[Design Pattern] Strategy Pattern

One-line pattern summary

A pattern that encapsulates algorithms with the same goal behind an interface so they can be swapped at runtime.

Typical Unity use cases

When AI attack behavior should change by situation.
When aiming or movement policies differ by character.

Parts (roles)

Strategy Interface
Concrete Strategy
Context

Unity example (C#)

The code below is a simplified Unity example based on the scenario described above.

using UnityEngine;

public interface IAimStrategy
{
    Vector3 GetAimPosition(Transform shooterTransform, Transform targetTransform);
}

public sealed class DirectAimStrategy : IAimStrategy
{
    public Vector3 GetAimPosition(Transform shooterTransform, Transform targetTransform)
    {
        return targetTransform.position;
    }
}

public sealed class LeadAimStrategy : IAimStrategy
{
    public Vector3 GetAimPosition(Transform shooterTransform, Transform targetTransform)
    {
        return targetTransform.position + targetTransform.forward * 0.5f;
    }
}

Advantages

Behavior is separated into smaller units, which reduces the impact of changes.
Adding or swapping rules is relatively safe.

Things to watch out for

As the number of objects and indirect calls increases, the flow can become harder to follow.
Ordering bugs should be pinned down with tests.

Interaction diagram

This shows the flow where a context swaps algorithms through a strategy interface.

flowchart LR

  context["PathContext"]
  selector["Select Strategy"]
  a_star["AStarStrategy"]
  flow_field["FlowFieldStrategy"]
  execute["FindPath()"]
  result["Path Result"]

  context --> selector
  selector -->|small map| a_star
  selector -->|large crowd| flow_field
  a_star --> execute
  flow_field --> execute
  execute --> result
  result --> context