Designing a reusable interaction framework: triggers, unlock conditions, directionality, and physics-driven doors in Blueprints.
This project is a Blueprint demo built in Unreal Engine 5.5 that implements eleven distinct door types. It was developed following this Udemy course as a systematic exploration of interactive object design — specifically, how to build door systems that are flexible, composable, and easy to extend.
You can watch the demo here: YouTube
The Core Abstraction: Separating the Door from Its Trigger
The most important architectural decision in a door system is whether the door knows how it opens, or whether something else tells it to open. Coupling the door directly to a specific trigger (e.g., “open when the player overlaps this volume”) produces a system that works for one case but breaks the moment requirements change.
The cleaner approach — and the one this demo explores — is to treat the door as a passive receiver of open/close signals, and to keep all trigger and unlock logic in separate actors or components. A pressure plate sends a signal. A lever sends a signal. A key collection system sends a signal. The door doesn’t care which one fired it. This separation is what makes it possible to build eleven door variants without eleven completely independent implementations.
Trigger Types
The demo covers three categories of triggers:
Proximity triggers — The pressure plate door opens when the player steps on it and closes when they leave. This is the simplest form of stateful trigger: it has two states (pressed / released) and maps them directly to door states (open / closed). The multi-pressure-plate door extends this by requiring all plates to be active simultaneously, which introduces AND logic into the trigger layer — a door that only opens when a set of conditions are all true at once. Objects can be placed on plates to hold them down, creating environmental puzzle potential.
Lever and combination triggers — A single lever is a binary toggle, straightforward to implement. The ordered combination of levers is more interesting: it requires tracking not just which levers are active, but the sequence in which they were activated. This is the first case where the trigger system needs internal state beyond a simple boolean — it needs a sequence buffer and a validation step. The button combination door operates similarly, comparing player input against a stored correct sequence.
Collection triggers — The key-collection door opens once all keys in the level have been picked up. This requires a counter or set maintained across the level, which the door (or a coordinating game state object) can query. It’s a good example of a trigger that doesn’t fire from a single interaction but from the accumulated result of multiple independent ones.
Directionality
Three of the door variants explore how a door responds to the player’s position relative to it:
The one-direction door only opens when approached from a specific side — useful for one-way corridors or airlocks. The opposite-direction door opens away from the player, which requires computing the player’s position relative to the door’s local forward vector at the moment of interaction and choosing the rotation direction accordingly. The one-side unlock door can only be triggered from one side, but once open, can be passed through from either direction.
All three require the door to know where the player is at interaction time, not just that an interaction occurred. This is the key distinction from pure trigger-based doors: directionality introduces spatial awareness into the door logic.
Physics-Driven Door
The swinging physics door is architecturally different from all the others. Every other door in the demo is animated — it follows a predefined timeline or interpolates to a target rotation. The swinging door is simulated: it responds to the player’s collision as a physics body, which means its behavior is emergent rather than scripted.
The tradeoff is control versus realism. An animated door is predictable and easy to design around. A physics door can overshoot, oscillate, or behave unexpectedly if constraints aren’t tuned carefully. For most game contexts, animated doors with good easing curves are the right choice. Physics doors are a better fit for environmental props where unpredictability adds to the feel.
Teleport Door
The teleport door is less about the door itself and more about the transition system behind it. When the player passes through, they are repositioned at a target location — typically another door actor in the level — with their orientation preserved relative to the exit. Getting the orientation transform right (especially when the entry and exit doors face different directions) is the main technical consideration here. A naive implementation that just teleports to a world position will produce disorienting rotation snaps.
Reflection
What makes this demo valuable as a learning exercise isn’t any single door type — it’s the process of recognizing that eleven surface-level variations reduce to a much smaller set of underlying patterns: trigger logic, condition evaluation, directionality, animation versus simulation. Once those patterns are clear, the implementation of any new door variant becomes a question of which combination of patterns it uses.
In a C++ context, this framework would map naturally to an interface-based design: an IInteractable interface for the door, a set of UTriggerComponent subclasses for the various trigger types, and a condition evaluator that the door queries before opening. The Blueprint version makes the same logical structure visible in node graph form — the wiring is just more explicit.
Nicolás Bertoa 
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