Visitor

Visitor

One-line pattern summary

A pattern that separates operations into visitors so the object structure can be extended without modifying it.

Typical Unity use cases

• When adding statistics or reward calculations across many unit types.
• When the structure is fixed but operations increase often.

Parts (roles)

• Visitor
• Element
• Accept

Unity example (C#)

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

public interface IUnitVisitor
{
    void Visit(PlayerUnit playerUnit);
    void Visit(EnemyUnit enemyUnit);
}

public interface IVisitableUnit
{
    void Accept(IUnitVisitor visitor);
}

public sealed class DamagePreviewVisitor : IUnitVisitor
{
    public int TotalPreviewDamage { get; private set; }

    public void Visit(PlayerUnit playerUnit) => TotalPreviewDamage += 5;
    public void Visit(EnemyUnit enemyUnit) => TotalPreviewDamage += 10;
}

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 object structure is traversed and operations are delegated to a visitor.