Event Systems in Games: Triggers, Conditions, Actions

An event system is how a game makes its objects react to things that happen, and it is built from three parts: a trigger, a condition, and an action. A trigger is what happens — a collision, a key press, a timer firing. A condition is a filter that must also be true, such as "and the player has no shield." An action is the response — subtract a life, play a sound, load the next level. Read together, an event rule is the sentence when this happens, if this is true, do that. Underneath sits the observer, or publisher-subscriber, pattern: one object emits an event, and any number of listeners respond, without the source needing to know who is listening.

This explainer covers the three parts of an event rule, the pattern underneath them, how the major engines express the same idea, and where event-driven logic shines and where it tangles.

The three parts of an event rule#

Every event-driven rule in a game, from a Construct event sheet to a block of code, comes down to the same three pieces.

• Trigger (the event) — the thing that happens. A collision begins or ends, a key is pressed, a timer reaches zero, an enemy enters a zone, a button is clicked, or a custom event you defined fires ("on boss defeated"). The trigger is the when.
• Condition — a check that must also pass before the response runs. "Only if the player has no shield," "only if health is greater than zero," "only if the enemy is facing left." Conditions are the whether. A rule can have no conditions, one, or several, and they are usually combined with AND (all must pass) or OR (any may pass).
• Action — what happens in response. Subtract a life, play a hurt sound, spawn particles, apply a force, load a level, set a variable. The action is the then. A rule can run several actions in sequence.

Put together, a rule is a sentence: when the player overlaps a spike, and the player has no shield, subtract a life and play a hurt sound. That is the whole model. If you want to see which engines make this model their primary way to build logic, the visual scripting engines comparison ranks them.

The pattern underneath: publish and subscribe#

The trigger-condition-action model is a friendly front end for a well-known software pattern called the observer pattern, also described as publish-subscribe. The idea is simple and powerful.

• A publisher owns an event and emits (or publishes, or broadcasts) it when something happens.
• Subscribers (listeners, observers) that registered interest in that event are notified and run their handlers in response.
• The publisher does not know who its subscribers are, or how many there are, or what they will do. It only says "this happened."

That last point is the whole reason the pattern exists: decoupling. When a player takes damage, the player object does not need to know about the health bar, the audio system, the particle effects, or the save system. It simply emits player_damaged. Whoever cares — the UI, the sound manager, the achievement tracker — listens and reacts on its own. Adding a new reaction (a new achievement, a new screen-shake effect) means subscribing a new listener, not editing the player object.

The same pattern appears in everyday software and hardware: a button click, a phone notification, a thermostat turning on when the temperature drops. A game just uses it more densely, because almost everything in a game is a reaction to something else.

How engines express the same idea#

The observer pattern is universal, but engines expose it through different interfaces. They all mean the same thing.

Event sheets (Construct, GDevelop). The trigger-condition-action model is literal. Each row is a rule: a trigger on the left, conditions beside it, actions to the right. Reading a sheet top to bottom reads like reading the game's rules. This is the most readable form of event-driven logic for a non-programmer.

Node events (Unreal, Unity). In a node-graph editor, an event node is the trigger — OnComponentBeginOverlap, Event Hit, a custom event, or a key-press node. Wires flow from it into branch nodes (the conditions) and into action nodes (the response). The graph is the same rule, drawn instead of listed. The node editor explainer covers how these graphs are read.

Signals (Godot). Godot calls its events signals. A node declares a signal (for example, health_changed) and emits it when relevant; other nodes connect to that signal and run a function when it fires. Godot's signal system is a textbook observer pattern, and it is one reason the engine could drop its visual scripting in version 4.0 — the underlying event model is already clean in code. (The comparison linked above covers that decision in detail.)

Code callbacks (everywhere). In text code the same thing is a callback, a delegate, or a method wired to a message. Unity uses messages such as OnCollisionEnter2D and OnTriggerEnter, plus C# events and delegates. The shape is identical: when X happens, run this function.

	Trigger	Condition	Action
Event sheet	"On collision" row	A condition cell	An action cell
Node graph	Event node	Branch node	Action node
Godot signal	signal emitted	An if in the handler	Code in the handler
Code callback	The message fires	An if statement	The handler body

Pick any row and translate it into any other; the meaning does not change.

How an event rule maps to code#

An event rule is not magic — it is a different view of code you could write by hand. The mapping is direct.

• A trigger is an event being raised — a signal emitted, a message sent, a callback invoked.
• A condition is an if statement inside the handler.
• An action is a function call (or several), run when the if passes.

On collision between player and enemy, if player has no shield, subtract one life and play a sound is, in code, a collision handler containing an if, and inside that if two function calls. This is why event-driven logic still rewards programming thinking: you are writing conditions and actions whether you type them or assemble them from blocks. The difference is the interface, not the logic.

Where event-driven logic shines#

Event systems earn their place by solving a specific, common problem: making many parts of a game react to one change without tangling them together.

• Decoupling. The source of an event stays ignorant of its listeners. New features attach as new subscribers, not as edits to existing code.
• Responsiveness. Logic runs when something happens, not on a fixed schedule. There is no need to check the same condition every frame.
• Readability. A rule expressed as trigger-condition-action reads like the behavior it produces, which helps designers reason about the game.
• Reuse. One event can drive many independent responses — a single "enemy defeated" trigger can update the score, drop loot, and check for level completion, each in its own handler.

Where it tangles#

Event-driven logic has one main failure mode, and it is worth naming plainly. When an event triggers a handler that emits another event, which triggers a handler that emits another, the chain becomes hard to follow — the spaghetti-events problem, a close cousin of spaghetti code. Tracing why something happened means walking a chain of publishers and subscribers scattered across the game.

A few habits keep this in check:

• One clear owner per event. Know which object is allowed to emit a given event.
• Prefer direct calls inside a system, events across systems. Two classes that already know each other do not need an event bus between them.
• Avoid cascades. If a handler exists only to re-emit another event, fold the work into the original handler.
• Clean up subscriptions. A listener that is never removed becomes a dangling handler and a memory leak.

How Egmatic uses events#

Egmatic is a 2D-first editor and engine built on the MonoGame runtime, and its node-based logic is event-driven at the core. You define triggers — a collision, an input, a timer, a custom game event — and wire them through conditions into actions, all in a visual graph that sits beside a real-time scene editor. The trigger-condition-action model is the native way to build behavior, not a translation layer over text code.

For 2D games, where most logic is exactly this shape ("when the player touches the spike, lose a life"), an event-driven graph reads like the rule it represents. That is the same reason event sheets work so well in Construct and GDevelop: the model fits the problem. For a broader look at the visual side, read about node-based game logic and building games without code.

Conclusion#

A game event system is the observer pattern applied to gameplay, expressed as three parts: a trigger (what happens), a condition (what must also be true), and an action (the response). Underneath, a publisher emits an event and any number of subscribers react, which keeps the source of an event cleanly separated from every part of the game that cares about it. Engines expose the same idea through event sheets (Construct, GDevelop), node events (Unreal, Unity), and signals (Godot) — different interfaces, identical meaning.

Event-driven logic shines when many parts must react to one change, and it tangles when events chain into events with no clear ownership. Build rules as sentences — when this, if that, do this — keep one owner per event, and the model will carry a project a long way before it ever needs more.

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Sources#

1. Observer pattern (definition and intent: publishers, subscribers, decoupling) — Wikipedia: Observer pattern
2. Publish-subscribe pattern (event-driven messaging) — Wikipedia: Publish–subscribe pattern
3. Event-driven programming (reacting to events, triggers, and handlers) — Wikipedia: Event-driven programming
4. Unreal Blueprints event nodes (Event Hit, OnComponentBeginOverlap, custom events) — Unreal Engine docs
5. Godot signals (emit, connect) as an observer-pattern system — Godot docs: Using signals
6. GDevelop events (conditions and actions as the game's logic) — GDevelop wiki: Events
7. Visual scripting engines and how they expose event logic — Egmatic blog