(function ($) {
    $.extend({
        timer: function (action,context,time) {
            var _timer;
            if ($.isFunction(action)) {
                (function () {
                    _timer = setInterval(function () {
                        if (!action(context)) {
                            clearInterval(_timer);
                        }
                    }, time);
                })();
            }
        }
    });
})(jQuery);

   
    画布
   
   
   
        #wrap
        {
            display: table;
            margin: 0 auto;
        }

        #cvs
        {
            display: table-cell;
            vertical-align: middle;
        }
   
   
        function drawRound(context) {           
            if (context.counterclockwise) {
                draw(context.x, context.y, context.r, context.start, context.start - Math.PI / 50, context.counterclockwise);
                context.start -= Math.PI / 50;
                return context.start > 0.5 * Math.PI;
            }
            else {
                draw(context.x, context.y, context.r, context.start, context.start + Math.PI / 50, context.counterclockwise);
                context.start += Math.PI / 50;
                return context.start < Math.PI;
            }
        }
        function draw(x, y, r, sAngle, eAngle, counterclockwise) {
            var cvs = document.getElementById("cvs");
            ctx = cvs.getContext("2d");
            ctx.strokeStyle = "#f00";
            ctx.beginPath();
            ctx.arc(x, y, r, sAngle, eAngle, counterclockwise);
            ctx.stroke();
        }
        $(function () {
            $.timer(drawRound, { x: 100, y: 100, r: 50, start: 1.5 * Math.PI, counterclockwise: true }, 200);
            $.timer(drawRound, { x: 100, y: 100, r: 60, start: 0, counterclockwise: false }, 200);
        });