ソースコード

using System;
using Kogane;
using UnityEngine;

public sealed class Example : MonoBehaviour
{
    [SerializeField] private Collider2D m_collider2D;

    private bool m_isHit;

    public event Action<Collider2D> OnEnter;
    public event Action<Collider2D> OnExit;

    private void OnTriggerStay2D( Collider2D other )
    {
        var isContains = m_collider2D.bounds.Contains( other.bounds );

        SetState( isContains, other );
    }

    private void OnTriggerExit2D( Collider2D other )
    {
        SetState( false, other );
    }

    private void SetState( bool isHit, Collider2D other )
    {
        if ( m_isHit == isHit ) return;

        var callback = isHit ? OnEnter : OnExit;
        callback?.Invoke( other );

        m_isHit = isHit;
    }
}

public static class BoundsExtensionMethods
{
    public static bool Contains( this Bounds self, Bounds other )
    {
        return self.Contains( other.min ) && self.Contains( other.max );
    }
}

使用例

using UnityEngine;

public sealed class SampleScene : MonoBehaviour
{
    [SerializeField] private Example m_example;

    private void Awake()
    {
        m_example.OnEnter += _ => Debug.Log( "OnEnter" );
        m_example.OnExit  += _ => Debug.Log( "OnExit" );
    }
}