Why Group use alway return 0 in the fist time run?

In the first time the project run, it alway return 0 for "used" variable; If i change the view, then come back first view, it will render and caculate used?

onBeforeCellRender: args => {
      if (args.cell.isParent) {
        var children = this.scheduler.control.rows.find(args.cell.resource).children();
        var total = children.length;
        var used = children.filter(function(row) { return !!row.events.forRange(args.cell.start, args.cell.end).length; } ).length;
        var available = total - used;
        var color = "white"
        var w = this.cellWidth / total;
        args.cell.properties.areas = [];
        args.cell.properties.areas.push({
          html: "" + used,
          // style: "text-align: center; font-size: 12px; font-weight: bold",
          top: 0,
          left: 0,
          right: 0,
          bottom: 0,
          backColor: color
        });
        
      }   
    },