Detecting Collisions

Running code when one entity collides with another

import {
  Behavior,
  Entity,
  EntityCollision,
  ColoredSquare,
} from "@dreamlab/engine";
import HealthBar from "./health-bar.ts"; // this import is not an api, the user will have to have or create a healthbar behavior for this example.
import PlayerBehavior from "./player.ts"; // this import is not an api, the user will have to have or create a player behavior for this example.

/*
  Handling Collisions in Game Entities:

  The `EntityCollision` signal in "@dreamlab/engine"
  helps detect when two entities collide. This signal provides details like whether the collision
  has just started and the other entity involved.

  Key Points:
  - **Listening for Collisions:** Use the `listen` method to react to collision events.
  - **Collision Filtering:** Handle collisions only with specific entities by checking their properties.
  - **Responding to Collisions:** For example, decrease health or destroy an entity when a collision occurs.

  Below is an `EnemyBehavior` that reduces health when hit by a bullet and increases the player's score if the enemy is destroyed.
*/

export default class EnemyBehavior extends Behavior {
  private healthBar!: HealthBar;

  onInitialize(): void {
    const health = Math.floor(Math.random() * 3) + 3;
    this.healthBar = this.entity.addBehavior({
      type: HealthBar,
      values: { maxHealth: health, currentHealth: health },
    });

    // Listen for collision event
    // EntityCollision only has: (public started: boolean, public other: Entity)
    this.listen(this.entity, EntityCollision, (e) => {
      if (e.started) this.onCollide(e.other);
    });
  }

  // Example of collision usage. We only want this entity to collide with the "Bullet" entity
  onCollide(other: Entity) {
    if (!other.name.startsWith("Bullet")) return;

    other.destroy();
    this.healthBar.takeDamage(1);
    if (this.healthBar.currentHealth <= 0) {
      const player = this.entity.game.world._.Player;
      player.getBehavior(PlayerBehavior).score += 100;
    }
  }

  // Another example of collision, in this example we change the color of its children ColoredSquare. This also works for ColoredPolygon
  // private onCollide(e: EntityCollision): void {
  //   // change wall color to balls color
  //   const wallSolidColor = e.other._.ColoredSquare;
  //   wallSolidColor.cast(ColoredSquare).color =
  //     this.entity._.ColoredSquare.cast(ColoredSquare).color;
  // }
}