Selecting multiple items with a two-finger pan gesture

At a glance

Project structure

Source bundle/
├── MultiselectGestureSample/
│   ├── AppDelegate.swift
│   ├── CollectionViewController.swift
│   ├── FillerModel.swift
│   ├── PhotoModel.swift
│   ├── SceneDelegate.swift
│   ├── TableViewController.swift
│   ├── CollectionViewCell.swift
│   └── Base.lproj/
│       ├── CollectionView.storyboard
│       └── LaunchScreen.storyboard
├── Configuration/
│   └── SampleCode.xcconfig
└── MultiselectGestureSample.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

Structure observations

• CollectionViewController and TableViewController are parallel feature roots rather than a call chain from AppDelegate.
• Model generation stays in PhotoModel and FillerModel; selection policy stays beside each UIKit controller.
• Framework data-source and delegate conformances are split into extensions for the collection example.

Overall architecture

flowchart LR
    PM["PhotoModel items"] --> CVC["CollectionViewController"]
    FM["FillerModel items"] --> TVC["TableViewController"]
    CVC --> CSEL["UICollectionView multiselect callbacks"]
    TVC --> TSEL["UITableView multiselect callbacks"]
    CSEL --> EDIT["Editing and selected-item UI"]
    TSEL --> EDIT

Reference code

MultiselectGestureSample/CollectionViewController.swift:104 — collection-view multiple-selection callback

    func collectionView(_ collectionView: UICollectionView, shouldBeginMultipleSelectionInteractionAt indexPath: IndexPath) -> Bool {
        // Returning `true` automatically sets `collectionView.isEditing`
        // to `true`. The app sets it to `false` after the user taps the Done button.
        return true
    }
    
    func collectionView(_ collectionView: UICollectionView, didBeginMultipleSelectionInteractionAt indexPath: IndexPath) {
        setEditing(true, animated: true)
    }

Interpretation

The sample demonstrates one UIKit capability twice. Each controller is its own adapter: it supplies model identifiers/cells, accepts the framework callback, and updates editing UI. AppDelegate and SceneDelegate only provide lifecycle plumbing.

Ownership and state

classDiagram
    CollectionViewController *-- PhotoModelArray : photos
    TableViewController *-- FillerModelArray : items
    CollectionViewController o-- UICollectionView : storyboard outlet
    CollectionViewCell *-- Bool : showSelectionIcons

Ownership evidence

MultiselectGestureSample/CollectionViewController.swift:12 — collection controller state

    @IBOutlet weak var collectionView: UICollectionView!
    
    private let photos = PhotoModel.generatePhotosItems(count: 100)
    private let sectionInsets = UIEdgeInsets(top: 1.0, left: 1.0, bottom: 1.0, right: 1.0)
    private var isPad = false

Composition denotes source-visible construction and retained value state. Aggregation denotes a stored or injected collaborator whose exclusive lifetime the source does not prove.

Class and protocol design

MultiselectGestureSample/CollectionViewController.swift:99 — framework delegate conformance

extension CollectionViewController: UICollectionViewDelegate {

    // MARK: - Multiple selection methods.

    func collectionView(_ collectionView: UICollectionView, shouldBeginMultipleSelectionInteractionAt indexPath: IndexPath) -> Bool {
        return true
    }
}

The source uses UIKit callback protocols, but defines no local substitutable protocol; this is callback-oriented, not an app-wide protocol-oriented architecture.

Access control

Reference code

MultiselectGestureSample/CollectionViewCell.swift:19 — cell-local selection state

    private var showSelectionIcons = false

    override func awakeFromNib() {
        super.awakeFromNib()
        // Turn `imageViewSelected` into a circle to make its background
        // color act as a border around the checkmark symbol.
    }

Declarations without an explicit modifier are module-internal. private is used for state and helpers that implement one type’s invariant; no public library surface is inferred for a single app target.

Logic ownership and placement

Design patterns

Naming conventions

• Types use UIKit role suffixes (CollectionViewController, TableViewController, CollectionViewCell) and content suffixes (PhotoModel, FillerModel).
• Callback names retain UIKit signatures; commands such as toggleSelectionMode and updateUserInterface describe intent.
• Private state names (isPad, showSelectionIcons) use Boolean phrasing.
• Files follow their primary type; collection data-source/delegate conformances stay in same-file extensions.

Architecture takeaways

• Treat the table and collection examples as siblings, not an AppDelegate-to-model dependency chain.
• Keep framework callback policy in the controller that owns the corresponding editing UI.
• Use private state to protect reusable-cell and layout invariants; no public API is needed.

Source map

• Official Apple documentation