11 - Projectile
In this lesson, you will create a Projectile that will move toward an Enemy. Upon reaching the Enemy, it will apply damage. If an enemy receives enough damage, it will be destroyed.
Challenge: Create a Projectile Prefab
Before your turrets can shoot a projectile, you must first create a Projectile Prefab to spawn.
- Create a new Prefab for your Projectile
- Add a model to the Prefab (I will be using
weapon-ammo-arrowbut you can use any model you like) - Create a
ProjectileMonoBehaviour Script - Add the
Projectilecomponent to the Prefab
When you're finished it should look similar to the image below:
Projectile Test Scene
It is often quicker to create a new scene to test your Prefabs and Scripts to see if they work in a constrained environment before putting them into your game.
- Create a new Folder called Test Scenes
- Right click, Create > Folder
- Create a Projectile Test Scene
- Rick click, Create > Scene > Scene
The Projectile Test
Within the Projectile Test Scene, you will set up a situation to determine if the Projectile is working as intended.
- A Projectile will have a Target that it will move toward.
- When it reaches the Target, it should deal damage to the Enemy.
- When it reaches the Target, it should be removed from the Scene.
- If the Enemy has sustained Damage equal to or greater than their Health, the enemy should be removed from the Scene.
Enemy Health and Damage
Before you are able to create your test, you will need to add health and damage to your Enemy.
- Create a new
HealthMonoBehaviour Script - Add a
float BaseHealthproperty - Add a
float Damageproperty
- Add a Health component to your Enemy Prefab
Projectile Properties
- In your
Projectilescript - Add a
float Speedproperty, set the default value to 2 - Add a
float Damageproperty, set the default value to 1 - Add a
Transform Targetproperty
Setting up the Test
- Open your Projectile Test Scene
- Add an Enemy to the Scene
- Set the Enemy's
BaseHealthto 2 - Add 2 Projectiles to the Scene
- Position the Projectiles such that one is further away from the Enemy
- Set the Projectiles Target to be the Enemy
When you're finished, your test scene should look similar to the video below:
Challenge: Implement Moving Projectile
- Add an
Updatemethod to yourProjectileScript - The
Projectileshould rotate to face the Target - The
Projectileshould move toward the Target atSpeedunits per second - If the
Projectileis sufficiently close to the Target display a "Hit" message in the console - Fix any
NullReferenceExceptionsthat you encounter
When you have finished the challenge, it should look similar to the video below:
Destroying Projectiles
When the Projectile has reached its target, it should remove itself from the scene. This can be done using the Object.Destroy(Object) method.
- Read the Friendly Manual: Unity - Scripting API: Object.Destroy
- Update your code to call
Object.Destroywhen theProjectilehas reach its Target
Note: You should destroy gameObject which is the Game Object the Projectile component is attached to.
- Enter Play Mode to test that your Projectiles are removed from the Hierarchy when they reach the Target
Applying Projectile
Next, you will add an ApplyHit(Projectile) method to your Health script that will add Projectile.Damage.
ApplyHit
- Open the
Healthscript - Add a
public void ApplyHit(Projectile) - Increase
Health.DamagebyProjectile.Damage
GetComponentInParent
Before the Projectile is destroyed, you must call Health.ApplyHit specifying that this Projectile has reached the Target. However, because Target is a Transform, you cannot call the ApplyHit method directly. Furthermore, the Target is actually a child of the Enemy Prefab. To find the Health component attached to the Parent object, you can use the Component.GetComponentInParent method to find the Health component.
- Read the Friendly Manual: Unity - Scripting API: Component.GetComponentInParent
- Open your
ProjectileScript - Update your code to use
Target.GetComponentInParent<Health>()to retrieve theHealthcomponent of theTarget - If a
Healthcomponent was found, callApplyHit(this)
- Verify that the Enemy's
Damageproperty updates properly in your Test Projectile Scene
Challenge: Destroy the Enemy
Finally, update your Health script to destroy the enemy and remove it from the Scene if it has taken damage greater than or equal to its BaseHealth.
When you have finished, your Test Projectile Scene should look similar to the video below:
Projectile Edge Cases
The test you have created is for the best case scenario. That is, the Enemy is destroyed after receiving 2 damage. What happens if there is a 3rd projectile?
- Add a 3rd Projectile to your scene that Targets the Enemy
- Place it in the Scene such that all 3 Projectiles reach the Enemy at different times
- Enter Play Mode, what happens?
Depending on how you implemented your Projectile, it may throw a NullReferenceException when the Target is destroyed. It may also simply stop moving when the Target is destroyed. If you check in the Inspector, you will see that the Target is marked as Missing. This is because it was destroyed.
As a developer, you will often need to account for situations that are not ideal but are technically possible during game play. These are often referred to as Edge Cases because they exist at the "boundaries" of what is expected. As you continue to develop your skills, you will begin to develop an intuition for Edge Cases and be able to anticipate many (but not all) of them.
Challenge: Fix the Edge Case
There are many possible ways to handle this particular Edge Case. One of the simplest solutions is to destroy the Projectile if it has no Target.
- Update your
Projectilescript such that if theTargetisnull, the Projectile is removed from the Scene
When you have successfully implemented this challenge, your scene should act similar to the video below:
What's Next?
With your Projectile Prefab working, it is time to add a script that allows your turrets to fire them.