Kiss My App: Collaborating using HTML5 Canvas and WebSockets

The Canvas element introduced in HTML5 is the most talked about feature in HTML5. It allows a developer to draw on a rectangular area and the ability to control each pixel in it. I'm not a very big fan of powerful graphics and animation on the web, however I wanted to try Canvas in conjunction with another popular feature - Web Sockets. The idea is to use the mouse events to draw on the Canvas and then send the coordinates to remote clients using Web Sockets. I have used the pusherapp Web Socket library in my example and this video shows how two clients can play a Tic-Tac-Toe game:




As you can see in this video, the mouse movements made on the canvas will draw lines on it and also cause lines to be drawn on another browser window.



HOW?:
There are various event handlers that can be used with the Canvas element. The one that I have used in this application are the mousedown and the mousemove events. When the user clicks on the Canvas the mousedown event is invoked. The point at the which the mouse was clicked will be marked as the starting point for the line. Now, when one moves the mouse, the mousemove event is invoked. The movement of mouse will then be used to draw the line on the canvas. The next mouse click is then used to mark the end of drawing and any mouse movement will not cause lines to be drawn on the Canvas. This is particularly useful when you want to draw multiple shapes or letters on the Canvas. To start drawing again click anywhere on the Canvas to mark the start point.

In both the cases i.e. on mouseclick and mousemove events, the X and Y coordinates along with the event type is sent over the Web Socket. On receiving the message on the Web Socket, the corresponding action is performed.

Code:

      var canvas, ctx, socket, clicked = true;  
     var oldX = 0, oldY = 0;  
      socket = new Pusher('{api_key}', 'test_channel');  
     /*  
      * Initialize the Canvas   
      * set the line width  
      * and add event listeners - mousedown and mousemove   
      */  
     $(document).ready(function(){  
         canvas = document.getElementById('myCanvas');  
         ctx = canvas.getContext('2d');  
         ctx.lineWidth = "6";  
          canvas.addEventListener('mousedown', drawOnCanvas, false);  
         canvas.addEventListener('mousemove', drawOnCanvas, false);  
     });  
     /*  
      * Function to be invoked when a mousedown or a mousemove event is received  
      */  
     function drawOnCanvas(evt){  
     /*  
      * Post a HTTP request to the server on mousedown and mousemove events.  
      * Post a HTTP request for mousemove only when the mouse is clicked.  
      */  
     if (evt.type == 'mousedown' || !(clicked)) {  
           $.post('send.cfm', {  
                x: evt.x,  
                y: evt.y,  
                eventType: evt.type  
           });  
        }  
     }  
     /*  
      * Bind the socket to an event  
      */  
     socket.bind('test_event', function(data){  
          if (data.eventType == 'mousedown') {  
          /*  
           * Change the start position to the coordinate where the mouse was clicked  
           * Set clicked to false, so that the next mousemove event can send the request to Pusher. See below.  
           */  
          if(clicked) {  
                ctx.moveTo(oldX, oldY);  
                clicked = false;  
          }  
          /*  
           * On second click, stop drawing any more lines on the Canvas until the next mousedown event is received.  
           * This is used to mark an end of a character or a diagram on the Canvas. Allows user to draw multiple characters or diagrams  
           */  
           else {  
                clicked = true;  
           }  
      }  
      /*  
       * In case of a mousemove event draw the line  
       */  
      else if(!clicked){  
           ctx.beginPath();  
           ctx.moveTo(oldX, oldY);  
           ctx.lineTo(data.x, data.y);  
           ctx.stroke();  
           clicked=false;  
      }  
      /*  
       * Update the next start position  
       */  
      oldX = data.x;  
      oldY = data.y;          
 });  
 

The performance of this application depends on the network speed. If you do see the application running slow, you can change the behavior such that the line is drawn if the difference between the new and the old coordinates is greater than some value.

Download code - http://goo.gl/tDDkY