Week 2: The Basic Enemy

For one of our enemies we wanted to start with having a simple behavior that would only move in one direction. The enemy would be moving from right to left and be randomly spawned in on a vertical line slightly outside the main view of the game.

As this would mostly be for learning and having an enemy that wouldn’t do much more than moving straight we decided that having it spawn in endlessly would be a good first step.

The enemy would give the player points for shooting and destroying it.

The enemy’s script for moving straight to the right was very easy to create as it only needs a speed and a transform on its rigidbody. In Unity the transform can do several things like scale the object and move it in different directions, among other functions.

Because of this it usually has a “transform.right” or something similar for moving but it doesn’t have a “transform.left” so I simply had to use the former statement and put it in the negative for the enemy to move in the left direction.

void FixedUpdate ()
{
        GetComponent<Rigidbody2D>().velocity = -transform.right * EnemySpeed;
}

 

The “EnemySpeed” is a public variable that can be changed in Unity.

The enemy also needed be be destroyed and give a score to the player so using Unity’s tag system I set up an if statement that if any collider with the tag of “bullet” would destroy the gameobject and add the scorevalue to the scoremanager we set up earlier.

void OnTriggerEnter2D(Collider2D other)
{
     if (other.tag == "Bullet")
     {
            ScoreManager.score += scoreValue;
            Destroy(gameObject);
     }
}

 

The simple enemy would spawn in at fixed intervals that could be determined in Unity after setting up a new script that would only handle it spawning.

public GameObject Enemies;
public Vector2 SpawnValues;
public int EnemyCount;
public float SpawnWait;
public float StartWait;
public float WaveWait;

 

Having these in the script we could choose the enemy prefab inside Unity and set up variables that would be needed for spawning in the enemy later.

When wanting to do spawning at set time intervals in Unity you can use StartCoroutine and IEnumerator.

void Start ()
{
      StartCoroutine(SpawnEnemies());
}

IEnumerator SpawnEnemies()
{
      yield return new WaitForSeconds(StartWait);
      while (true)
      {
            for (int i = 0; i < EnemyCount; i++)
            {
             Vector2 SpawnPos = new Vector2(SpawnValues.x,
             Random.Range(-SpawnValues.y, SpawnValues.y));
             Instantiate(Enemies, SpawnPos, Quaternion.identity);
             yield return new WaitForSeconds(SpawnWait);
            }
            yield return new WaitForSeconds(WaveWait);
        }
}

 

Here is the simple script for spawning endless waves of the enemy you choose in Unity.

So Vector2 here sets up a location on a 2D plane from the values we set in Unity and the Random.Range is pretty easy to figure out as it places the enemy randomly on the vertical plane.

To spawn them in, Instantiate is used and it need the gameobject, the position and a rotation of the object so “Quaternion.identity” is used because it is “no rotation”.

The “yield return new WaitForSeconds” is also pretty easy to see what it does as it set the time between the different wait times you have set up.