Decrementing a variable in a script in which it was not declared.

Question

I am trying to control the number of enemies that can be in my game at any given time. Within the script that controls spawning I have a public static variable named enemyCount (the script is attached to multiple objects) that is incremented by 1 every time an enemy is spawned, and when enemyCount reaches the maximum allowed enemies then enemies stop spawning. How can I decrement this variable by 1 within my script that controls when an enemy loses health and ultimately dies? Everything I have tried up to this point has given me a NullReferenceException: Object reference not set to an instance of an object.

I have left in an example of what I tried to decrement the value, and that gave me the exception.

Script that controls enemy spawn: public class SpawnerManager : MonoBehaviour { public PlayerHealth playerHealth; public GameObject enemy; public Transform spawnPoint;

 public float spawnTime;
 private static int enemyCount = 1;

 public int EnemyCount
 {
     get { return enemyCount; }
     set { enemyCount += value; }
 }

 private void OnEnable()
 {
     StartCoroutine(ExecuteAfterTime());
 }

 IEnumerator ExecuteAfterTime()
 {
     spawnTime = Random.Range(3, 7);
     yield return new WaitForSeconds(spawnTime);

     if (enemyCount < 5)
     {
         Instantiate(enemy, spawnPoint.position, spawnPoint.rotation);
         enemyCount++;
     }

     StartCoroutine(ExecuteAfterTime());
 }

}

Script that controls enemy health, death, etc: public class EnemyHealth : MonoBehaviour { public int startingHealth = 100; public int currentHealth; public float sinkSpeed = 2.5f; public int scoreValue = 10;

 public AudioClip deathClip;

 Animator anim;
 AudioSource enemyAudio;
 ParticleSystem hitParticles;
 CapsuleCollider capsuleCollider;
 SpawnerManager spawnerManager;

 private bool isDead;
 private bool isSinking;


 void Awake ()
 {
     anim = GetComponent <Animator> ();
     enemyAudio = GetComponent <AudioSource> ();
     hitParticles = GetComponentInChildren <ParticleSystem> ();
     capsuleCollider = GetComponent <CapsuleCollider> ();
     spawnerManager = GetComponent<SpawnerManager>();

     currentHealth = startingHealth;
 }


 void Update ()
 {
     if(isSinking)
     {
         transform.Translate (-Vector3.up * sinkSpeed * Time.deltaTime);
     }
 }


 public void TakeDamage (int amount, Vector3 hitPoint)
 {
     if(isDead)
         return;

     enemyAudio.Play ();

     currentHealth -= amount;
         
     hitParticles.transform.position = hitPoint;
     hitParticles.Play();

     if(currentHealth <= 0)
     {
         Death ();
     }
 }


 public void Death ()
 {
     isDead = true;

     capsuleCollider.isTrigger = true;

     anim.SetTrigger ("Dead");

     enemyAudio.clip = deathClip;
     enemyAudio.Play ();

     spawnerManager.EnemyCount = -1;
 }


 public void StartSinking ()
 {
     GetComponent <UnityEngine.AI.NavMeshAgent> ().enabled = false;
     GetComponent <Rigidbody> ().isKinematic = true;
     isSinking = true;
     ScoreManager.score += scoreValue;
     Destroy (gameObject, 2f);
 }

}

Answer 1

Best Answer

Answer by UnityCoach · Mar 01, 2017 at 08:55 PM

As this is a static property, you need to access it at class level, not instance level. You probably meant to write :

 SpawnerManager.EnemyCount = -1;

instead of

 spawnerManager.EnemyCount = -1;

as spawnerManager is obviously an instance of SpawnerManager

Add comment · Show 3 · Share

UnityCoach · Mar 01, 2017 at 08:58 PM 0

Share

Sorry, I misread you code.

In fact, what appears odd to me is this :

 public int EnemyCount
  {
      get { return enemyCount; }
      set { enemyCount += value; }
  }

I would expect that :

 public int EnemyCount
  {
      get { return enemyCount; }
      set { enemyCount = value; }
  }

UnityCoach UnityCoach · Mar 01, 2017 at 09:00 PM 0

Share

And then :

 spawner$$anonymous$$anager.EnemyCount -= 1;

Mat7itan · Mar 01, 2017 at 09:08 PM 0

Share

Thank you! This fixed it.

I have set { enemyCount += value; } so that the value that the variable enemyCount is set to has 1 subtracted from it when an enemy dies. If set { enemyCount = value; } then enemyCount becomes -1 and will essentially allow infinite enemy spawns as long as the player keeps killing them because enemyCount will never reach the value for the maximum number of enemies.

Decrementing a variable in a script in which it was not declared.

1 Reply

Your answer

Welcome to Unity Answers

Follow this Question

Related Questions