canvas绘制虚线

var canvas = document.getElementByIdx_x_x_x("canvas"),
ctx = canvas.getContext("2d");
function drawDashLine(ctx, x1, y1, x2, y2, dashLength){
var dashLen = dashLength === undefined ? 5 : dashLength,
xpos = x2 - x1, //得到横向的宽度;
ypos = y2 - y1, //得到纵向的高度;
numDashes = Math.floor(Math.sqrt(xpos * xpos + ypos * ypos) / dashLen);
//利用正切获取斜边的长度除以虚线长度，得到要分为多少段;
for(var i=0; i小于numDashes; i++){
if(i % 2 === 0){
ctx.moveTo(x1 + (xpos/numDashes) * i, y1 + (ypos/numDashes) * i);
//有了横向宽度和多少段，得出每一段是多长，起点 + 每段长度 * i = 要绘制的起点；
}else{
ctx.lineTo(x1 + (xpos/numDashes) * i, y1 + (ypos/numDashes) * i);
}
}
ctx.stroke();
}
ctx.lineWidth = 2;
ctx.strokeStyle = "#0000ff";
drawDashLine(ctx, 0, 0, 400, 400, 8)