Games aren't much fun until you have aliens running around on screen! In this game, we will make use of two types of movements:
- **Keyboard/Mouse movement**: when the user interacts with the keyboard or mouse to move an object on the screen.
- **Game induced movement**: when the game moves an object with a certain time interval.
So how do we move things on a screen? It's all about cartesian coordinates: we change the location (x,y) of the object and then redraw the screen.
Typically you need the following steps to accomplish *movement* on a screen:
1.**Set a new location** for an object; this is needed to perceive the object as having moved.
2.**Clear the screen**, the screen needs to be cleared in between draws. We can clear it by drawing a rectangle that we fill with a background color.
3.**Redraw object** at new location. By doing this we finally accomplish moving the object from one location to the other.
Here's what it can look like in code:
```javascript
//set the hero's location
hero.x += 5;
// clear the rectangle that hosts the hero
ctx.clearRect(0, 0, canvas.width, canvas.height);
// redraw the game background and hero
ctx.fillRect(0, 0, canvas.width, canvas.height)
ctx.fillStyle = "black";
ctx.drawImage(heroImg, hero.x, hero.y);
```
✅ Can you think of a reason why redrawing your hero many frames per second might accrue performance costs? Read about [alternatives to this pattern](https://www.html5rocks.com/en/tutorials/canvas/performance/).
## Handle keyboard events
You handle events by attaching specific events to code. Keyboard events are triggered on the whole window whereas mouse events like a `click` can be connected to clicking a specific element. We will use keyboard events throughout this project.
To handle an event you need to use the window's `addEventListener()` method and provide it with two input parameters. The first parameter is the name of the event, for example `keyup`. The second parameter is the function that should be invoked as a result of the event taking place.
Here's an example:
```javascript
window.addEventListener('keyup', (evt) => {
// `evt.key` = string representation of the key
if (evt.key === 'ArrowUp') {
// do something
}
})
```
For key events there are two properties on the event you can use to see what key was pressed:
-`key`, this is a string representation of the pressed key, for example `ArrowUp`
-`keyCode`, this is a number representation, for example `37`, corresponds to `ArrowLeft`.
✅ Key event manipulation is useful outside of game development. What other uses can you think of for this technique?
### Special keys: a caveat
There are some *special* keys that affect the window. That means that if you are listening to a `keyup` event and you use these special keys to move your hero it will also perform horizontal scrolling. For that reason you might want to *shut-off* this built-in browser behavior as you build out your game. You need code like this:
```javascript
let onKeyDown = function (e) {
console.log(e.keyCode);
switch (e.keyCode) {
case 37:
case 39:
case 38:
case 40: // Arrow keys
case 32:
e.preventDefault();
break; // Space
default:
break; // do not block other keys
}
};
window.addEventListener('keydown', onKeyDown);
```
The above code will ensure that arrow-keys and the space key have their *default* behavior shut off. The *shut-off* mechanism happens when we call `e.preventDefault()`.
## Game induced movement
We can make things move by themselves by using timers such as the `setTimeout()` or `setInterval()` function that update the location of the object on each tick, or time interval. Here's what that can look like:
```javascript
let id = setInterval(() => {
//move the enemy on the y axis
enemy.y += 10;
})
```
## The game loop
The game loop is a concept that is essentially a function that is invoked at regular intervals. It's called the game loop as everything that should be visible to the user is drawn into the loop. The game loop makes use of all the game objects that are part of the game, drawing all of them unless for some reason shouldn't be part of the game any more. For example if an object is an enemy that was hit by a laser and blows up, it's no longer part of the current game loop (you'll learn more on this in subsequent lessons).
Here's what a game loop can typically look like, expressed in code:
```javascript
let gameLoopId = setInterval(() =>
function gameLoop() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = "black";
ctx.fillRect(0, 0, canvas.width, canvas.height);
drawHero();
drawEnemies();
drawStaticObjects();
}, 200);
```
The above loop is invoked every `200` milliseconds to redraw the canvas. You have the ability to choose the best interval that makes sense for your game.
## Continuing the Space Game
You will take the existing code and extend it. Either start with the code that you completed during part I or use the code in [Part II- starter](your-work).
- **Moving the hero**: you will add code to ensure you can move the hero using the arrow keys.
- **Move enemies**: you will also need to add code to ensure the enemies move from top to bottom at a given rate.
## Recommended steps
Locate the files that have been created for you in the `your-work` sub folder. It should contain the following:
```bash
-| assets
-| enemyShip.png
-| player.png
-| index.html
-| app.js
-| package.json
```
You start your project the `your_work` folder by typing:
```bash
cd your-work
npm start
```
The above will start a HTTP Server on address `http://localhost:5000`. Open up a browser and input that address, right now it should render the hero and all the enemies; nothing is moving - yet!
### Add code
1.**Add dedicated objects** for `hero` and `enemy` and `game object`, they should have `x` and `y` properties. (Remember the portion on [Inheritance or composition](../README.md) ).
*HINT*`game object` should be the one with `x` and `y` and the ability to draw itself to a canvas.
>tip: start by adding a new GameObject class with its constructor delineated as below, and then draw it to the canvas:
and add a `createHero()` function to do a similar process for the hero.
```javascript
function createHero() {
hero = new Hero(
canvas.width / 2 - 45,
canvas.height - canvas.height / 4
);
hero.img = heroImg;
gameObjects.push(hero);
}
```
and finally, add a `drawGameObjects()` function to start the drawing:
```javascript
function drawGameObjects(ctx) {
gameObjects.forEach(go => go.draw(ctx));
}
```
Your enemies should start advancing on your hero spaceship!
---
## 🚀 Challenge
As you can see, your code can turn into 'spaghetti code' when you start adding functions and variables and classes. How can you better organize your code so that it is more readable? Sketch out a system to organize your code, even if it still resides in one file.
While we're writing our game without using frameworks, there are many JavaScript-based canvas frameworks for game development. Take some time to do some [reading about these](https://github.com/collections/javascript-game-engines).
