Attacking the Furthest Enemy in Range (Tower Defence)

In the previous article, I demonstrated how to efficiently track the furthest enemy using a max heap. In this article, we’ll see how to integrate that into a game mechanic.

Event-Driven Approach

The existing implementation uses an event-driven architecture. For this article, we will focus on the Enemy events. Those events will trigger spin-off actions.

Each enemy can experience various events. Below is an example of a life cycle that the enemy may go through:

For the article, I am interested in the two events:

• enemyMoved: Fired when an enemy's position changes.
• enemyRemoved*: Fired when an enemy is removed from the game (e.g. defeated).

(* I plan to adjust event names in the future since an enemy can be removed for different reasons.)

Plan

I created an Event Model diagram to visualise how different events interact. This helps with understanding how things connect.

For every event, I have a command that triggers it. (So, an event is the outcome of a command.) In some cases, as a result of an event, we will need to update data (green sticky notes depict this). A combination of all three together is a vertical slice.

My focus will be on the green sticky note "Enemies within tower's range".

Implementation

The goal is for us to update available enemies whenever an enemy is within the tower's range and remove them if not.

We will be working with a tower class. In this class, we have a variable to store enemies.

export class Tower implements ITower {
    public enemies = new MaxHeap()

    constructor(id: number, coords: Coordinate) {
        this.id = id
        this.coords = coords

        // listeners will go here...
    }

Placing event listeners in the Tower class centralises the logic, reducing the need to maintain mappings between towers and enemies. While this adds some complexity to the class, it ensures better encapsulation and simplifies debugging, which is an easier direction to take for now.

Action: Adding Enemies Within Range

Test: Adding an Enemy

First, we’ll write a test to verify that an enemy within range is added to the tower's enemies heap:

it('should add an enemy to the tower when enemy is within range', () => {
    const tower = new Tower(1, { col: 0, row: 1 });
    const enemy = new TinyEnemy();
    enemy.currentPosition = { col: 0, row: 1 };

    triggerEnemyMovedEvent(enemy);

    expect(tower.enemies.length()).toBe(1);
});

Implementation: Adding an Enemy

Here’s the corresponding implementation:

window.addEventListener("enemyMoved", event => {
    const enemy: Enemy = event.detail.enemy;

    if (enemyWithinRange(this, enemy)) {
        this.enemies.insertOrUpdate(enemy.id, enemy.distanceTraveled);
    }
});

Whenever enemyMoved is triggered, we will check if an enemy should be added to the heap. I already have enemyWithinRange function, it's a matter of adding insertOrUpdate call.

---

Action: Prevent Adding Out-of-Range Enemies

Test: Ignoring Enemies Out of Range

Next, we ensure that enemies outside the tower's range are not added:

export class Tower implements ITower {
    public enemies = new MaxHeap()

    constructor(id: number, coords: Coordinate) {
        this.id = id
        this.coords = coords

        // listeners will go here...
    }

Implementation: Ignoring Enemies Out of Range

This scenario is already covered by our earlier check using enemyWithinRange, so no additional code is required.

---

Action: Removing Enemies Out of Range

Test: Removing an Out-of-Range Enemy

Now we test that enemies leaving the range are removed from the tower's visibility:

it('should add an enemy to the tower when enemy is within range', () => {
    const tower = new Tower(1, { col: 0, row: 1 });
    const enemy = new TinyEnemy();
    enemy.currentPosition = { col: 0, row: 1 };

    triggerEnemyMovedEvent(enemy);

    expect(tower.enemies.length()).toBe(1);
});

Implementation: Removing an Out-of-Range Enemy

window.addEventListener("enemyMoved", event => {
    const enemy: Enemy = event.detail.enemy;

    if (enemyWithinRange(this, enemy)) {
        this.enemies.insertOrUpdate(enemy.id, enemy.distanceTraveled);
    }
});

If the enemy used to be within the range then we can remove it.

---

Action: Removing an Enemy from the Game

Test: Handling enemyRemoved Event

Lastly, we ensure that enemies removed from the game are also removed from the tower's heap:

it('should not add an enemy to the tower if enemy is out of range', () => {
    const tower = new Tower(1, { col: 0, row: 1 });
    const enemy = new TinyEnemy();
    enemy.currentPosition = { col: 0, row: 99 };

    triggerEnemyMovedEvent(enemy);

    expect(tower.enemies.length()).toBe(0);
});

Implementation: Handling enemyRemoved

it('should remove an enemy from the tower when it moves out of range', () => {
    const tower = new Tower(1, { col: 0, row: 1 });
    const enemy = new TinyEnemy();
    enemy.currentPosition = { col: 0, row: 1 };

    // enemy within range
    triggerEnemyMovedEvent(enemy);
    expect(tower.enemies.length()).toBe(1);

    // enemy outside of the range
    enemy.currentPosition = { col: 0, row: 99 };
    triggerEnemyMovedEvent(enemy);

    expect(tower.enemies.length()).toBe(0);
});

Whenever an event is triggered, if the enemy is within the range then we can remove them.

Conclusion

By combining an event-driven approach with a max heap, we achieved an efficient way for towers to prioritise enemies dynamically. The implementation ties seamlessly into the game's event system, ensuring real-time updates and responsiveness.

Additionally, when it comes to testing, using an event-driven approach removes the need to tie internal code to test. Hence making tests less brittle. We can refactor code behind behaviour in whatever way we want, and as long as events/listeners are set up correctly, tests should still pass.

This implementation can now pave the way to:

• Adding attack functionality (now that we know who to attack)
• Swapping how data for enemies is stored

Feel free to adapt this approach for your own tower defence games.

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