Best Practices: Godot GDScript - Event Bus

This guide covers a pattern to maintain signal connections in Godot projects of growing sizes.

Thanks to Ombarus for sharing this design pattern. He presented it in a devlog video on his YouTube channel.

Events bus: Observer pattern for Godot

Maintaining signal connections isn’t the easiest in Godot, especially when wiring them via code. Godot 3.1 doesn’t offer any visual cues for signals connected through code as opposed to signals connected with the editor. Connecting signals through the editor or via code have different advantages.

Connecting signals through the editor’s node tab

• You can see an icon next to nodes that are connected to another node in the scene tree.
• The code doesn’t get cluttered with connection declarations.

Connecting signals via code

• You can connect any node, including those you create at runtime, as well as those that aren’t present in the current scene.
• You can search for calls to the connect() method globally in the project.
• Contrary to the editor, you’re not limited to using Node only. Any Object can define, emit, and connect signals. See the Object class’s docs.

Using an Event singleton to avoid spaghetti code

With these guidelines and in our work, we’re trying to decouple code to create independent, reusable, and scalable systems. This comes at a cost: we lose the ability to connect signals across separate systems easily, and it tends to lead to spaghetti code. The Event singleton is a pattern to reduce this effect at the expense of introducing a global dependency.

We couldn’t directly connect a signal in a deeply nested node from DialogSystem to a nested node in the Board tree branch while following the decoupling guidelines.

One solution is to declare the signal connection between those systems in a script attached to the Game node. The problem is that we can lose track of connections since they’re not declared in the scripts attached to the nodes that need these connections themselves.

In a complex system, you might have hundreds of signals emitted and connected all over the place. To manage this we can use dedicated Events singletons (autoloaded scripts):

Here’s an example Events.gd:

signal party_walk_started(msg)
signal party_walk_finished(msg)

...

signal dialog_system_proceed_pressed(msg)
signal dialog_system_cancel_pressed(msg)

...

signal battle_started(msg)
signal battle_finished(msg)

This singleton’s only purpose is to lists signals that can be emitted and connected to. A “deeply” nested node like $Game/Party/Godette/Walk could then emit the appropriate signal directly using the global Events node: Events.emit_signal("party_walk_started", {destination = destination}). Other nested nodes could connect to these signals: Events.connect("party_walk_started", self, "_on_Party_walk_started") etc.

This way, we can encapsulate signal connections in the related nodes instead of managing them in the code of some parent script like Game.