QML 怎么在gridview中用Index定位? 怎么在代理中设置背景?

import QtQuick 1.0

Item {
    id: me
     100
    height: 300

    GridView {
        id: grid
        anchors.fill: parent

        model: 60
        cellHeight: 20
        cellWidth: 20

        property int colcount: Math.floor(width / cellWidth)

        delegate: Rectangle {
             grid.cellWidth
            height: grid.cellHeight
            color: {
                var rowIndex = Math.floor(index / grid.colcount);
                switch (rowIndex % 3) {
                case 0:
                    return "yellow";
                case 1:
                    return "blue";
                case 2:
                    return "grey";
                }
            }

            Text {
                anchors.centerIn: parent
                font.pixelSize: 15
                text: index
            }
        }
    }
}