In previous article we displayed also game objects. Now it’s time to add some movement.

We will use mouse click to navigate an avatar around the stage. It seems pretty straightforward, but there are several gotchas. Let’s explore them.

First of all we need to revisit the topic of Vuex store which we mentioned in the first article. It’s a very simple yet powerful design. All information about the state of the game is stored in the store. When you’d like to change something you dispatch a commit from your code and store methods will create a new state. In Vuex store these methods are known as mutations.

Let’s design simple mutation moveObject where we will send game object and two vectors in file js/game.js.

const store = new Vuex.Store({
...
    mutations: {
        moveObject(state, { gameObject, vectorX, vectorY }) {
            gameObject.x += vectorX;
            gameObject.y += vectorY;
        },
    }
});

You may wonder about interesting syntax of moveObject. The second attribute is an object which contains three attributes. If you write code this way it will allow you to pass more than one attribute to mutation function.

Values of vectorX and vectorY will be -1, 0, 1. We can send this values from method in component playground.

Vue.component('playground', {
    ...
    methods: {
        moveGameObject: function(gameObject, vectorX, vectorY) {
            store.commit("moveObject", { gameObject, vectorX, vectorY });
        },

This code here is pretty straightforward. It’s just transporting information about a move from component to the store. Now we need to tell HTML component to listen on click and send it to component. We will use v-on:click to send the event to JavaScript method mouseClicked. Note: @click is shorthand for v-on:click.

<script type="text/x-template" id="playground-template">
  <div @click="mouseClicked">

Now we’re missing just the last piece of the puzzle to make this thing work. It’s the method mouseClicked. This one will be a little bit tricky. Let’s split the method into several parts. In the first part, we will need to determine coordinates where a user clicked relative to the component. This requires some computation because the object of event does not contain the information about relative coordinates.

methods: {
        ...
        mouseClicked: function(event) {
            let rect = event.currentTarget.getBoundingClientRect();
            let scrollLeft = window.pageXOffset || document.documentElement.scrollLeft;
            let scrollTop = window.pageYOffset || document.documentElement.scrollTop;
            let top = rect.top + scrollTop;
            let left = rect.left + scrollLeft;

So we have the coordinates of click relative to the component. We would like to know whether a user clicked left, right, below or above the avatar. We need this information to determine in which direction avatar should move. We will retrieve coordinates of the first object (avatar) from the store. Then we will compute delta so we can decide the direction.

            let avatar = this.$store.state.gameObjects[0];
            let deltaX = (event.clientX - left - 32)/64 - avatar.x;
            let deltaY = (event.clientY - top - 32)/64 - avatar.y;

You may wonder why we’re dividing coordinates by 64. It’s size of tile from previous articles.

Based on delta we can decide what to do. First of all, it’s good to eliminate case when we should not do anything.

            // Ignore clicks within avatar box
            if (Math.abs(deltaX) < 0.5) {
                deltaX = 0;
            }

            if (Math.abs(deltaY) < 0.5) {
                deltaY = 0;
            }

           // Do not move in case of click inside the icon of avatar
            if ((deltaX === 0) && (deltaY === 0)) {
                return;
            }

Let’s add code for deciding in which direction should the main character move.

          if (Math.abs(deltaX) > Math.abs(deltaY)) {
                if (deltaX > 0) {
                    this.moveGameObject(avatar, 1, 0);
                } else {
                    this.moveGameObject(avatar, -1, 0);
                }
            } else {
                if (deltaY > 0) {
                    this.moveGameObject(avatar, 0, 1);
                } else {
                    this.moveGameObject(avatar, 0, -1);
                }
            }
        }

The result should look like this and it should react on mouse click:

The source code of this article is stored at Github branch article-04.

In the next article, we will add checks so the avatar does not move through other objects or walls.

You can find more HTML5 games at https://georgik.rocks/tag/games

Back to tutorial Sokoban in Vue.js