @export properties but the parent — not the Inspector — sets them. Best for game-specific dependencies that change per level.

See references/scene-injection.md for the parent-injects-children pattern (GDScript + C#).

7. Testing with Dependency Injection

Injecting fakes / test doubles is what makes nodes testable. For autoloads: mock-replace before the test scene loads. For @export injection: swap the export to a test double. For Service Locator: register a fake under the same key.

See references/testing-with-di.md for GUT-based test patterns showing each injection technique.

8. When to Use What

Quick decision guide:

Does every scene in the project need it?
  YES → Autoload singleton
  NO  ↓

Is the dependency known at edit-time and wired in the Inspector?
  YES → @export injection
  NO  ↓

Does the dependency need to be swapped at runtime (plugins, A/B testing)?
  YES → Service Locator
  NO  ↓

Does a parent scene own both the consumer and the dependency?
  YES → Scene injection
  NO  → Reconsider — either promote to autoload or restructure ownership

9. Anti-patterns

Autoload for everything

# BAD — GameManager, EnemySpawner, InventorySystem, DialogueSystem all as autoloads.
# Every node in the game is coupled to every other system at module level.
# Test one component → must initialise all autoloads.

# GOOD — Only AudioManager, Settings, and SceneTransition are autoloads.
# EnemySpawner is a node in the Level scene, injected into enemies that need it.

// BAD — GameManager, EnemySpawner, InventorySystem, DialogueSystem all as autoloads.
// Every node in the game is coupled to every other system at module level.
// Test one component → must initialise all autoloads.

// GOOD — Only AudioManager, Settings, and SceneTransition are autoloads.
// EnemySpawner is a node in the Level scene, injected into enemies that need it.

Deep dependency chains

# BAD — Player needs HealthComponent, which needs AudioManager,
# which needs SoundBank, which needs FileSystem...
# A change deep in the chain breaks everything above it.

# GOOD — flatten: HealthComponent takes only AudioManager (or a narrow interface).
# Each node declares only immediate dependencies.

// BAD — Player needs HealthComponent, which needs AudioManager,
// which needs SoundBank, which needs FileSystem...
// A change deep in the chain breaks everything above it.

// GOOD — flatten: HealthComponent takes only AudioManager (or a narrow interface).
// Each node declares only immediate dependencies.

Circular dependencies

# BAD
# PlayerController._ready() calls ServiceLocator.get_service("inventory")
# InventorySystem._ready() calls ServiceLocator.get_service("player")
# Neither can fully initialise because the other isn't ready yet.

# GOOD — break the cycle with a signal.
# InventorySystem emits item_used; PlayerController connects to it.
# PlayerController never holds a reference to InventorySystem at all.

// BAD
// PlayerController._Ready() calls ServiceLocator.GetService("inventory")
// InventorySystem._Ready() calls ServiceLocator.GetService("player")
// Neither can fully initialise because the other isn't ready yet.

// GOOD — break the cycle with a signal.
// InventorySystem emits ItemUsed; PlayerController connects to it.
// PlayerController never holds a reference to InventorySystem at all.

Service Locator as a god object

# BAD — everything is registered: enemies, UI panels, individual nodes.
# ServiceLocator becomes a second, untyped scene tree.

# GOOD — only register stable, long-lived services (audio, analytics, save system).
# Short-lived nodes are wired by their parent via scene injection.

// BAD — everything is registered: enemies, UI panels, individual nodes.
// ServiceLocator becomes a second, untyped scene tree.

// GOOD — only register stable, long-lived services (audio, analytics, save system).
// Short-lived nodes are wired by their parent via scene injection.

Forgetting null checks after injection

# BAD — crashes if the @export was never set in the editor
func take_damage(amount: int) -> void:
    audio.play_sfx("hurt")   # NullReferenceError if audio was not wired

# GOOD — guard or assert clearly
func take_damage(amount: int) -> void:
    assert(audio != null, "HealthComponent: audio dependency was not injected")
    audio.play_sfx("hurt")

# OR — treat it as optional
func take_damage(amount: int) -> void:
    if audio != null:
        audio.play_sfx("hurt")

// BAD — crashes if the [Export] was never set in the editor
public void TakeDamage(int amount)
{
    _audio.PlaySfx("hurt");   // NullReferenceException if _audio was not wired
}

// GOOD — guard or assert clearly
public void TakeDamage(int amount)
{
    if (_audio == null)
    {
        GD.PushError("HealthComponent: audio dependency was not injected");
        return;
    }
    _audio.PlaySfx("hurt");
}

// OR — treat it as optional
public void TakeDamage(int amount)
{
    _audio?.PlaySfx("hurt");
}

10. Checklist

• Autoloads are used only for genuinely global services (audio, settings, platform)
• Nodes declare their dependencies explicitly (@export or a public property) rather than calling get_node on distant relatives
• @export fields are validated (assert or null check) before use
• Service Locator services call unregister in _exit_tree() / _ExitTree()
• Scene injection is done in the parent's _ready(), after children are fully initialised
• No circular dependencies between services or autoloads
• Each node depends only on its immediate collaborators — no deep chains
• Test stubs/mocks are plain nodes that implement the same interface as the real service
• C# @export ([Export]) dependencies are disconnected / cleared in _ExitTree() if they hold event subscriptions
• Service Locator is not used to store scene-specific or short-lived nodes