javascript用jscex画圆

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JavaScript

在支持html5的浏览器中执行下面代码：

<!DOCTYPE HTML>
<html>
<body>

<canvas id="myCanvas" width="480" height="300" style="border:1px solid #c3c3c3;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript ">

    var c = document.getElementById("myCanvas");
    var cxt = c.getContext("2d");
    var x = 150;
    var y = 150;
    var r = 100;
    cxt.moveTo(x - r, y);
    for (var i = x - r; i < x + r + 1; i++) {
        var tempY = Math.pow(r * r - (x - i) * (x - i), 1 / 2);
        cxt.lineTo(i, y + tempY);
    }
    cxt.moveTo(x - r, y);
    for (var i = x - r; i < x + r + 1; i++) {
        var tempY = Math.pow(r * r - (x - i) * (x - i), 1 / 2);
        cxt.lineTo(i, y - tempY);
    }
      cxt.stroke();

</script>

</body>
</html>

在Canvas 里显示如下：

但是，我们明明是在画圆，怎么没有看到画圆的过程？javascript就是这样，解释完就画完了，而不会呈现解释的过程，这也是为什么javascript不用考虑多线程问题，仅仅UI线程。那么怎么看到画圆的过程？

jscex闪亮登场！

<!DOCTYPE HTML>
<html>
<body>
<canvas id="myCanvas" width="480" height="300" style="border:1px solid #c3c3c3;">
Your browser does not support the canvas element.
</canvas>
    <script language="javascript" type="text/javascript" src="lib/uglifyjs-parser.js"></script>
    <script language="javascript" type="text/javascript" src="src/jscex.js"></script>
    <script language="javascript" type="text/javascript" src="src/jscex.builderBase.js"></script>
    <script language="javascript" type="text/javascript" src="src/jscex.async.js"></script>
<script type="text/javascript">
    var c = document.getElementById("myCanvas");
    var cxt = c.getContext("2d");
    var x = 150;
    var y = 150;
    var r = 100;
    var drawAsync = eval(Jscex.compile("async", function () {
        cxt.moveTo(x - r, y);
        for (var i = x - r; i < x + r + 1; i++) {
            $await(Jscex.Async.sleep(10));
            var tempY = Math.pow(r * r - (x - i) * (x - i), 1 / 2);
            cxt.lineTo(i, y + tempY);
            cxt.stroke();
        }
        cxt.moveTo(x - r, y);
        for (var i = x - r; i < x + r + 1; i++) {
            $await(Jscex.Async.sleep(10));
            var tempY = Math.pow(r * r - (x - i) * (x - i), 1 / 2);
            cxt.lineTo(i, y - tempY);
            cxt.stroke();
        }

    }));
    drawAsync().start();
</script>

</body>
</html>