Pong Game

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Revision as of 04:20, 13 November 2013 by Shane (talk | contribs) (Creating from content formerly in Module 7)
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The Pong Game Interface

For the remainder of this module, your work will be in three files:

  • app.js (or whatever you called your Node.JS file)
  • client.html
  • client.js

What you need to do is to build a system that enables two players who are both viewing client.html to choose each other (e.g., from a lobby) and play a game of Pong.

You should be wondering, "how do I interact with the Pong game?" The public interface is as follows:

  • Methods:
    • pong.init() - initialize the game client.
    • pong.resetBall(x, y) - arbitrarily set the position of the ball. Defaults to the center. Cancels all animations currently acting on the ball.
    • pong.launch(angle, direction) - launch the ball from its current position in the specified angle (-89 to +89) and direction (-1 for left or 1 for right).
    • pong.updateOpponentPaddle(position) - set the position of the opponent paddle to position. (Hint: the position value should have been generated by the opponent's paddlemove event)
    • pong.ready() - returns true if the game is not in a "paused" state.
    • pong.setScore(score) - display score on the game canvas. score is an object literal containing two fields: left and right.
  • Events (all fired on window):
    • paddlemove - the player's paddle has moved.
      This event will fire only after the paddle has moved 15 pixels or more.
      • event.detail.position - the new y coordinate of the paddle.
    • paddlehit-left - the ball has hit the left side of the screen.
      • event.detail.hit - true if the ball hit the player's paddle; false otherwise.
      • event.detail.angle - if the ball was reflected, the new angle of the ball's rightward movement.
      • event.detail.position - the position at which the ball hit the left side

Problems with Our Interface

There are at least two major problematic issues with our interface:

  • We are firing events on the window. We should have taken the time to make our Pong class implement the EventTarget interface directly.
  • We are putting more trust in the client than we should, because the client can fake any angle and position upon the paddlehit-left event and manipulate the game in his or her favor. The trajectory should be computed on the Node.JS side.

Just be aware of the above issues as you complete this lab. You won't be required to fix them, although if you do (and if you recompile your modified code with Lime) you may be a candidate for extra credit.

Enabling a Player to Pause

You do not need to implement the pausing part of your Pong game for the purposes of Module 7; it is here for reference.

Implementing a game pause is more difficult for a network game than it is for a game that runs on just one computer. There is one situation during a Pong game when it makes sense to enable a user to pause the game, though: after a user has lost a round, but before they have fired the next ball at the opponent. At this time, you may display the play/pause button like so:


When your timeout for firing the new ball is executed, test whether or not the canvas is ready. If it is, fire the ball and hide the play/pause button; otherwise, continue waiting:

var launchNewBall = function(angle, direction){
	if (pong.ready()) {
		// Hide the pause/resume button
		// Launch the ball on our local canvas
		pong.launch(angle, direction);
		// Tell our opponent the angle and direction of the new launch (this probably won't be exactly the same with your implementation)
		socket.emit("launch", {
			angle: angle,
			direction: direction
		// Game is paused; keep waiting
		setTimeout(launchNewBall, 500, angle, direction);

Example: Starting a Pong Game

Once two players have formed a pair, one of them needs to initialize the game with a certain angle and direction for the ball. Here is how you might go about doing this:

// generate an initial angle between -60 degrees and +60 degrees:
var initAngle = -60 + 120*Math.random();

// randomly choose the initial direction of the ball:
var initDirection = Math.random() < 0.5 ? -1 : 1;

// show the game canvas:

// initialize the game canvas:

// move the ball to the center:

// set the ball into motion:
pong.launch(initAngle, initDirection);

// tell the opponent about the ball's initial angle and direction.  For example:
socket.emit("launch", {
	angle: initAngle,
	direction: initDirection

Example: When a Player Reflects the Ball

When the ball hits a player's side, the paddlehit-left event fires on the window. Here is how you might listen for this event in your client.js and transmit this information over your socket:

window.addEventListener("paddlehit-left", function(e){
	// e.detail.hit will be true if the client hit the ball with his/her paddle.
	if (e.detail.hit) {
		console.log("HIT PADDLE.  New angle: %f", e.detail.angle); // log a message to the console
		// Tell our opponent via Socket.IO about the ball's new angle and position:
		socket.emit("reflect", {
			angle: e.detail.angle,
			position: e.detail.position
		// Note: The game automatically launches the ball and determines the angle based on the ball's position relative to the paddle position.  We therefore do not need to call pong.launch() in here (but our opponent will, as shown below).
		console.log("MISSED PADDLE");
		// in here, we will update the score, check for victory condition, launch a new ball (perhaps after a timeout), etc.

Here is what you might have in app.js inside of your Socket.IO callback function:

// Listen for the "reflect" message from the client
socket.on("reflect", function(data){
	// Tell our opponent about the reflection.
	// Note: In this sample implementation, we store the opponent in `socket.opponent`.
	// You might use a different mechanism for storing the opponent's socket.
	socket.opponent.emit("reflect", data);

Finally, here is what you might have on the opponent's end (hint: this is also in client.js):

// Listen for the "reflect" message from the server
socket.on("reflect", function(data){
	// In order to make up for any network lag, immediately move the ball to its correct position:
	pong.resetBall(960, data.position);
	// Finally, launch the ball on our local game client:
	pong.launch(data.angle, -1);