Implementing Peek and Pop

At a glance

Project structure

PeekPopNavigation/
├── ColorsViewControllerBase.swift
├── ColorsViewControllerCode.swift
├── ColorsViewControllerStoryboard.swift
├── ColorItemViewController.swift
├── Models/
│   ├── ColorData.swift
│   ├── ColorItem.swift
│   └── Notifications.swift
└── Base.lproj/Main.storyboard

Structure observations

• The base controller contains all table, navigation, and notification behavior so each variant shows only its integration difference.
• ColorsViewControllerStoryboard is intentionally empty; storyboard configuration is the implementation.
• Model events are named in Notifications.swift, separate from the mutable model types that emit them.

Overall architecture

flowchart LR
    Storyboard["Storyboard variant"] --> Base["ColorsViewControllerBase"]
    Code["ColorsViewControllerCode"] --> Base
    Base --> Data["ColorData"]
    Base --> Table["Color list"]
    Code --> PreviewDelegate["UIViewControllerPreviewingDelegate"]
    PreviewDelegate --> Preview["ColorItemViewController preview"]
    Preview -->|commit| Nav["Navigation controller push"]
    Base -->|normal segue| Preview
    Data --> Preview
    Item["ColorItem"] --> Preview
    Preview --> QuickActions["Star / Delete preview actions"]
    QuickActions --> Item
    QuickActions --> Data

Reference code

PeekPopNavigation/ColorsViewControllerCode.swift:25 — the code variant builds the preview with the same ColorData and ColorItem references used by the list, then commits that configured controller.

func previewingContext(_ previewingContext: UIViewControllerPreviewing,
                       viewControllerForLocation location: CGPoint) -> UIViewController? {
    guard let indexPath = tableView.indexPathForRow(at: location),
        let cell = tableView.cellForRow(at: indexPath) else { return nil }

    previewingContext.sourceRect = cell.frame

    guard let viewController = storyboard?.instantiateViewController(
        withIdentifier: "ColorItemViewController") as? ColorItemViewController
        else { preconditionFailure("Expected a ColorItemViewController") }

    viewController.colorData = colorData
    viewController.colorItem = colorData.colors[indexPath.row]
    return viewController
}

func previewingContext(_ previewingContext: UIViewControllerPreviewing,
                       commit viewControllerToCommit: UIViewController) {
    navigationController?.pushViewController(viewControllerToCommit, animated: true)
}

Interpretation

Preview is not a copy of feature state. The preview and eventual committed controller share the original object references, so quick actions immediately affect the same catalog observed by the table screen.

Ownership and state

classDiagram
    ColorsViewControllerBase *-- ColorData : creates one per instance
    ColorData *-- ColorItem : owns array
    ColorsViewControllerBase --> NotificationCenter : observes
    ColorItem --> NotificationCenter : posts updated
    ColorData --> NotificationCenter : posts deleted
    ColorsViewControllerCode --|> ColorsViewControllerBase
    ColorsViewControllerStoryboard --|> ColorsViewControllerBase
    ColorItemViewController o-- ColorData : injected reference
    ColorItemViewController o-- ColorItem : injected reference
    ColorItem ..|> Equatable

Ownership evidence

PeekPopNavigation/Models/ColorItem.swift:10 — each mutable property emits a model event; the owning data array remains in ColorData.

class ColorItem {
    // ...
}

Class and protocol design

There is no app-defined protocol. The code variant conforms directly to the UIKit previewing delegate; the base-class seam is inheritance because the sample compares two setup mechanisms around identical behavior.

Access control

Reference code

PeekPopNavigation/ColorsViewControllerBase.swift:15 — the base exposes its stable model container to subclasses rather than widening anything to a public framework API.

class ColorsViewControllerBase: UITableViewController {
    // ...
}

Logic ownership and placement

Design patterns

Naming conventions

• Base, Code, and Storyboard suffixes make the comparison structure explicit.
• ColorData names the collection owner; ColorItem names one mutable entity.
• Notifications use past-tense events: colorItemUpdated and colorItemDeleted.
• previewingContext names are inherited from UIKit; domain actions remain toggleStar and delete.

Architecture takeaways

• Factor shared feature behavior away from alternative integration mechanisms when teaching or testing platform APIs.
• Inject the same model reference into preview and committed presentation when actions must have immediate effect.
• Emit change events from mutation owners, but scope observers by object identity when multiple catalogs exist.
• Internal access is sufficient for an app target; absence of access modifiers should not be described as a public API.

Source map

• Official Apple documentation