Updating image UI with unity

Question

I was trying to make a program that updated the amount of hearts the player has every turn (full, half and empty hearts). When I was doing this I instantiated a gameobject of that prefab as a variable and then assigned it to my UI panel in unity. However (I'm not sure but I think that) the variables used in the update just before are still being referenced after being destroyed in the next loop giving me the error:

MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it.

Here is the update loop:

void Update()
{
    if (HP <= 0)
    {
        anim.SetBool("Death", true);
        Destroy(GameObject.Find("Hearts"));
        Destroy(GameObject.Find("Inventory"));
        text.alignment = TextAnchor.LowerCenter;
        text.text = "\n YOU DIED";
        text.fontSize = 150;
        text.color = new Color(255, 0, 0);
    } else {
        foreach (Transform child in GameObject.Find("Hearts").transform)
        {
            Destroy(child.gameObject);
        }
        for (var i = 0; i<(int)HP; i++)
        {
           GameObject Heart = Instantiate(heart, new Vector3(0, 0, 0), Quaternion.identity) as GameObject;
            Heart.transform.SetParent(GameObject.Find("Hearts").transform);
        }
        if (HP - (float)((int)HP) == 0.5F) {
            GameObject HalfHeart = Instantiate(halfheart, new Vector3(0, 0, 0), Quaternion.identity) as GameObject;
            HalfHeart.transform.SetParent(GameObject.Find("Hearts").transform);
        }
        for (var i =0; i<Mathf.Floor(MaxHP-HP); i++)
        {
            GameObject EmptyHeart = Instantiate(emptyheart, new Vector3(0, 0, 0), Quaternion.identity) as GameObject;
            EmptyHeart.transform.SetParent(GameObject.Find("Hearts").transform);
        }
    }

Is there a way to instantiate a prefab without making a variable?, or a way to make the variable reference temporary so it only lasts one update? Thank you for your help in advance!

It seems like you are destroying and reinstantiating your health bar every frame if I'm understanding this correctly. You will want to make this more event driven (i.e. update the healthbar only once at the moment when the HP changes). — Alex Myers
Could you please show us the code where HP increases/decreases? — Alex Myers
1. You shouldn't be using Find in Update! 2. You shouldn't use Find multiple times ... if something use this only once, store the result and reuse the reference later! 3. You shouldn't destroy and re-instantiate stuff all the time in Update this is very very performance intense and a waste of resources. Instantiate stuff only once... — derHugo

Kalle Halvarsson Kalle Halvarsson · Accepted Answer · 2019-04-27T20:23:37

The problem is that once HP goes below zero, every subsequent update will enter the first if-statement and try to delete the "Hearts" and "Inventory" objects over and over. You can solve this by adding a bool called isDead and change the statement to if (HP <= 0 && !isDead) and then set isDead = true inside the block. This will prevent it from entering it twice.

Frankly though, your way of solving things is entirely backwards. As others have pointed out, deleting and instantiating objects every frame is very inefficient, and Transform.Find is also slow. You don't really need to destroy anything at all - you can rather just have a list of hearts and enable/disable an appropriate amount whenever the HP changes. You can have a single half-heart at the end of the list and enable/disable it when appropriate - if you are using a HorizontalLayoutGroup, it will still align correctly. You might want to make it so that you can only change the HP using a property or function (something like ModifyHealth(float amount)), and put the logic for updating the hearts display in there.

Updating image UI with unity

1 Answers