IGListKit

To use models with IGListKit, they must conform to the ListDiffable protocol. This requires implementing two methods:

1. diffIdentifier(): Returns a unique identifier for the object (e.g., a database ID or a combination of unique fields). This allows IGListKit to track the identity of an object across updates.
2. isEqual(toDiffableObject:): Compares the content of two objects with the same identifier. If this returns false, the UI will be updated to reflect the changes.

Important Requirements:

• If two models have the same diffIdentifier, they must be considered equal by isEqual(toDiffableObject:) if their content hasn't changed.
• For ListBindingSectionController, a good equality check is critical; whenever a property changes, isEqual must return false to trigger a cell refresh.

final class UserViewModel: ListDiffable {
  let username: String
  let timestamp: String

  init(username: String, timestamp: String) {
    self.username = username
    self.timestamp = timestamp
  }

  func diffIdentifier() -> NSObjectProtocol {
    return "user" as NSObjectProtocol
  }

  func isEqual(toDiffableObject object: ListDiffable?) -> Bool {
    guard let object = object as? UserViewModel else { return false }
    return username == object.username && timestamp == object.timestamp
  }
}

To enable IGListKit's diffing algorithm to identify inserts, deletes, updates, and moves, your data models must conform to ListDiffable.

1. diffIdentifier(): Return a unique identifier (e.g., a primary key) that represents the identity of the object. This identifier must never change for the life of the object.
2. isEqual(toDiffableObject:): Return true if the content of the object is the same as another, and false if the content has changed. Returning false triggers a cell reload.

Important: Always use immutable models. If you mutate an existing object instead of providing a new instance, the diffing algorithm may fail to detect changes because the old and new instances will appear identical.

extension User: ListDiffable {
  func diffIdentifier() -> NSObjectProtocol {
    return primaryKey
  }

  func isEqual(toDiffableObject object: Any?) -> Bool {
    if let object = object as? User {
      return name == object.name
    }
    return false
  }
}

A ListBindingSectionController is a specialized section controller that manages a collection of view models derived from a single top-level model. It automates the process of mapping models to cells and provides animated, one-way updates.

Workflow

1. Top-level Model: The section controller receives one main model (e.g., a Post).
2. Decomposition: The controller's data source decomposes that main model into an array of smaller 'view models' (e.g., UserViewModel, ImageViewModel, Comment).
3. Binding: The controller maps each view model to a specific UICollectionViewCell type and provides its size.

Implementation Steps

Subclass ListBindingSectionController<T> (where T is your top-level model) and conform to ListBindingSectionControllerDataSource. You must implement three methods:

• sectionController(_:viewModelsFor:): Transform the top-level model into an array of ListDiffable view models.
• sectionController(_:sizeForViewModel:at:): Return the CGSize for each view model type.
• sectionController(_:cellForViewModel:at:): Dequeue and return the appropriate cell for each view model type.

final class PostSectionController: ListBindingSectionController<Post>, ListBindingSectionControllerDataSource {
  override init() {
    super.init()
    dataSource = self
  }
}