Table views, cell reuse & diffable data source

How it works

• UITableViewDataSource (what to show) — required: tableView(_:numberOfRowsInSection:), tableView(_:cellForRowAt:); optional numberOfSections(in:). UITableViewDelegate (behaviour) — didSelectRowAt, heightForRowAt, willDisplay. Both held weak.
• Reuse: register(_:forCellReuseIdentifier:) once, then dequeueReusableCell(withIdentifier:for:) — always returns a cell (crashes if unregistered). Old dequeueReusableCell(withIdentifier:) may return nil. A cell leaving the screen goes to a per-identifier reuse queue; prepareForReuse() runs before it comes back.
• Stale content = a recycled cell still shows the last row’s image/check/colour, because cellForRowAt set it only on some paths, or an async image load finished after reuse. Fix: set every property on every path; in prepareForReuse reset transient state + cancel the load; on completion check the cell still shows the same ID.
• Manual updates: change the model first, then performBatchUpdates with insertRows/deleteRows. Counts must add up or NSInternalInconsistencyException (“invalid number of rows”).
• UICollectionView = same data source/delegate/reuse, plus a layout object: UICollectionViewFlowLayout or UICollectionViewCompositionalLayout (item → group → section); lists via UICollectionLayoutListConfiguration (iOS 14).
• Diffable (iOS 13): UITableViewDiffableDataSource / UICollectionViewDiffableDataSource<SectionID, ItemID>; both generic params Hashable; created with a cellProvider closure. NSDiffableDataSourceSnapshot is a struct: appendSections, appendItems(_:toSection:), deleteItems, moveItem(_:beforeItem:), then apply(_:animatingDifferences:). Edit the live one via dataSource.snapshot(). Map positions with itemIdentifier(for:) / indexPath(for:).
• Same ID, new content: reconfigureItems (iOS 15) re-runs the config on the existing cell — cheap, keeps state. reloadItems (iOS 13) throws the cell away and dequeues a new one. Prefer reconfigure unless the cell type changes.
• CellRegistration (iOS 14): typed cell + config closure, no string IDs; dequeueConfiguredReusableCell(using:for:item:). Headers need supplementaryViewProvider (collection) or a subclass overriding titleForHeaderInSection (table).

Example

enum Section: Hashable { case main }
let reg = UICollectionView.CellRegistration
    <UICollectionViewListCell, Todo.ID> { cell, _, id in
  var c = cell.defaultContentConfiguration()
  c.text = store[id]?.title        // read LIVE data by id
  cell.contentConfiguration = c    // set EVERY property
}
ds = UICollectionViewDiffableDataSource<Section, Todo.ID>(
  collectionView: cv) { cv, ip, id in
  cv.dequeueConfiguredReusableCell(using: reg, for: ip, item: id) }
var snap = NSDiffableDataSourceSnapshot<Section, Todo.ID>()
snap.appendSections([.main])
snap.appendItems(todos.map(\.id))  // ids UNIQUE
ds.apply(snap, animatingDifferences: true)

Picture 1 — the reuse pool

Picture 2 — snapshot diff: IDs, not content

Interview traps

• “Diffable detects content changes.” No — it diffs identifiers only. Same ID + new title = no update until reconfigureItems/reloadItems.
• Duplicate IDs anywhere in the snapshot (across sections too) → runtime crash. Never use a display name as the ID.
• Caching an IndexPath across an apply — positions shift; convert to the ID with itemIdentifier(for:) immediately.
• Config closure capturing a model copy → reconfigure shows old data. Look it up by ID.
• Apply from main or always the same background queue — never mix.
• prepareForReuse is not the place to set content — that is cellForRowAt.

Remember

“The pool gives you a used cell — set everything, assume nothing.” “Snapshot = IDs; content = your store; reconfigure = repaint.”

Likely questions

1. Why do old images appear? — reuse; reset/cancel in prepareForReuse, set all in cellForRowAt.
2. What must identifiers be? — Hashable and unique in the snapshot.
3. Reconfigure vs reload? — same cell re-configured vs new cell dequeued.
4. What did diffable replace? — performBatchUpdates index math + its crashes.