拖动实现，即时累加轨迹两点距离

function DragBase() {
    var
            prevX, prevY,
            toX, toY;

    this.start = function (x, y) {
        prevX = x;
        prevY = y;
        toX = 0;
        toY = 0;
    };

    this.move = function (x, y, fn) {
        toX = x - prevX;
        toY = y - prevY;

        fn(toX, toY);

        prevX = x;
        prevY = y;
    };
}