ios - 从 collectionView 返回什么(_ :viewForSupplementaryElementOfKind:at:) when you want to return nothing?

Question

我有一个 UICollectionView，它有部分标题，但没有部分页脚。因此，我没有定义的页脚 View 。 Apple's documentation声明 您不得从此方法返回 nil。

func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
    switch kind {
    case UICollectionElementKindSectionHeader:
        let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind,
                                                                         withReuseIdentifier: "MyHeaderView",
                                                                         for: indexPath) as! MyHeaderView
        switch indexPath.section
        {
        case 0:
            headerView.label_title.text = "SOME HEADER"
        case 1:
            headerView.label_title.text = "ANOTHER HEADER"
        default:
            headerView.label_title.text = "UNKNOWN HEADER"
        }
        return headerView
    default:
        assert(false, "Unexpected element kind") // Not a good idea either
    }
    return nil // NOPE, not allowed
}

Answer 1

以上都不适合我。在我的例子中，我在同一个 View Controller 中有两个 UICollectionView 对象。首先是水平的，选择一个显示下面的 UICollectionView 的项目，它是垂直的并且包含部分标题。

来自 Apple docs :

This method must always return a valid view object. If you do not want a supplementary view in a particular case, your layout object should not create the attributes for that view. Alternatively, you can hide views by setting the isHidden property of the corresponding attributes to true or set the alpha property of the attributes to 0. To hide header and footer views in a flow layout, you can also set the width and height of those views to 0.

所以像往常一样将 headerView 出队，因为如果你不这样做，只返回一个 UICollectionReusableView() 的实例，Xcode 会提示标题 View 没有出队。然后，如果您出于某种原因不想显示它(对我来说，直到用户从上方的水平 Collection View 中选择一个项目)- 将 headerView 的宽度和高度设置为 0.0 并返回它。

let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: collectionHeaderReuseIdentifier, for: indexPath) 
if objects.isEmpty {
    headerView.frame.size.height = 0.0
    headerView.frame.size.width = 0.0
    return headerView
}
// Configure the header view here if needed
return headerView