Trouble accessing a UI gameobject and modifying when entering a trigger.

Unity Engine
1

I’m trying to set up a health bat that goes down whenever the player takes damage. Just for testing purposes, I created an empty game object and used that as a trigger to decrease the health bar when the player walks into it.

Here’s the code attached to the health bar UI

using UnityEngine;
using System.Collections;
using UnityEngine.UI;


public class HealthBarController : MonoBehaviour {

    public Image healthBar;
    private float startHealth = 1.0f;
    private float currentHealth;

    // Use this for initialization
    void Start ()
    {
        currentHealth = startHealth;

        GetHealth();
    }

    void GetHealth ()
    {
        healthBar.fillAmount = currentHealth;
    }
}

and here’s the code on the trigger that I want to use to decrease the player’s health bar when they enter it.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class TestDamageScript : MonoBehaviour
{
    HealthBarController HPcontroller;

    void Start ()
    {
        HPcontroller = GetComponent<HealthBarController>();
    }

    void OnTriggerEnter2D (Collider2D entity)
    {
        if (entity.tag == "Player")
        {
            HPcontroller.healthBar.fillAmount -= 0.5f;
        }
    }

}

whenever I run the scene, everything works as it should until I enter the trigger where it throws back:

NullReferenceException: Object reference not set to an instance of an object
TestDamageScript.OnTriggerEnter2D (UnityEngine.Collider2D entity) (at Assets/Scripts/TestDamageScript.cs:18)

I know the error is probably my logic behind the HPcontroller.healthBar.fillAmount. I’m just not sure what to do to start fixing it.

2

I think fillAmount have value from 0 to 1 :smile:.

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class TestDamageScript : MonoBehaviour
{
    HealthBarController HPcontroller;
    void Start ()
   {
        HPcontroller = GetComponent<HealthBarController>();
   }
    void OnTriggerEnter2D (Collider2D entity)
    {
        if (entity.tag == "Player")
        {
          var value = HPcontroller.healthBar.fillAmount - 0.5f;
          if(value < 0)
                value = 0;
          HPcontroller.healthBar.fillAmount = value;
        }
    }
}