UICollectionView无法滚动

Question

我在视图中添加了UICollectionView，而不是滚动。

private func addFriendCollectionView() {
    let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsets(top: 20, left: 10, bottom: 10, right: 10)
    layout.itemSize = CGSize(width: 90, height: 50)

    let positionRectangle = CGRect(x: 0, y: 100, width: (mainContainerView?.frame.width)!, height: 100)

    friendsCollection = UICollectionView(frame: positionRectangle, collectionViewLayout: layout)
    friendsCollection?.registerClass(FriendCollectionViewCell.self, forCellWithReuseIdentifier: "friendCell")
    friendsCollection?.backgroundColor = UIColor.whiteColor()

    friendsCollection?.dataSource = self
    friendsCollection?.delegate = self

    self.mainContainerView?.addSubview(friendsCollection!)
}

但如果我改变

private func addFriendCollectionView() {
    let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsets(top: 20, left: 10, bottom: 10, right: 10)
    layout.itemSize = CGSize(width: 90, height: 50)

    let positionRectangle = CGRect(x: 0, y: 100, width: (mainContainerView?.frame.width)!, height: 100)

    friendsCollection = UICollectionView(frame: positionRectangle, collectionViewLayout: layout)
    friendsCollection?.registerClass(FriendCollectionViewCell.self, forCellWithReuseIdentifier: "friendCell")
    friendsCollection?.backgroundColor = UIColor.whiteColor()

    friendsCollection?.dataSource = self
    friendsCollection?.delegate = self
// Add collection view to root view 
    self.view.addSubview(friendsCollection!)
}

一切正常。

如何向UICollectionView添加滚动？按照代码显示如何容器视图

private func addBackGroundView() {
    let imageRect = CGRect(x: 5, y: 70, width: self.view.frame.width - 10, height: self.view.frame.height - 75)
    backGroundImage = InviteFriendsBackGroundImageView(frame: imageRect)
    backGroundImage!.image = InviteFriendsData.sharedInstance.image

    self.view.addSubview(backGroundImage!)
}

private func addMainContainerView() {
    let mainContainerPosition = CGRect(x: 0, y: 50, width: self.view.frame.width - 10, height: self.view.frame.height - 80)

    mainContainerView = UIView(frame: mainContainerPosition)
    mainContainerView?.backgroundColor = UIColor.whiteColor()

    backGroundImage?.addSubview(mainContainerView!)
}