09 - Enemy Spawner

In this section, you will implement an Enemy Spawner that repeatedly spawns new Enemies for your player to destroy!

High Level Overview

  • Creates an Enemy
  • Sets the Enemies first Target Waypoint
  • Waits for 5 seconds before repeating

Challenge: Enemy Spawner Prefab

Before implementing the Enemy Spawner script, create a Prefab that can be placed on your map where Enemies will spawn. Then, place one in your world at the start of your path.

When you're finished, it will look similar to the image below. Note: You can hide your Waypoints from being viewed in the Scene View by clicking the Eye Icon in the Hierarchy next to the parent container. This can be helpful when placing your Enemy Spawner.

example-enemy-spanwer-prefab.png

EnemySpawner Properties

  • Create an EnemySpawner MonoBehaviour Script with the following properties
    • Waypoint StartTarget - This will be the first Target Waypoint for the spawned Enemy
    • EnemyMovement Enemy - This will be the Enemy that will be spawned
    • float Delay - This will be the number of seconds between spawns
  • Serialize each property so they can be specified in the Inspector

enemy-spawner-properties.png

Spawn Method

To keep your code organized, add a Spawn() method that creates an Enemy and sets the Enemy's first Target waypoint. To create a Game Object during game play, you can call Object.Instantiate. This will add the specified Game Object to the Scene.

first-draft-spawn.png

You likely noticed that the Target property of your EnemyMovement cannot be set because it is private. Before your program will compile, you need to modify the EnemyMovement script to allow the Target property to be set.

  • Update your EnemyMovement.Target property to have a public setter

Note: It is considered a good practice to use private access modifiers whenever possible. This reduces the potential search space for bugs related to that property being modified. When you change it to public the property can be accessed anywhere in your code which mean you can introduce bugs related to the property anywhere in your code.

update-target-to-public.png

With the EnemyMovement.Target updated, your EnemySpawner should now compile

Test the Spawn Method

For now, test your Spawn method by calling it once in Start()

  • Call Spawn() from the EnemySpawner.Start method

call-spawn-in-start.png

Setting up the Enemy Spawner

Before you can test your Spawn() method, you need give your Prefab the Enemy Spawner component and specify the initial values.

  • In the Inspector add a Enemy Spawner component to your Enemy Spawner prefab.
  • Set the initial StartingWaypoint from your scene's Hierarchy

set-starting-waypoint.png

You might be tempted to set the Enemy property using the Enemy object in your Scene. However, if this object is destroyed, it will break your Spawner. Instead, you can use a Prefab from your Project which won't be destroyed in the Scene.

  • Delete the Enemy that is currently in your Hierarchy
  • Set the Enemy property to use the Enemy Prefab from your Project View

set-enemy-with-prefab.png

With your Enemy Spawner wired up, enter Play Mode to verify that a new Enemy is spawned. If all went well, you should see a new Enemy(Clone) in your Hierarchy as well as an Enemy moving between your Waypoints.

verify-spawn-method.webp

InvokeRepeating

You can use Unity's MonoBehaviour.InvokeRepeating method to repeatedly call a method by its name.

invoke-repeating.png

Testing the EnemySpawner

Enter Play Mode to test your Enemy Spawner. You may find it helpful to set the Delay to a smaller number for testing. Note: Because you use Delay in your Start method, you cannot change the Delay of the Enemy Spawner while in Play Mode.

If all went well, your game should look similar to the video below:

verify-repeating-spawn.webp

Challenge: Limit Number of Spawns

In this challenge, you will update your Enemy Spawner to only spawn a specified number of Enemies. After the specified number of Enemies have spawned, you will use MonoBehaviour.CancelInvoke() to stop the EnemySpawner.

  • Read the Friendly Manual: Unity - Scripting API: MonoBehaviour.CancelInvoke
  • Add a property SpawnsRemaining to your EnemySpawner
    • Set the default value to 5.
  • In your Spawn() method, decrement SpawnsRemaining
  • When SpawnsRemaining is 0, call CancelInvoke()

When you have successfully implemented this challenge, your game should look similar to the video below.

verify-repeating-spawn 1.webp

Edge Case: Disabling the Spawner

If the EnemySpawner becomes disabled, InvokeRepeating will continue to execute. This is typically not the desired behaviour. It is considered a best practice to call CancelInvoke() in your OnDisable() method to prevent strange side effects that might occur when using InvokeRepeating.

ondisable-cancelinvoke.png

Verify that your enemies stop spawning if you disable the EnemySpawner:

verify-ondisable.webp

Challenge: Continue Spawning OnEnable

You may have noticed that when you re-enable the Enemy Spawner, the remaining Enemies do not continue spawning. This is because Start() is only called once per Game Object the first time it becomes active in the Scene. If you would like to run code every time a Game Object becomes active, you an use OnEnable().

When implemented correctly, your game should act similar to the video below:

verify-onenable.webp

What's Next

Now that you are able to spawn enemies into you game. It is time to design a turret that rotates to follow nearby enemies: 10 - Rotating Turret

Table Of Contents
09 - Enemy Spawner
High Level Overview
Challenge: Enemy Spawner Prefab
EnemySpawner Properties
Spawn Method
Test the Spawn Method
Setting up the Enemy Spawner
InvokeRepeating
Testing the EnemySpawner
Challenge: Limit Number of Spawns
Edge Case: Disabling the Spawner
Challenge: Continue Spawning OnEnable
What's Next