collectionView高度无效

Question

我对collectionView有一些奇怪的问题。

首先，我有一个UIView子类，里面有collectionView。看起来像这样

class UsersListMenu:UIView, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout{

lazy var collectionView:UICollectionView = {
    var layout = UICollectionViewFlowLayout()
    layout.scrollDirection = .Horizontal
    var collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.delegate = self
    collectionView.dataSource = self
    collectionView.translatesAutoresizingMaskIntoConstraints = false
    return collectionView
}()

let cellId = "cellId"
let menuArrayItems = ["Подходящие", "Онлайн", "Новые", "Рядом", "Избранные"]

override init(frame: CGRect) {
    super.init(frame: frame)

    addSubview(collectionView)

    collectionView.leadingAnchor.constraintEqualToAnchor(leadingAnchor).active = true
    collectionView.trailingAnchor.constraintEqualToAnchor(trailingAnchor).active = true
    collectionView.topAnchor.constraintEqualToAnchor(topAnchor).active = true
    collectionView.bottomAnchor.constraintEqualToAnchor(bottomAnchor).active = true

    collectionView.registerClass(UsersListMenuCell.self, forCellWithReuseIdentifier: cellId)
    collectionView.backgroundColor = .whiteColor()
}

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellId, forIndexPath: indexPath) as! UsersListMenuCell
    cell.label.text = menuArrayItems[indexPath.item]
    return cell
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 5
}

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    return CGSizeMake(100, 40)
}

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets {
    return UIEdgeInsetsZero
}

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAtIndex section: Int) -> CGFloat {
    return 0
}

}

如您所见，单元格高度设置为40.然后我在ViewController中创建该视图的实例

class UsersListViewController: UIViewController {

var menu:UsersListMenu = {
    var menu = UsersListMenu()
    menu.translatesAutoresizingMaskIntoConstraints = false
    return menu
}()

override func viewDidLoad() {
    super.viewDidLoad()
    view.addSubview(menu)

    menu.topAnchor.constraintEqualToAnchor(view.topAnchor, constant: 80).active = true
    menu.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor).active = true
    menu.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor).active = true
    menu.heightAnchor.constraintEqualToConstant(40).active = true
}
}

菜单看起来空白的问题 - 细胞无法看到

除非我将视图控制器中的菜单高度设置为100

menu.heightAnchor.constraintEqualToConstant(100).active = true

然后看起来像这样

这里发生了什么？