Adopting hover support for Apple Pencil

At a glance

Project structure

ApplePencilSampleApp/ApplePencilSampleApp/
├── AppDelegate.swift
├── SceneDelegate.swift
├── ViewController.swift
├── Drawing/
│   ├── HoverDrawView.swift
│   └── DrawGestureRecognizer.swift
└── Base.lproj/Main.storyboard

Structure observations

• The empty host controller confirms that interaction logic belongs to the reusable view.
• A custom long-press recognizer exists only to retain the current touch/event needed for coalesced Pencil samples.

Overall architecture

flowchart LR
    Host["ViewController"] --> Canvas["HoverDrawView"]
    Pencil["Apple Pencil input"] --> DrawGR["DrawGestureRecognizer"]
    Pencil --> HoverGR["UIHoverGestureRecognizer"]
    DrawGR --> Canvas
    HoverGR --> Canvas
    Canvas --> Live["CAShapeLayer live stroke or preview"]
    Canvas --> Bitmap["CGContext committed strokes"]
    Bitmap --> Layer["UIView.layer contents"]
    HoverGR -->|zOffset| Preview["Preview opacity and tail"]
    Preview --> Live

Reference code

ApplePencilSampleApp/ApplePencilSampleApp/Drawing/HoverDrawView.swift:65 — the view creates and retains both recognizers because they drive one shared drawing state machine.

private func configureGestureRecognizers() {
    if drawGestureRecognizer == nil {
        let drawGesture = DrawGestureRecognizer(target: self, action: #selector(drawGesture(_:)))
        drawGesture.minimumPressDuration = 0
        addGestureRecognizer(drawGesture)
        drawGestureRecognizer = drawGesture
    }
    if hoverGestureRecognizer == nil {
        let hoverGesture = UIHoverGestureRecognizer(target: self, action: #selector(hoverGesture(_:)))
        addGestureRecognizer(hoverGesture)
        hoverGestureRecognizer = hoverGesture
    }
}

Interpretation

Hover and draw update the same path/layer, but only completed drawing is flattened into the bitmap. The view guards hover while drawing so preview state cannot overwrite an active stroke.

Ownership and state

classDiagram
    UIViewController *-- HoverDrawView : storyboard hierarchy
    HoverDrawView *-- DrawGestureRecognizer : creates
    HoverDrawView *-- UIHoverGestureRecognizer : creates
    HoverDrawView *-- CAShapeLayer : creates live layer
    HoverDrawView *-- CGContext : creates bitmap context
    HoverDrawView *-- UIBezierPath : owns current path
    DrawGestureRecognizer o-- UITouch : weak currentTouch
    DrawGestureRecognizer o-- UIEvent : weak currentEvent

Ownership evidence

ApplePencilSampleApp/ApplePencilSampleApp/Drawing/HoverDrawView.swift:23 — all mutable drawing and preview state is private to the canvas.

private var bitmapContext: CGContext?
private var bitmapContextSize = CGSize(width: 0, height: 0)
private var bezierPath = UIBezierPath()
private var shapeLayer: CAShapeLayer?
private var drawGestureRecognizer: DrawGestureRecognizer?
private var hoverGestureRecognizer: UIHoverGestureRecognizer?
private var isPreviewing = false
private var previewAlpha = 1.0
private var isDrawing = false
private var strokePathTail: [CGPoint] = []

Class and protocol design

No app-defined protocol abstraction is present. Subclassing is used where UIKit requires custom view and gesture behavior.

Access control

Reference code

ApplePencilSampleApp/ApplePencilSampleApp/Drawing/DrawGestureRecognizer.swift:13 — only the bridge values cross the two-type boundary, and both are weak.

class DrawGestureRecognizer: UILongPressGestureRecognizer {
    weak var currentTouch: UITouch?
    weak var currentEvent: UIEvent?

    override func reset() {
        super.reset()
        currentTouch = nil
        currentEvent = nil
    }
}

Logic ownership and placement

Design patterns

Naming conventions

• HoverDrawView names both supported interaction modes; DrawGestureRecognizer names its focused adapter role.
• Boolean state uses isPreviewing and isDrawing; thresholds use max... and fade....
• Rendering commands use update, reset, and current to distinguish mutation from derivation.
• Drawing/ groups the only feature-specific view and recognizer.

Architecture takeaways

• Put tightly coupled input and rendering state in the custom canvas instead of a passive host controller.
• Keep hover preview transient and commit only completed strokes to durable backing storage.
• Use weak references when exposing UIKit-owned touch/event objects beyond an override.
• Release bitmap/layer resources when the view leaves its hierarchy.

Source map

• Official Apple documentation