Lecture 2 – More Canvas with input and JavaScript + WebSockets and node.js

Written by Matthew Shelley for Professor Wei Shi Lecture 2 – More Canvas with input and JavaScript + WebSockets and node.js

More Canvas with Input

In This Example... • User Input • Keyboard Events • Mouse (click) events for canvas • Touch events on mobile devices are also included • Game Objects • Represented by coloured rectangles, which can be clicked • Canvas • Inheritance • Render game objects to canvas (only once) • Click game objects through canvas based on z-index

Overview of Files • userinput_canvas.htm • Very basic page, which serves to include the necessary JavaScript, CSS, and library files • userinput_canvas.css • Disables selecting and dragging of the canvas and any images, due to undesirable behaviour • userinput_canvas.js • Creates canvas, draws to it, and handles clicking of game objects within

Structure of .js File • Constants • A few constants are stored atop the .js file • UserInputAndCanvasExample • Namespace for the entire demo, which exposes a single public method, init() • $(document).ready(function() {...}); • Called when the document has finished loading

UserInputAndCanvasExample • m_keydownFunc [variable / function] • Takes in a ‘key code’ for the key just pressed • BaseCanvas [class] • Basic canvas object, which references a jQuery canvas element to respond to clicks and touches • GameObject [class] • A simple object, which can be rendered and clicked • GameWorld [class] • Extends BaseCanvas to manage and render objects • m_gameWorld [variable / object] • Reference to a GameWorld object

User Input • m_keydownFunc(e) • Assigned to the document body through init() • Uses a switch statement to handle each key press • e.which returns the key code • For key values, see: • http://www.cambiaresearch.com/articles/15/javascript-char-codes-key-codes

User Input • Each canvas object has a virtual function to handle click and touch events: • canvasElementClickOrTouchCallback(e) • Assigned in the constructor for each canvas • To fetch the relative position of a click or touch within a canvas, pass along e to: • getClickOrTouchPosition(e)

Game Objects • Represent a coloured rectangle, which can be drawn and clicked on through the canvas • isPointInside(point) • Checks if a point exists within a given area • handleClick() [virtual] • Response for a click

Canvas • BaseCanvas • Creates a canvas element, which is wrapped by jQuery, and assigns the click callback • Assign ‘self’ reference to DOM element • Click method simply avoids ‘bubble up’ • Note: this method treats ‘this’ as the DOM element • Disables dragging and selecting through DOM methods; supposedly jQuery has problems • Does not have a render() method, as there is nothing for an empty canvas to render

Canvas • GameWorld • Extends BaseCanvas to handle GameObject creation, clicking, and drawing • When clicked, the object with the highest zIndex that contains the ‘target’ has its handleClick() method called • createGameObject • Creates a game object and then stores it in an array based on its zIndex • renderOneFrame • Draws all game objects exactly once

Useful Links • Most of the code from the example: • http://www.ibm.com/developerworks/web/library/wa-games/index.html • jQuery: • http://jquery.com/ • Class: • http://ejohn.org/blog/simple-javascript-inheritance/ • jCanvas: • http://calebevans.me/projects/jcanvas/index.php

More JavaScript

JavaScript Object Notation • Variables in JavaScript are dynamically-typed • A variable can be a string, then an integer, then an array, then a function, then an object, etc. • Objects are also dynamic, as new properties can be added or deleted as necessary • Properties are just variables, too! Similarly, they are dynamic and can be objects as well. • Objects can be seen as ‘associative arrays’ or ‘dictionaries’; they assign values to properties

JavaScript Object Notation • An ‘empty’ object can be created like so: • var x = {}; // an empty associative array • An object can have some initial properties: • var x ={a: 1, b: 2}; • var y = {‘a’: 29, “b”: 3}; • var z = {a: 18, “b” 33}; • Any of these formats is valid • Any string can be used with no limit on length

JavaScript Object Notation • To read a property: • x.a • x[‘a’] • x[“a”] • To set a property: • x.a = 7; • x[‘a’] = 17; • x[“a”] = 81;

JavaScript Object Notation • To create a property: • Simply set its value as though it already existed • To delete a property: • delete x.a; • delete x[‘b’]; • delete x[“c”] • To check if a property exists: • (a in x) OR (‘b’ in y) OR (“c” in z) • You cannot do “not in” or “!in” or some variant • x.hasOwnProperty(‘a’) OR y.hasOwnProperty(“b”) • You can do !x.hasOwnProperty(...)

JavaScript Object Notation • To traverse each property: for (key in obj) // this is a for-each loop { if (obj.hasOwnProperty(key)) obj[key].doSomething(); } • No assumptions can be made on key ordering • We refer to this entire notation as JSON

Common jQuery • jQuery is an additional library for JavaScript that is commonly used in client-side applications • The primary $(...) method is complex • As per the name jQuery, it queries the document for DOM elements, which meet certain criteria, and then returns their ‘jQuery-wrapped’ versions • $(“body”) returns the body element, for instance • $(“img”) returns every img tag • jQuery simplifies the majority of traditional DOM-manipulation methods

Common jQuery • To create an element: • jqElement = $(document.createElement("canvas")) ; • jqElement = $(“<canvas>”); // create raw HTML • jqElement = $(“<canvas></canvas>”); • To add an element to another: • jqElement.append(objectToAdd); • $(“body”).append(“<canvas>”); // append raw HTML • To remove an element: • jqElement.remove(); // also removes any children

Common jQuery • To set an attribute of a jQuery element: • jqElement.attr(“attr”, “value”); • Note that if the selector retrieves more than 1 element all of those element will be updated! • $(“img”).attr(“width”, 320); // all image widths change • To append callbacks: • jqElement.on(“click”, someFunc); • jqElement.click(function() {...});

BREAK

WebSockets

WebSockets • WebSockets work similar to sockets, but instead they specifically communicate between a web browser and a web server • Messages are sent and received between the two • The protocols ws:// and wss:// are built atop of http:// and https://, respectively • Ports 80 and 443 are similarly used • WebSockets require both server and client support • So, you might need a virtual private server

WebSockets – Echo Example • Launch the websocket-echo.htm example • A connection is made with the server ws://echo.websocket.org/ • A message is sent to the server • The same message is received • Both messages are displayed for comparison • Afterward, the client disconnects

Useful Links • About WebSockets: • http://www.websocket.org/aboutwebsocket.html • Websocket Echo Example: • http://www.websocket.org/echo.html

Node.js

Software Installation • First, please download and install node.js • http://nodejs.org/ • Click ‘Install’ or go to the ‘Downloads’ page • Once downloaded, run the installer • Second, please download PuTTY • http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

Introduction to Node.js “Node.js is a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices.” - nodejs.org

Using Node.js • With command prompt, navigate to the folder containing your code • Use the cd command on Windows • To run a file named ‘example.js’: • node example.js • Server-side JavaScript code • We are not writing code for a web browser, but rather code that will be handled by the server

Node.js – Example 1 • The first example displays “hello,” and then “world” 2 seconds later to command prompt

Node.js – Example 2 • We create an http server, which listens to requests on port 8000 and responds to all incoming connections similar to before • First, run the example via Command Prompt • Then, connect to localhost:8000 via PuTTY • “Hello” followed by “world” 2 seconds later should appear in PuTTY’s console

Node.js – Example 3 • This example creates a TCP ‘echo’ server that simply sends back each message that the server receives • Like before, load the example in command prompt and then connect to localhost:8000 through PuTTY • Type a message in PuTTY to see it returned

Node.js – Example 4 • The final example creates a simple chat room where multiple users can write and messages, which are then broadcast to everyone else • This example demonstrates how to • store multiple connections; • filter out who receives what messages; and, • handle clients that disconnect • Multiple PuTTY clients are necessary

Socket.IO • Socket.IO builds on top of node.js to improve network connectivity on many devices, while providing some ‘interface’ improvements • http://socket.io/ • To install socket.io, use command prompt (assuming node.js has been installed): • npm install socket.io

Socket.IO - Example • Run “node socketio-example.js” via command prompt, like usual • Launch “socketio-example.htm” in a web browser, preferably Google Chrome • Using developer tools, it is possible to see messages logged to the console • These messages contain messages received by the client that were sent by the server

Disclaimer • Unless you have a Virtual Private Server or a host that supports websockets, node.js or some variant, you will not be able to use any of these services outside of your computer • As an alternative, one can use AJAX to call server-side code, such as PHP • We will look into AJAX in the next lecture

Useful Links • node.js • http://www.nodejs.org/ • “Introduction to Node.js with Ryan Dahl” • http://www.youtube.com/watch?v=jo_B4LTHi3I • “How do I get started with node.js?” • http://stackoverflow.com/questions/2353818/how-do-i-get-started-with-node-js • “Advanced HTML5 JavaScript: Down 'n Dirty” • http://www.youtube.com/watch?v=Pm6Ch4qoNe8&feature=relmfu

Lecture 2 – More Canvas with input and JavaScript + WebSockets and node.js