ios - UICollectionViewController中collectionView的内容为何存在初始偏移？如何消除？

今天在做UICollectionView的时候发现一个奇怪的问题。在初始化UICollectionViewController后，对其属性collectionView进行重新layout，但是content的起始位置和collectionView的顶部始终存在一个offset。

如图所示：

图中可以明显的看出这个初始存在的offset的height＝statusbarHeight + navigationbarHeight。可以理解的是如果不去对collectionView重新布局，那么这个content的起始位置正好在NavigationBar的下面，这是极其合理的。但是如果需要重新布局，那这个content的初始偏移就很讨厌了。我找了半天没有找到有这个配置的地方，故来此地求助。
（通过contentInset可以改变这个起始的offset，但还是想知道这个offset是在哪配置的）

附上代码：

class MyCollectionViewController: UICollectionViewController {   
    init() {
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = UICollectionViewScrollDirection.Vertical
        
        super.init(collectionViewLayout: layout)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = UIColor.whiteColor()
        self.collectionView!.registerClass(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
        self.collectionView!.delegate = self
        self.collectionView!.dataSource = self
        self.collectionView!.showsVerticalScrollIndicator = false
        
//        self.collectionView!.snp_makeConstraints { (make) in
//            make.bottom.equalTo(self.view).offset(-self.view.frame.height*0.1)
//            make.left.right.equalTo(self.view)
//            make.top.equalTo(self.view).offset(self.view.frame.height*0.2)
//        }
    }

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

    // MARK: UICollectionViewDataSource

    override func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
        // #warning Incomplete implementation, return the number of sections
        return 1
    }


    override func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        // #warning Incomplete implementation, return the number of items
        return 100
    }

    override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath)
        
        // Configure the cell
        let cellColor = UIColor(red: CGFloat(arc4random() % 100)/100, green: CGFloat(arc4random() % 100)/100, blue: CGFloat(arc4random() % 100)/100, alpha: 1.0)
        cell.backgroundColor = cellColor
        
        return cell
    }
}

extension MyCollectionViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAtIndex section: Int) -> CGFloat {
        return 5;
    }
    
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
        let sideLength = CGFloat((self.collectionView!.frame.width - 15) / 4)
        return CGSizeMake(sideLength, sideLength)
    }
        
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAtIndex section: Int) -> CGFloat {
        return 5.0
    }
}