【Unreal Engine】Loose Coupling with Blueprint Interface and Reducing Casting

Problems with Casting

When developing with Unreal Engine's Blueprint, you tend to heavily use "Cast To" nodes to access functionality of other Actors or Components. This is intuitive and easy to understand, but as games become complex, this Casting can cause design problems.

Important: The Real Problem with Casting is "Hard References"

The type-checking process of Casting itself (pointer comparison) is very fast—negligible CPU load. The real problem is creating "Hard References."

When you use Cast To BP_Enemy, that Blueprint holds a Hard Reference to the BP_Enemy class. This means:

• All assets referenced by BP_Enemy (meshes, textures, sounds, etc.) get loaded into memory
• Increased load times
• Increased memory usage
• "Dependency explosion" from chains of references

This is known as the "Blueprint Hard Reference Problem."

This article thoroughly explains practical use of Blueprint Interface, a powerful Unreal Engine feature for breaking free from Casting dependency and achieving more efficient, extensible designs, with concrete Blueprint examples.

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What is Blueprint Interface?

Blueprint Interface is a design tool for defining contracts for specific functionality (methods). It's similar to abstract classes with pure virtual functions in C++ or interfaces in other languages.

Blueprint Interface Basics

Interfaces themselves have no data or implementation. They simply define rules like "Actors implementing this Interface must be able to respond to functions (messages) with these names."

Benefits:

1. No Casting Required: The caller doesn't need to know what class the recipient is—just that it implements the Interface is enough to send messages.
2. Loose Coupling: Dependencies between caller and implementer are weak, making changes to one side less likely to affect the other.
3. Polymorphism: Different class Actors implementing the same Interface can execute their own unique logic in response to the same message.

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Replacing Casting with Blueprint Interface

Let's look at replacing Casting with Interface in a simple example where the player deals damage to objects they touch.

Step 1: Create the Interface

First, define the contract for receiving damage.

1. Right-click in Content Browser, select "Blueprint" -> "Blueprint Interface."
2. Name it BPI_Damageable.
3. Open BPI_Damageable, create a new function named ApplyDamage.
4. Add an input pin DamageAmount (Float).

Step 2: Implement the Interface

Next, implement this Interface in Actors that receive damage (e.g., enemies, destructible objects).

1. Open the enemy Blueprint (e.g., BP_Enemy).
2. Open "Class Settings" in the top-left.
3. In the "Interfaces" section, click "Add" and select BPI_Damageable.
4. Return to the Event Graph, right-click the ApplyDamage function and select "Implement Event."
5. Write damage processing (e.g., reduce health variable, play effects) in the implemented event node.

Step 3: Call the Interface (Eliminating Casting)

Finally, send messages from the player side via Interface.

Example Using Casting:

// Player's Overlap event, etc.
// ...
// Cast Other Actor To BP_Enemy
// If Cast succeeds, call BP_Enemy's TakeDamage function
// If Cast fails, do nothing (or try another Cast)

This method performs type checking every time Cast To BP_Enemy is executed.

Example Using Blueprint Interface:

// Player's Overlap event, etc.
// ...
// Call Message: ApplyDamage on Other Actor
// (Is Valid check not required, but best practice to verify reference is valid)

The ApplyDamage message node internally checks whether the target implements BPI_Damageable, and if so, executes that processing. No explicit Cast To node is needed.

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Blueprint Interface Applications

Application: Building an Interaction System

Interfaces are perfect for building generic interaction systems.

1. Create BPI_Interactable and define an OnInteract function.
2. Implement BPI_Interactable in all interactable Actors: doors, switches, items.
3. The player simply calls Message: OnInteract on the Actor in their line of sight, regardless of that Actor's class.

Common Mistakes and Best Practices

Mistake	Best Practice
Cramming functions into Interface	Keep Interfaces focused on a single purpose (e.g., damage, interaction, movement) and split functionality.
Casting before calling Interface	Use Does Implement Interface node to check if Interface is implemented, avoid Casting.
Using Interface for data storage	Interfaces are functionality contracts, not places to store data (variables). Keep data in Actor Blueprints.
Over-relying on Interface return values	For complex data exchange, consider using Event Dispatchers or Delegates instead of Interfaces.

C++ Usage

In C++, create Interface classes with the I[InterfaceName] naming convention and call functions via Interface using ImplementsInterface() or Cast<I[InterfaceName]>(Actor). In C++, Interface overhead is smaller than Blueprint, making it a powerful optimization tool.

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Choosing Between Casting and Interface

Blueprint Interface is an important tool for achieving loose coupling and dependency optimization in Unreal Engine development.

Item	Casting	Blueprint Interface
Dependencies	Creates Hard Reference (impacts memory/load time)	Soft Reference (no dependency created)
Coupling	High (depends on specific class)	Low (depends only on functionality contract)
Extensibility	Low (needs Cast for each new class)	High (just implement Interface)
CPU Processing Speed	Fast (pointer comparison only)	Slightly slower (Interface lookup cost)

Guidelines for Choosing

• Casting: Between closely related classes within the same module, or when the reference target is definitely loaded
• Interface: Between different modules, generic functionality calls, when you want to break dependencies

You don't need to completely eliminate Casting, but especially for communication between different modules or generic functionality calls, actively consider replacing with Blueprint Interface. This improves not only load time and memory usage but also achieves more maintainable, extensible designs.