HealthUI.cs

using System.Collections;
using System.Collections.Generic;
using DG.Tweening;
using UnityEngine;
using UnityEngine.UI;

public class HealthUI: MonoBehaviour {
    Image heartComponent;
    static Sprite fullHeart;
    static Sprite emptyHeart;
    [SerializeField] EntityType entityType;
    List<Image> hearts;
    int lastHeart;
    CanvasGroup canvasGroup;

    void Awake() {
        heartComponent = GetComponentInChildren<Image>();
        if (heartComponent == null) {
            Debug.LogError("HealthUI is missing default heart Image component.");
        }

        if (fullHeart == null) {
            fullHeart = Resources.Load<Sprite>("Heart-full");
            if (fullHeart == null) {
                Debug.LogError("Could not load Full Heart image.");
            }
        }
        if (emptyHeart == null) {
            emptyHeart = Resources.Load<Sprite>("Heart-empty");
            if (emptyHeart == null) {
                Debug.LogError("Could not load Empty Heart image.");
            }
        }

        canvasGroup = gameObject.AddComponent<CanvasGroup>();
        canvasGroup.blocksRaycasts = false;
        canvasGroup.interactable = false;
        canvasGroup.alpha = 1f;
    }

    void Start() {
        if (entityType == EntityType.Player) {
            Player.I.Health.OnDamage += RemoveHeart;
            Player.I.Health.OnDied += FadeOut;
            WaveManager.I.OnGameComplete += FadeOut;
            InitUI(Player.I.Health.HP);
        } else if (entityType == EntityType.Enemy) {
            var enemyHealth = GetComponentInParent<Health>();
            if (enemyHealth == null) {
                Debug.LogError("Could not grab enemy health component.");
            }
            enemyHealth.OnDamage += RemoveHeart;
            enemyHealth.OnDied += FadeOut;
            InitUI(enemyHealth.HP);
        }
    }

    void Update() {
        if (entityType == EntityType.Enemy) {
            transform.FaceCamera();
        }
    }
    
    void InitUI(int health) {
        hearts = new() { heartComponent };
        var size = heartComponent.rectTransform.sizeDelta;
        //One child already exists.
        for (int i = 0; i < health - 1; i++) {
            var newChild = Instantiate(heartComponent, transform);
            newChild.transform.SetParent(transform);
            hearts.Add(newChild);
        }
        lastHeart = hearts.Count - 1;
    }

    void RemoveHeart() {
        if (lastHeart < 0) {
            Debug.LogError("trying to remove heart ui when no hearts are left!");
            return;
        }
        hearts[lastHeart].sprite = emptyHeart;
        lastHeart--;
    }

    void FadeOut() {
        new Timeout(() => canvasGroup.DOFade(0f, 0.3f).SetEase(Ease.InOutSine), 0.3f, this);
    }
}