[Design Pattern] State Pattern

One-line pattern summary

A pattern that separates behavior by state into objects and swaps behavior based on state transitions.

Typical Unity use cases

When player movement and combat states keep increasing.
When if-else state branching becomes too large.

Parts (roles)

Context
State Interface
Concrete State

Unity example (C#)

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

public interface ICharacterState
{
    void Tick(PlayerStateMachine stateMachine);
}

public sealed class IdleState : ICharacterState
{
    public void Tick(PlayerStateMachine stateMachine)
    {
        if (stateMachine.MoveInput > 0f)
        {
            stateMachine.ChangeState(new RunState());
        }
    }
}

public sealed class RunState : ICharacterState
{
    public void Tick(PlayerStateMachine stateMachine)
    {
        if (stateMachine.MoveInput <= 0f)
        {
            stateMachine.ChangeState(new IdleState());
        }
    }
}

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 the context delegates to the current state object and performs transitions.

flowchart LR

  context["PlayerContext"]
  idle["IdleState"]
  run["RunState"]
  jump["JumpState"]
  input["Input"]
  output["Animation / Logic"]

  input --> context
  context -->|delegate| idle
  idle -->|move key| run
  run -->|jump key| jump
  jump -->|landing| idle
  idle --> output
  run --> output
  jump --> output