Testing a Custom UICollectionViewCompositionalLayout.list

In iOS 14 Apple added a collection view compositional layout that lets us create collection views that look and behave like UITableViews.
It is based on UICollectionViewCompositionalLayout and as a result allows to create very complex collection view layouts with only little code.

I believe that we should test our view controllers.
For example I often write tests that assure that setting up a table view cell works as intended.
An example for such a test looks like this:

func test_cellForRow_populatesCell() {
  // given
  sut.names = ["foo"]
  
  // when
  let indexPath = IndexPath(row: 0, section: 0)
  let cell = tableView.dataSource?.tableView(tableView, cellForRowAt: indexPath)
  
  // then
  XCTAssertEqual(cell?.textLabel?.text, sut.names.first)
}

(This code is taken from my book Test-Driven iOS Development - Gimme The Code).

Testing the population of a table view cell is quite easy.
We setup the data source, get the cell from it and assert that the relevant data is set to the label in the cell.

The list layout in UICollectionViewCompositionalLayout works quite differently.
I don't want to go into detail in this post how you can setup a collection view with the new list layout.
Let's safe that for a later post.
All you need to know is that you setup a UICellConfigurationState and tell the cell that it needs to update it's configuration.
The cell then calls the method updateConfiguration(using:) and you override that method to setup the cell.

You can see how this works in CustomCellListViewController.swift in the sample code provided by Apple here.

When I tried to write a test for this, I ran into the problem that updateConfiguration(using:) is called after my test is finished.
So it looks like to me that UIKit postpones the call onto the next run loop.
This means to be able to test the population of the collection view cell, I needed move the assertion to the next run loop as well.
The Apple engineer @_mochs answered to my posts, that it should be sufficient to call layoutIfNeeded on the cell to trigger populating the cell.

So, this works:

func test_cellForItem_returnsConfiguresCell() {
    let item = Item(category: .music, title: "Foo", description: "Bar")
    sut.items = [item]
    sut.loadViewIfNeeded()

    let indexPath = IndexPath(item: 0, section: 0)
    let cell = collectionView.dataSource?.collectionView(collectionView, cellForItemAt: indexPath) as! CustomListCell

    let listContentConfiguration = cell.listContentView.configuration as! UIListContentConfiguration
    XCTAssertEqual(listContentConfiguration.text, item.title)
}

By the way, this is a test for the sample code provided by Apple.
I've added tests to better understand what's going on.

Do you know a better way to test this?
How would you test that the collection view cell in the list layout is populated properly?

If you have questions or remarks about this post, please let me know on Twitter: @dasdom.

(First published on dasdom.github.io)