Transcript Lecture 2 PPT - ScriptedPixels

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/javas
cript-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/pu
tty/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=Pm6Ch4qoNe
8&feature=relmfu