Chain of Responsibility

One-line pattern summary

A pattern that passes a request through a chain of handlers, where each handler performs its responsibility in sequence.

Typical Unity use cases

• When shields, buffs, and resistances must be applied sequentially in damage calculation.
• When input filters need to process in stages.

Parts (roles)

• Handler
• Concrete Handler
• Next Handler

Unity example (C#)

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

public abstract class DamageModifierHandler
{
    private DamageModifierHandler nextHandler;

    public DamageModifierHandler SetNext(DamageModifierHandler nextHandler)
    {
        this.nextHandler = nextHandler;
        return nextHandler;
    }

    public int Handle(int incomingDamage)
    {
        int updatedDamage = ModifyDamage(incomingDamage);
        return nextHandler == null ? updatedDamage : nextHandler.Handle(updatedDamage);
    }

    protected abstract int ModifyDamage(int incomingDamage);
}

public sealed class ShieldDamageHandler : DamageModifierHandler
{
    protected override int ModifyDamage(int incomingDamage)
    {
        return System.Math.Max(0, incomingDamage - 20);
    }
}

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 request moves through a chain and each handler takes responsibility if possible.

flowchart LR

  request["Cast Skill Request"]
  auth["AuthHandler"]
  cooldown["CooldownHandler"]
  cost["CostHandler"]
  execute["ExecuteHandler"]
  reject["Rejected"]
  complete["Executed"]

  request --> auth
  auth -->|fail| reject
  auth -->|pass| cooldown
  cooldown -->|fail| reject
  cooldown -->|pass| cost
  cost -->|fail| reject
  cost -->|pass| execute
  execute --> complete