Here's a outline of how to drag-draw a rectangle on canvas:
In mousedown:
- save the starting mouse position
- set a flag indicating the drag has begun
In mousemove:
- clear the canvas of the previous rectangle
- calculate the rectangle width/height based on the starting vs current mouse position
- draw a rectangle from the starting XY to the current mouse position
In mouseup:
- clear the dragging flag because the drag is over
Here's example code and a Demo: http://jsfiddle.net/m1erickson/6E2yd/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
#canvas{border:1px solid red;}
</style>
<script>
$(function(){
// get references to the canvas and context
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// style the context
ctx.strokeStyle = "blue";
ctx.lineWidth=3;
// calculate where the canvas is on the window
// (used to help calculate mouseX/mouseY)
var $canvas=$("#canvas");
var canvasOffset=$canvas.offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var scrollX=$canvas.scrollLeft();
var scrollY=$canvas.scrollTop();
// this flage is true when the user is dragging the mouse
var isDown=false;
// these vars will hold the starting mouse position
var startX;
var startY;
function handleMouseDown(e){
e.preventDefault();
e.stopPropagation();
// save the starting x/y of the rectangle
startX=parseInt(e.clientX-offsetX);
startY=parseInt(e.clientY-offsetY);
// set a flag indicating the drag has begun
isDown=true;
}
function handleMouseUp(e){
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown=false;
}
function handleMouseOut(e){
e.preventDefault();
e.stopPropagation();
// the drag is over, clear the dragging flag
isDown=false;
}
function handleMouseMove(e){
e.preventDefault();
e.stopPropagation();
// if we're not dragging, just return
if(!isDown){return;}
// get the current mouse position
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
// clear the canvas
ctx.clearRect(0,0,canvas.width,canvas.height);
// calculate the rectangle width/height based
// on starting vs current mouse position
var width=mouseX-startX;
var height=mouseY-startY;
// draw a new rect from the start position
// to the current mouse position
ctx.strokeRect(startX,startY,width,height);
}
// listen for mouse events
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
$("#canvas").mouseout(function(e){handleMouseOut(e);});
}); // end $(function(){});
</script>
</head>
<body>
<h4>Drag the mouse to create a rectangle</h4>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
- clear the