State

State

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.