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Crash Course 2: Invaders - Part 12

by Hectate (Updated on 2015-05-02)

Part 12: Destroying the Enemy Ships

Now we put those bullets to use! We want to give the enemy some health so it can take a few hits, and then destroy it once it takes too much damage.

Step 83: While still in the Bullet's events we should be looking at the Actor - Type collision event. We're going to use the same collision to also tell the Enemy Ship that it was hit by a bullet.

From the palette, find the Trigger Event [text] in all behaviors for [Actor] block (found in Behavior > Triggers) and snap it above the Kill Self block (we want it to be triggered first).

Type "Hit" into the text field, and then grab the blue Actor Of Type block from the collision event wrapper above and insert it into the Actor spot. The event should look like the image below.

Step 84: Switch to the Enemy Ship actor and access the Events tab for it. Click Add Event and select Advanced > Custom Event.

Step 85: The event wrapper will appear, in the blank field type "Hit". This will cause the previous "tell" block we added (to the bullet) to trigger any code that we put in this event when triggered.

Now, if the enemy was just going to take one hit and be destroyed we could put a kill self block in here and be done with it. However, we actually want the enemy ship to take multiple hits. To track how many hits it can take, we're going to need an attribute to store a "Health" value.

Step 86: Create a new Attribute, select Number for its type, and name it Health Points (click the Attributes button on the Palette, then the Create an Attribute button).

Step 87: Add another event to the Enemy Ship. Create a When Creating Event (click + Add Event, then mouse over Basics, then click When Creating).

Step 88: Now lets actually set the initial Health value. Click on Attributes -> Setters in the Palette and drag over the Set Health Points to [ ] block, snap it inside the When Created Event, and type 3 into the empty field.

Step 89: Now that each Enemy Ship will have 3 health points when they're created, lets do some damage to them when they're hit.

Switch back to the Enemy Ship's Custom Event, and then grab the Increment Number By [ ] block (found in Numbers & Text > Math in the pallete) and put it inside the custom event. Switch the "increment" field to "decrement", and select "Health Points" in the number field (if not there already). Finally, in empty field type "1".

Step 90: Finally, we need to add an Event that checks the number of Health Points the Enemy Ship has left and destroys it if it has 0 (or fewer, in case it gets hit more than once very quickly).

To start, click the + Add Event button, mouse over Basics again, and select When Updating. Next, drag an if block inside the Always block (in Flow -> Conditions in the Palette).

Step 91: Now we add a "Less Than Or Equal To" block (under Flow -> Conditions again, it looks contains a "<=" between two numbers) to the empty field in the if block and then check whether Health Points (found in Attributes > Getters) is less than or equal to 0 or not.

Step 92: Here's where we put the Kill Self block (under Actor > Properties, if you recall).

Now our enemy ships will be killed if they are hit by three or more bullets! Feel free to test this out now. That said, its not very fun for them to just disappear; let's fix that!

Step 92: We want to use our Explosion sound effect we uploaded earlier. If we add a [Play] [Sound] block (found in Sound & Images > Sound) and select our Explosion sound effect from the dropdown menu (click the block and then select the Explosion sound we uploaded earlier), that sound will play right as our Actor is removed from the Scene.

Step 93: Now we need to add an effect to our Enemy Ship Actor Type so the player can tell when it’s hit. Switch back to the Custom Event. We’re going to use blocks under Actor -> Effects in the Palette

For the effects, grab these three blocks:

  • Apply Effect [ ] to [Actor]
  • Make Negative
  • Remove All Effects From [Actor]

We also need to have a Do After timer block (Flow > Time in the palette).

Put the Make Negative block inside the Apply Effect block's blank space. Snap the Do After block in just below this and type ".1" into the blank for the time. Finally, put the Remove All Effects block inside the timer.

With all this in place, test the game. If everything is working, it’s time to create a way for our player to win the game.


Click here to go to Part 13.

Disclaimer: Use comments to provide feedback and point out issues with the article (typo, wrong info, etc.). If you're seeking help for your game, please ask a question on the forums. Thanks!


The Enemy Ships don't disappear when ever a bullet hits them I checked the collision groups thrice but the enemy doesn't destroy itself.
0 11 years, 8 months ago
Oh no! The enemies don't dissapear!

OOPS, Nevermind, hadn't changed increment to decrement.

0 11 years, 8 months ago
What does the "make negative" effect do? Is this what makes the enemy actor light up when a bullet hits it? Is there a way to alter/edit this effect?
0 11 years, 8 months ago
Sorted it, went into Enemy Ship, Properties checked Group - was fine, but Bullets was not. Onwards!
0 11 years, 9 months ago
For some reason cannot get the bullets to hit the enemies, checked the Collision Groups, but that is ok. Might start this bit again
0 11 years, 9 months ago
To add the explosion sound effect when bullets hit an enemy ship, add a Play Sound block (under "Sound") right before the Decrement Health block and set the second dropdown to "Explosion."
2 11 years, 10 months ago

If anyone is having a problem with collision detection with the enemies, you may want to check your Collision Group settings. This can be found on the Dashboard under Game>Collision Groups. Be sure to check the Enemies group and the Bullets group. Under Enemies, the Bullets button should be toggled green. Likewise, under Bullets, Enemies should be toggled green.

I don't know if I just plain missed it while reading or if it was something unclear, but that drove me a little nuts for a while since the logic seemed right XD!

Hope that helps!

1 11 years, 10 months ago
Thanks it works!
0 11 years, 10 months ago

Why the enemies is not there when i played it??

0 11 years, 11 months ago
i got error:
Behavior: ActorEvents_2 at line 50
Syntax error: expecting identifier before rightparen.
if((_HealthPoints <= ))

Behavior: ActorEvents_2 at line 51
Syntax error: expecting rightparen before leftbrace.

Behavior: ActorEvents_2 at line 51
Syntax error: expecting rightparen before leftbrace.

0 11 years, 11 months ago

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