Disabling the pull-down gesture for a sheet

At a glance

Project structure

SheetPresentations/
├── RootViewController.swift
├── EditViewController.swift
│   └── EditViewControllerDelegate
├── AppDelegate.swift
└── Base.lproj/Main.storyboard

Structure observations

• Committed display state and draft editing state have different owners.
• RootViewController wires the sheet collaboration during segue preparation.
• EditViewController contains both text-editing and dismissal-protection policy because both depend on hasChanges.

Overall architecture

flowchart LR
    Root["RootViewController"] -->|"segue: inject originalText"| Edit["EditViewController"]
    Root -->|"EditViewControllerDelegate"| Edit
    Presentation["UIPresentationController"] -->|"attempted dismiss"| Edit
    Edit --> Draft["originalText + editedText"]
    Draft --> Dirty["hasChanges"]
    Dirty -->|"true"| Guard["isModalInPresentation"]
    Edit -->|"save"| Root
    Edit -->|"cancel / discard"| Root
    Root -->|"commit editedText"| Saved["Root text"]
    Root -->|"dismiss"| Presentation

Reference code

SheetPresentations/RootViewController.swift:32 — the composition point connects both delegate directions and copies committed text into the edit transaction.

override func prepare(
    for segue: UIStoryboardSegue,
    sender: Any?
) {
    guard segue.identifier == "Edit" else { return }

    let navigationController =
        segue.destination as! UINavigationController
    let editor =
        navigationController.topViewController
            as! EditViewController

    navigationController.presentationController?
        .delegate = editor
    editor.delegate = self
    editor.originalText = text
}

Interpretation

The root is the commit owner; the editor is a draft/session owner. Setting the editor as presentation-controller delegate lets the same owner that knows whether changes exist decide what an attempted gesture should do.

Ownership and state

classDiagram
    RootViewController *-- String : committed text
    UINavigationController *-- EditViewController : presented child
    EditViewController *-- String : originalText
    EditViewController *-- String : editedText
    EditViewController --> RootViewController : weak delegate
    UIPresentationController --> EditViewController : adaptive delegate
    EditViewController o-- UITextView : weak IBOutlet
    EditViewController o-- UIBarButtonItem : weak outlets
    EditViewController --> UIAlertController : temporary confirmation

Ownership evidence

SheetPresentations/EditViewController.swift:20 — the editor owns the original/draft pair, derives dirty state, and keeps the commit recipient weak.

var originalText = "" {
    didSet {
        editedText = originalText
    }
}

var editedText = "" {
    didSet {
        viewIfLoaded?.setNeedsLayout()
    }
}

var hasChanges: Bool {
    originalText != editedText
}

weak var delegate: EditViewControllerDelegate?

Class and protocol design

The app-defined protocol is a useful semantic boundary: the editor reports outcome without reaching into root storage or dismissing via a global presenter. Class-only conformance enables the weak reference.

Access control

Reference code

SheetPresentations/EditViewController.swift:53 — one derived flag drives both save availability and system interactive-dismissal policy.

override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()

    textView.text = editedText
    saveButton.isEnabled = hasChanges
    isModalInPresentation = hasChanges
}

func textViewDidChange(_ textView: UITextView) {
    editedText = textView.text
}

Logic ownership and placement

Design patterns

Naming conventions

• RootViewController and EditViewController distinguish committed display from draft workflow.
• Delegate callbacks use subject/outcome names: editViewControllerDidCancel and editViewControllerDidFinish.
• originalText, editedText, and hasChanges make transaction phases explicit.
• confirmCancel(showingSave:) describes UI intent; a richer enum could replace the Boolean if more entry paths existed.

Architecture takeaways

• Keep committed data and editable draft in separate owners.
• Derive dirty state from original and edited values instead of storing a second flag.
• Let the dirty-state owner control isModalInPresentation and attempted-dismiss confirmation.
• Use a weak, class-bound delegate to return commit/cancel outcomes.
• Narrow draft mutation and confirmation helpers in production while keeping the semantic delegate visible.

Source map

• Official Apple documentation