Blog about App development for iOS

Back before iOS 14 if you want to dequeue a cell for reuse you needed to register it first. It was a tedious two-step process that we all are used to.


collectionView.register(Cell.self, forCellWithReuseIdentifier: "Foo")

And dequeue

collectionView.dequeueReusableCell(withReuseIdentifier: "Foo", for: indexPath) as! Cell

iOS 14 brings us a very welcomed cell registration through an iOS 14 only UICollectionView extension:

public struct CellRegistration<Cell, Item> where Cell : UICollectionViewCell

If we are to make use of this new feature, we will need a data source. For our example, we will use UICollectionViewDiffableDataSource available since iOS 13.

iOS 14 gives us CellRegistration, a new mechanism to dequeue a reusable UICollectionViewCell.

We will need three elements when calling CellRegistration’s handle: The UICollectionView where we render our cells. The Position of the cell: our IndexPath An Item, which is our model with all the information the cell will render: ItemIdentifierType. Since we are using a diffable data source, our model must adopt Hashable.

UICollectionViewDiffableDataSource has as parameter a CellProvider, which is a closure that builds a UICollectionViewCell and has as all the elements needed to build our collection view an IndexPath and our Item, or model

UICollectionView, IndexPath and ItemIdentifierType (our model)

More on how to build it below.

As for UICollectionView, we have now dequeueConfiguredReusableCell, which builds a UICollectionViewCell.

dequeueConfiguredReusableCell is special. It requires an IndexPath, an Item (our model), and a CellRegistration. CellRegistration is a struct which knows how to build and configure a UICollectionViewCell. It is initialized with a registration handler. This registration handler is a closure that accepts three parameters: Cell, IndexPath, and Item. The cell must be a UICollectionViewCell and Item is our Model.

CellRegistration can also be built with a UINib. In that case, the first parameter if its constructor is the nib, and the second one, the closure that configures our cell.

let cellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, MyAwesomeModel> 
{ (cell, indexPath, model) in
	var content = cell.defaultContentConfiguration()
	content.text = model.awesomeText
	cell.contentConfiguration = content

This CellRegistration's configuration handler is to be called whenever we want to dequeue our reusable cell.

dataSource = UICollectionViewDiffableDataSource<Section, Model>(collectionView: collectionView) 
{ (collectionView: UICollectionView, indexPath: IndexPath, item: Model) -> UICollectionViewCell? in
	collectionView.dequeueConfiguredReusableCell(using: cellRegistration, for: indexPath, item: item)

Easy isn't it?

So what happens is that our data source dequeues through dequeueConfiguredReusableCell a cell, passing a CellRegistration, an IndexPath, and an Item (our model) and CellRegistration calls its closure, which configures the UICollectionViewCell according to our model and the index path.

But why is this better?

Traditionally we always dequeue reusable cells with an identifier

collectionView.dequeueReusableCell(withReuseIdentifier: "MyCell", for: indexPath)

which we need to register before

swift collectionView.register(MyCell.self, forCellWithReuseIdentifier: "MyCell")

Now our data source (diffable or not) just needs to call collectionView.dequeueConfiguredReusableCell and that takes care of it all... Provided you have a CellRegistration, which is the element that binds everything together.

The separation is clear, and it is now possible to have a central place where we define how our cells are being set up.

But there is even more. We can apply all this to Supplementary Views such as headers or footers. Apple's WWDC sample code has even examples for badges that are rendered on top of our cells. For this we have a new UICollectionView function called dequeueConfiguredReusableSupplementary that behaves like dequeueConfiguredReusableCell but instead of returning a UICollectionViewCell returns a UICollectionReusableView

Published on August 1, 2020

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