invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:) isn't called on a custom NSCollectionViewLayout subclass

Question

Link to the relevant sample project

Description

I’m trying to create a custom NSCollectionViewLayout subclass that uses use Auto Layout to determine its items’ sizes. This layout positions items from top-to-bottom, where all items share the same width, but each item's height is deretmined by its content. Think of it as a UITableView layout for macOS.

Assumption

The initial layout attributes are calculated in the layout's prepare() method, and given to the collection view through layoutAttributesForElements(in:). These attributes are used by the collection view to determine which items will need to be displayed. These items are be provided by the collection view's delegate, and each item's apply(_:) method is called to to set up its view properties.

The collection view will then call the item's preferredLayoutAttributesFitting(_:) method to caclulate its content's fitting size. The returned attributes will then be passed into the collection view layout's shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:) method, where the layout determines whether or not it needs to make adjustments in response to the new attributes, and returns that boolean decision to the collection view.

If the layout decides it needs to be invalidated, the collection view will then call invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:) to get specific information from the layout for how it should update itself. When the collection view then calls invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:), the layout can apply those changes.

This process is then repeated until shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:) returns false for each layout item—prompting the collection view to display the items on the screen.

Premise

The ListLayout subclass of NSCollectionViewLayout is able to create an initial set of layout attributes for each NSCollectionViewItem with an "estimated" initial height of 10 points. We are expecting to use Auto Layout to determine each item's actual height, so this initial value shouldn't matter.

The layout is then able to provide the correct set of those initial attributes through layoutAttributesForElements(in:).

The TextFieldItem subclass of NSCollectionViewItem contains a single NSTextField instance, which is set to both its view and textField properties. This view has its translatesAutoresizingMaskIntoConstraints property set to false, and has a required vertical content compression resistance priority. The item's apply(_:) method is overriden to set the text field's preferredMaxLayoutWidth to the width provided by the given attributes. The item's preferredLayoutAttributesFitting(_:) method is overriden to set the preferred attribute's height to equal the text field's intrinsicContentSize property.

The collection view is provided instances of TextFieldItem populated with multiple lines of text.

It is expected that each TextFieldItem will have its preferredLayoutAttributesFitting(_:) method called after its apply(_:) method, followed by a call to both the layout's shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:) and invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:) methods.

However, neither preferredLayoutAttributesFitting(_:), nor shouldInvalidateLayout(forPreferredLayoutAttributes:withOriginalAttributes:), nor invalidationContext(forPreferredLayoutAttributes:withOriginalAttributes:) are ever called when the playground is run. As a result, each item's height remains at that initial "estimated" height.

Question

There must be something I’m missing about NSCollectionViewLayout and Auto Layout, but I haven't found anything in the documentation to indicate that.

I've tried manually calling these methods at various different points in the collection view's lifecycle, but nothing has been able to correctly trigger the layout to adjust its caclulated attributes.

Is there a property that needs to be set on the collection view, its layout, or the items to indicate that preferredLayoutAttributesFitting(_:) should be called? Is Auto Layout only supported for subclasses of NSCollectionViewFlowLayout and NSCollectionViewGridLayout? Or am I misunderstanding the lifecycle of an NSCollectionViewLayout instance?

Answer 1

I have the same problem, this is my solution, it works perfectly. Add the code below to the subclass of UICollectionViewCell.

override func awakeFromNib() {

    super.awakeFromNib()

    self.contentView.translatesAutoresizingMaskIntoConstraints = false

    let leftConstraint = contentView.leftAnchor.constraint(equalTo: leftAnchor)
    let rightConstraint = contentView.rightAnchor.constraint(equalTo: rightAnchor)
    let topConstraint = contentView.topAnchor.constraint(equalTo: topAnchor)
    let bottomConstraint = contentView.bottomAnchor.constraint(equalTo: bottomAnchor)
    NSLayoutConstraint.activate([leftConstraint, rightConstraint, topConstraint, bottomConstraint])
}