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More game logic

If each event is described in as much detail as before, it's going to be quite a long tutorial. Let's make the description a little briefer for the next events. Remember, the steps to add a condition or action are:

  1. Double-click to insert a new event, or click an Add action link to add an action.
  2. Double-click the object the condition/action is in.
  3. Double-click the condition/action you want.
  4. Enter parameters, if any are needed.

From now on, events will be described as the object, followed by the condition/action, followed by any parameters. For example, the event we have just inserted could be written:

Add condition SystemEvery tick

Add action PlayerSet angle towards position, and for X: Mouse.X, Y: Mouse.Y

Get the player to shoot

When the player clicks, they should shoot a bullet. This can be done with the Spawn an object action in Player, which creates a new instance of an object at the same position and angle. The Bullet movement we added earlier will then make it fly out forwards. Make the following event:

Condition: MouseOn clickLeft clicked (the default)

Action: PlayerSpawn another object, then for Object, choose the Bullet object. Leave the other parameters as they are.

Your event should now look like this:

If you run the game, you can shoot bullets! However you may notice the bullets shoot from the middle of the player, rather than from the end of the gun. Let's fix that by placing an image point at the end of the gun. An image point is just a position on an image that you can spawn objects from, and we can reference it in the Spawn another object action.

Right-click the player in the Project Bar and select Edit animations.

The image editor for the player reappears. Click the origin and image points tool:

...and then the side pane turns in to a list of image points:

Notice the object origin appears in the list. That's the "hotspot" or "pivot point" of the object. If you rotate the object, it spins around the origin. That's also what's used when you spawn an object at image point 0, as our action did. We want to add another image point to represent the gun, so right-click in the list and select Add a new image point. A new item appears in the list, and an icon appears over the image to indicate where this image point is. Left-click at the end of the player's gun to place the image point there:

Close the image editor. Double-click the Spawn an object action we added earlier, and change the Image point to 1. The event should now look like below - note it says Image point 1 now:

Preview the game again. The bullets now shoot from the end of your gun! The bullets don't do anything yet, though. Hopefully, however, you'll start to realise that once you get to grips with the event system, you can put together your game's logic very quickly.

Let's make the bullets kill monsters. Add the following event:

Condition: BulletOn collision with another objectMonster.

Action: MonsterDestroy

Action: BulletSpawn another objectExplosion

Action: BulletDestroy

Here's what the finished event looks like.

The explosion effect

Preview the game, and try shooting a monster. Oops, the explosion has that big black border!

You might have predicted it'd look like that from the start, and wondered if our game was really going to end up like that! Don't worry, it won't. Click the Explosion object in the Project Bar. Its properties appear in the Properties Bar on the left. In the Effects section, set its Blend mode to Additive. Now try the game again.

Why does this work? Without going too much in to the nuts and bolts, ordinary images are pasted on top of the screen. With the additive blend mode, each pixel is instead added (as in, summed) with the background pixel behind it. Black is a zero pixel value, so nothing gets added - you don't see the black background. Brighter colors add more, so appear more strongly. It's great for explosions and lighting effects.

Making monsters a little smarter

Right now the monsters just wander off the layout to the right. Let's make them a bit more interesting. First of all, let's start them at a random angle.

Condition: SystemOn start of Layout

Action: MonsterSet angle to random(360)

They will still wander off forever when they leave the layout, never to be seen again. Let's keep them inside. What we'll do is point them back at the player when they leave the layout. This does two things: they always stay within the layout, and if the player stands still, monsters end up coming right for them!

Condition: MonsterIs outside layout

Action: MonsterSet angle toward position X: Player.X Y: Player.Y

Here are what the two finished events look like.

Run the game. If you hang around for a while, you'll notice the monsters stay around the layout too, and they're going in all kinds of directions. It's hardly AI, but it'll do!

Now, suppose we want to have to shoot a monster five times before it dies, rather than instant death like it is at the moment. How do we do that? If we only store one "Health" counter, then once we've hit a monster five times, all the monsters will die. Instead, we need each monster to remember its own health. We can do that with instance variables.

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