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JavaScript

Konrad Pilch
Konrad Pilch
2,435 Points

Adding movement

HI,

How can i add movement to my ball?

    //Variables
    var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");
    var ballRadius = 25;
    var x = canvas.width/2; 
    var y = canvas.height/2;
    var dx = 5;
    var dy = 5;

    //Movement
    var upPressed = false;
    var rightPressed = false;
    var downPressed = false;
    var leftPressed = false;






    //Ball
    function drawBall() {
        ctx.beginPath();
        ctx.arc(x, y, ballRadius, 0, Math.PI*2);
        ctx.fillStyle = ("red");
        ctx.fill();
        ctx.closePath();
    }

    //Draw
    function draw() {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        drawBall();

        //Ball Movement


        requestAnimationFrame();
    }

    draw();

I know that the left is number 37, top 38, right 39 and down 40, and i get the kind of logic of if or swich statement, but how can i make it to move, the ball? and in future id like it to jump as well : p

2 Answers

Clayton Perszyk
MOD
Clayton Perszyk
Treehouse Moderator 48,565 Points

Hey Konrad,

I was able to get the ball to move up, down, left, right. Here is the code; I added comments to explain what I did. If you have any questions, I would be glad to answer them.

//Variables
var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");
var ballRadius = 25;
var x = canvas.width/2;
var y = canvas.height/2;

// add event listener to move x or y
// up, down, left, right
// use the event's keyCode property to get the
// correct key code
document.addEventListener('keydown', function(event){
  switch (event.keyCode) {
    case 40:
       y += 1;
       break;
    case 38:
       y -= 1;
       break;
    case 37:
       x -= 1;
       break;
    case 39:
       x += 1;
       break;
    default:
      throw new Error("Not a valid key.");
  }
})

// draw ball is the same as your original
function drawBall() {
    ctx.beginPath();
    ctx.arc(x, y, ballRadius, 0, Math.PI*2);
    ctx.fillStyle = ("red");
    ctx.fill();
    ctx.closePath();
}

//Draw
function draw() {
    // clear the canvas, so a new ball can be drawn
    ctx.clearRect(0, 0, canvas.width, canvas.height);

   // draw ball with potentially new  x and y coordinates
    drawBall();

    // pass the draw function as an argument
    requestAnimationFrame(draw);
}

draw();

I haven't tried to make it bounce yet; I'm not sure what you are planning for the implementation (e.g. if a user clicks the ball, it should bounce up and down x times, or bounce across the screen, etc.).

I hope this helps.

Best,

Clayton

Konrad Pilch
Konrad Pilch
2,435 Points

Thank you :) One question, how does this work?

document.addEventListener('keydown', function(event){
  switch (event.keyCode) {
Konrad Pilch
Konrad Pilch
2,435 Points

And what about this technique? Im trying to learn OOP by making a mario like game : p

http://nokarma.org/2011/02/27/javascript-game-development-keyboard-input/

Clayton Perszyk
Clayton Perszyk
Treehouse Moderator 48,565 Points

The callback function, which is the second argument passed to the addEventListener method, receives an object that corresponds to the event, and contains information relevant to that event. You can call it whatever you want but event makes the most sense. This object contains several properties, among them the keyCode property, which is used in this case to decide which switch statement to run. If you want to inspect it, you can put console.log(event) in the code and when run, open up the developer console in the browser and see what the event object contains.

Here are some helpful resources:

Clayton Perszyk
Clayton Perszyk
Treehouse Moderator 48,565 Points

As far as oop in javascript goes, here are some resources Iv'e found helpful:

I would suggest taking the code you have right now and apply oop principles to it (i.e. make a Ball constructor, etc.). Then you can take what you've learned and apply it a mario style game.

simhub
simhub
26,543 Points

hi Konrad,
here a little (very simple!) bounce function to keep the ball inside the canvas

    //Bounce
         function bounce () {
            if ((x + ballRadius) > canvas.clientWidth) x -= 1;
            if ((x - ballRadius) < 0) x += 1;
            if ((y + ballRadius) > canvas.clientHeight) y -= 1;
            if ((y - ballRadius) < 0) y += 1;
        };
Konrad Pilch
Konrad Pilch
2,435 Points

thank you :D Do you know how could i add gravity to it so the ball starts on the canvas?

I need to select the bottom of the canvas, and then say to it that if it touches the ball, then there should be a collision that the ball will not be able to go throw it something liek atha?

Im trying to make a gravity, and then the movement, and now i was simply trying to figure out how to implement keys to a drawing canvas.

This stuff is reallly exciting.