我想做一个抽屉效果的demo，左视图是个scrollview，用了KGFloatingDrawer第三方控件实现了抽屉，然后在左视图的viewController中设置scrollview代码如下：

swiftimport UIKit

class LeftViewController: UIViewController {

    var leftScrollView:UIScrollView?
    override func viewDidLoad() {
        super.viewDidLoad()
        //set scrollview
        leftScrollView = UIScrollView()
        leftScrollView?.backgroundColor = UIColor.yellowColor()
        leftScrollView?.frame = self.view.frame
        leftScrollView?.frame.size.width = 280
        leftScrollView?.contentSize = CGSize(width: 280, height: 2000)
        leftScrollView?.indicatorStyle = UIScrollViewIndicatorStyle.White
        leftScrollView?.showsVerticalScrollIndicator = true
        leftScrollView?.scrollEnabled = true
        self.view.addSubview(leftScrollView!)

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}

但是不能滚动，没有滚动条，是为什么呢？
运行图如下