When you “move” an object, what you really need to do is erase the object and redraw it
First code a function that will redraw the rect at a specified x,y
function draw(x,y){
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.beginPath();
ctx.fillStyle="skyblue";
ctx.strokeStyle="gray";
ctx.rect(x,y,30,20);
ctx.fill();
ctx.stroke();
}
Then handle mousedown events and call the draw function
This example uses jquery for cross-browser compatibility, but you can always recode using native javascript.
// listen for all mousedown events on the canvas
$("#canvas").mousedown(function(e){handleMouseDown(e);});
// handle the mousedown event
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#downlog").html("Down: "+ mouseX + " / " + mouseY);
// Put your mousedown stuff here
draw(mouseX,mouseY);
}
Here is code and a Fiddle: http://jsfiddle.net/m1erickson/GHSG4/
<!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>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
function draw(x,y){
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.beginPath();
ctx.fillStyle="skyblue";
ctx.strokeStyle="gray";
ctx.rect(x,y,30,20);
ctx.fill();
ctx.stroke();
}
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
$("#downlog").html("Down: "+ mouseX + " / " + mouseY);
// Put your mousedown stuff here
draw(mouseX,mouseY);
}
$("#canvas").mousedown(function(e){handleMouseDown(e);});
// start the rect at [10,10]
draw(10,10);
}); // end $(function(){});
</script>
</head>
<body>
<p>Click to redraw the rect at the mouse position</p>
<p id="downlog">Down</p>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
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