Supporting gesture interaction in your apps

At a glance

Project structure

Source bundle/
├── GestureRecognizer/
│   ├── AppDelegate.swift
│   ├── SceneDelegate.swift
│   ├── ViewController.swift
│   ├── ResetGestureRecognizer.swift
│   ├── Base.lproj/
│   │   ├── LaunchScreen.storyboard
│   │   └── Main.storyboard
│   └── Info.plist
├── Configuration/
│   └── SampleCode.xcconfig
└── GestureRecognizer.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

Structure observations

• ResetGestureRecognizer is the reusable recognition engine; ViewController owns visual piece transforms.
• Standard gesture actions and the custom recognizer converge on the same controller-owned UI state.
• The controller’s UIKit delegate methods define coexistence and touch-filtering policy.

Overall architecture

flowchart LR
    TOUCH["Touch stream"] --> STANDARD["Pan, pinch, rotation recognizers"]
    TOUCH --> RESET["ResetGestureRecognizer"]
    RESET --> STATE["possible / began / changed / ended / failed"]
    STANDARD --> VC["ViewController actions"]
    STATE --> VC
    VC --> PIECES["Piece position, scale, rotation, reset"]

Reference code

GestureRecognizer/ViewController.swift:16 — custom recognizer attachment

    override func viewDidLoad() {
        super.viewDidLoad()
        initializePieces()
        let resetGestureRecognizer = ResetGestureRecognizer(
            target: self,
            action: #selector(resetPieces(_:)))
        view.addGestureRecognizer(resetGestureRecognizer)
        resetGestureRecognizer.delegate = self
        resetGestureRecognizer.cancelsTouchesInView = false
    }

Interpretation

Recognition and presentation are separate. The recognizer reduces raw touches into UIKit states; the controller receives target-action callbacks and applies view transforms. Its delegate conformance decides which recognizers may run together and which touches the reset recognizer receives.

Ownership and state

classDiagram
    UIView *-- ResetGestureRecognizer : attached recognizer
    ViewController --> ResetGestureRecognizer : target and delegate
    ResetGestureRecognizer *-- UITouch : trackedTouch
    ResetGestureRecognizer *-- CGPointArray : touchedPoints
    ViewController o-- UILabel : storyboard pieces

Ownership evidence

GestureRecognizer/ResetGestureRecognizer.swift:12 — recognizer-local state

class ResetGestureRecognizer: UIGestureRecognizer {

    private var trackedTouch: UITouch?
    private var touchedPoints = [CGPoint]()

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesBegan(touches, with: event)
    }
}

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

GestureRecognizer/ResetGestureRecognizer.swift:59 — recognizer state transition

    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesEnded(touches, with: event)
        let count = countHorizontalTurning(touchedPoints: touchedPoints)
        state = count > 2 ? .ended : .failed
    }

    override func reset() {
        super.reset()
        trackedTouch = nil
        touchedPoints.removeAll()
        state = .possible
    }

UIGestureRecognizerDelegate is a framework callback seam. There is no local protocol abstraction; reuse comes from the custom recognizer subclass.

Access control

Reference code

GestureRecognizer/ResetGestureRecognizer.swift:14 — private recognition state

    private var trackedTouch: UITouch?
    private var touchedPoints = [CGPoint]()

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesMoved(touches, with: event)
        guard state != .failed else { return }
    }

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

• ResetGestureRecognizer names the user intent and UIKit role, not its horizontal-turn implementation.
• Action methods use gesture verbs (panPiece, pinchPiece, rotatePiece, resetPieces).
• Boolean/state names follow UIKit vocabulary (cancelsTouchesInView, recognizer state).
• The custom recognizer has its own file; lifecycle types remain separate.

Architecture takeaways

• Put raw-touch accumulation and recognition transitions inside the recognizer subclass.
• Keep view transforms and gesture coexistence policy on the screen controller.
• Private recognition state prevents callers from forcing inconsistent transitions.

Source map

• Official Apple documentation