canvas文字超过宽自动换行

绘制自动换行的字符串

    drawText(ctx, txtwid, t, x, y, w){
      var chr = t.split("");
      var temp = "";				
      var row = [];
      ctx.font = "20px Arial";
      ctx.fillStyle = "black";
      ctx.textBaseline = "middle";
      for(var a = 0; a < chr.length; a++){
        if( ctx.measureText(temp).width < w ){
          ;
        }
        else{
          row.push(temp);
          temp = "";
        }
        temp += chr[a];
      }
      row.push(temp);
      for(var b = 0; b < row.length; b++){
        ctx.setFontSize(txtwid);
        ctx.setFillStyle("#333333");
        ctx.fillText(row[b],x,y+(b+1)*20);
      }
    },