html5 canvas 画一个矩形，在矩形里添加文字。 文字超出矩形怎样自动换行？？？_html/css_WEB-ITnose

   var canvas = document.getElementById("demo")   var ctx = canvas.getContext('2d');   ctx.font = "32pt Arial";   ctx.beginPath();   ctx.lineWidth = "1";   ctx.strokeStyle = "blue";   ctx.moveTo(100, 100);   ctx.lineTo(100, 400);   ctx.lineTo(400, 400);   ctx.lineTo(400, 100);   ctx.closePath();   ctx.stroke();   ctx.strokeText("hello world hello world hello world hello world hello world", 120, 200);   ctx.strokeStyle = "red";   ctx.stroke();