Customizing and resizing sheets in UIKit

At a glance

Project structure

Postcards/Postcards/
├── AppDelegate.swift
├── SceneDelegate.swift
├── ViewController.swift
├── SettingsViewController.swift
├── PresentationHelper.swift
├── Base.lproj/Main.storyboard
└── Assets.xcassets/

Structure observations

• Main postcard editing and modal presentation live in ViewController.
• Settings controls and live mutation of the settings sheet live in SettingsViewController.
• PresentationHelper is a small shared state holder; despite “Helper,” it stores policy rather than performing presentation.

Overall architecture

flowchart LR
    Main["ViewController"] -->|"present popover"| Settings["SettingsViewController"]
    Main -->|"present popover"| ImagePicker["UIImagePickerController"]
    Main -->|"present popover"| FontPicker["UIFontPickerViewController"]
    Main -->|"configure"| Sheet["adaptiveSheetPresentationController"]
    Settings --> Shared["PresentationHelper.sharedInstance"]
    Main --> Shared
    Settings -->|"updateSheet()"| Sheet
    ImagePicker -->|"delegate image"| Main
    FontPicker -->|"delegate font"| Main
    Sheet --> Detent["small / medium / large detents"]

Reference code

Postcards/Postcards/ViewController.swift:40 — the main controller constructs the settings presentation, defines a fractional custom detent, and applies shared sheet policy.

if let settingsViewController = storyboard?
        .instantiateViewController(withIdentifier: "settings")
        as? SettingsViewController {
    settingsViewController.modalPresentationStyle = .popover
    if let popover =
            settingsViewController.popoverPresentationController {
        popover.barButtonItem = sender
        let sheet = popover.adaptiveSheetPresentationController
        sheet.detents = [
            .custom(identifier: .small) { context in
                0.3 * context.maximumDetentValue
            },
            .medium(),
            .large()
        ]
        sheet.prefersGrabberVisible = true
    }
    present(settingsViewController, animated: true)
}

Interpretation

ViewController is the presentation coordinator: it knows which controller is being shown, its anchor, and the supported detents. PresentationHelper carries user-selected policy across independently created presentations; SettingsViewController edits that policy and applies it live.

Ownership and state

classDiagram
    PresentationHelper *-- PresentationHelper : sharedInstance
    PresentationHelper *-- SheetPolicy : mutable settings
    ViewController o-- UIImageView : storyboard outlet
    ViewController o-- UITextView : storyboard outlet
    ViewController *-- NSLayoutConstraint : current image aspect ratio
    ViewController --> SettingsViewController : creates then presents
    UIViewController *-- SettingsViewController : presentation lifetime
    SettingsViewController o-- UISegmentedControl : weak outlet
    SettingsViewController o-- UISwitch : weak outlets
    SettingsViewController --> PresentationHelper : shared state
    UIPopoverPresentationController *-- UISheetPresentationController : adaptive sheet

Ownership evidence

Postcards/Postcards/PresentationHelper.swift:10 — a custom identifier and one static instance hold the policy read by both controllers.

extension UISheetPresentationController.Detent.Identifier {
    static let small =
        UISheetPresentationController.Detent.Identifier("small")
}

class PresentationHelper {
    static let sharedInstance = PresentationHelper()
    var largestUndimmedDetentIdentifier:
        UISheetPresentationController.Detent.Identifier = .medium
    var prefersScrollingExpandsWhenScrolledToEdge = false
    var prefersEdgeAttachedInCompactHeight = true
    var widthFollowsPreferredContentSizeWhenEdgeAttached = true
}

sharedInstance is conventional shared ownership, not a strictly enforced singleton: the initializer is still internal, so the module could create additional PresentationHelper objects.

Class and protocol design

There is no app-defined protocol. The sample uses framework delegate protocols for picker results, while presentation settings are shared through a concrete object.

Access control

Reference code

Postcards/Postcards/SettingsViewController.swift:71 — settings mutate the already-presented adaptive sheet by copying policy from the shared object.

func updateSheet() {
    guard let sheet =
        popoverPresentationController?
            .adaptiveSheetPresentationController else {
        return
    }
    let settings = PresentationHelper.sharedInstance
    sheet.largestUndimmedDetentIdentifier =
        settings.largestUndimmedDetentIdentifier
    sheet.prefersScrollingExpandsWhenScrolledToEdge =
        settings.prefersScrollingExpandsWhenScrolledToEdge
    sheet.prefersEdgeAttachedInCompactHeight =
        settings.prefersEdgeAttachedInCompactHeight
}

Logic ownership and placement

Design patterns

Naming conventions

• SettingsViewController names its UI role; the root ViewController is generic because the sample app has one main screen.
• PresentationHelper is broad; SheetPresentationSettings would describe its stored-state role more precisely.
• Boolean properties mirror UIKit names exactly, making assignment obvious.
• Actions use “show” for presentation and “Changed” for control-to-policy updates.

Architecture takeaways

• Keep presentation construction at the controller that owns context and anchors.
• Store only genuinely shared sheet policy in a shared object; keep per-presentation detents with the presenting flow.
• A custom detent closure is a strategy for converting available height into policy.
• Use animateChanges for live detent changes owned by the sheet controller.
• The sample’s all-internal access is convenient; production code should narrow mutable settings and per-controller constraint state.

Source map

• Official Apple documentation