Understanding Diffable Data Sources in iOS

Managing data in table views and collection views has traditionally been a complex task involving multiple delegate methods and careful state management. Diffable data sources revolutionize this approach by providing a streamlined, declarative way to handle data updates in iOS applications.

Understanding the Fundamentals

A diffable data source extends either UITableViewDataSource or UICollectionViewDataSource, utilizing two generic Hashable types: SectionIdentifierType and ItemIdentifierType. These type constraints define the structure of your data presentation.

The section identifier typically represents different sections within your view. For simple single-section layouts, an enumeration works perfectly:

enum Section {
    case main
}

The item identifier represents the actual data models that populate individual cells. A typical model might look like:

struct CellModel {
    let title: String
    let imageIdentifier: String
}

When initializing a UITableViewDiffableDataSource, you provide both a table view reference and a cell provider closure:

public typealias CellProvider = (_ tableView: UITableView, _ indexPath: IndexPath, _ itemIdentifier: ItemIdentifierType) -> UITableViewCell?
 
public init(tableView: UITableView, cellProvider: @escaping CellProvider)

The cell provider function handles cell dequeuing, configuration, and return logic. Here's a practical implementation:

func makeDataSource(tableView: UITableView) -> UITableViewDiffableDataSource<Section, CellModel> {
    UITableViewDiffableDataSource(tableView: tableView, cellProvider: cellProvider)
}
 
func cellProvider(tableView: UITableView, indexPath: IndexPath, model: CellModel) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: Cell.reuseIdentifier, for: indexPath) as! Cell
    cell.textLabel.text = model.title
    cell.imageView.image = UIImage(named: model.imageIdentifier)
    return cell
}

Since the data source requires an initialized table view, using a lazy property is recommended:

lazy var dataSource = makeDataSource(tableView: tableView)

This approach differs significantly from traditional data sources, where the table view would query the data source for information. Instead, diffable data sources receive direct access to the view, eliminating the need for complex state sharing between components.

Working with Snapshots

Snapshots represent the core innovation of diffable data sources. An NSDiffableDataSourceSnapshot captures the current state of data you want to display. Updating your interface becomes as simple as creating a new snapshot and applying it.

Snapshots use the same generic types as their corresponding data sources and provide intuitive methods for state manipulation:

// NSDiffableDataSourceSnapshot<Section, CellModel> provides...
func appendSections(_ identifiers: [Section])
func appendItems(_ identifiers: [CellModel], toSection: Section?)
func insertItems(_ identifiers: [CellModel], afterItem: CellModel)
func moveItem(_ identifier: CellModel, afterItem: CellModel)
func deleteItems(_ identifiers: [CellModel])

Applying changes involves creating a snapshot, configuring it, then telling the data source to apply it:

typealias Snapshot = NSDiffableDataSourceSnapshot<Section, CellModel>
var snapshot = Snapshot()
snapshot.appendSections([.main])
snapshot.appendItems(models, toSection: .main)
dataSource.apply(snapshot, animatingDifferences: true)

The magic happens during the apply operation. The data source leverages the Hashable conformance of section and item types to calculate differences between the current and new snapshots, applying only necessary changes. If no data changed, no work occurs. If items simply reordered, only those specific cells update.

Handling Selection Events

Managing selection in diffable data source-backed views remains straightforward through delegate methods. The data source provides convenient methods to retrieve items and sections from index paths:

func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    guard let item = dataSource.itemIdentifier(for: indexPath) else { return }
    // Handle the selected item
}

Similarly, you can retrieve section identifiers using sectionIdentifier(for: Int) when needed.

Key Advantages

Diffable data sources deliver several significant benefits over traditional approaches. They provide clean, concise code for powering efficient table and collection views. The apply(snapshot) API creates a clear, single entry point for view updates, eliminating the need to maintain state across multiple locations.

View models can focus purely on output generation, where their output directly becomes input for apply(snapshot). This separation of concerns greatly simplifies architecture and reduces bugs related to state synchronization.

Critical Implementation Notes

Proper implementation of Hashable and Equatable protocols is essential. Custom hashing or equality implementations must accurately reflect model state changes. Incorrect implementations lead to unexpected display behavior, as the diffing algorithm relies on these protocols to determine what actually changed.

Ensure your model objects properly distinguish between different states through their hash values and equality comparisons. This attention to detail ensures the diffable data source can accurately compute and apply only necessary updates.

Summary

Diffable data sources represent a significant advancement in iOS data presentation, offering a declarative approach that simplifies state management and improves performance. By understanding snapshots, proper type constraints, and implementation details, developers can create more maintainable and efficient applications. The key lies in proper Hashable conformance and leveraging the snapshot-based update model to create clean, predictable data flow patterns.