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dpearson2699/swift-ios-skills

Agent skills for iOS, iPadOS, Swift, SwiftUI, and modern Apple framework development.

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SKILL.mdskills/pencilkit/

name:
pencilkit
description:
Add Apple Pencil drawing, canvas views, tool pickers, and ink serialization using PencilKit. Use when building drawing apps, annotation features, handwriting capture, signature fields, or any Apple Pencil-powered experience on iOS/iPadOS/visionOS.

PencilKit

Capture Apple Pencil and finger input using PKCanvasView, manage drawing tools with PKToolPicker, serialize drawings with PKDrawing, and wrap PencilKit in SwiftUI. Targets Swift 6.3 / iOS 26+.

Contents

  • Setup
  • PKCanvasView Basics
  • PKToolPicker
  • PKDrawing Serialization
  • Exporting to Image
  • Stroke Inspection
  • SwiftUI Integration
  • PaperKit Relationship
  • Common Mistakes
  • Review Checklist
  • References

Setup

PencilKit requires no entitlements or Info.plist entries. Import PencilKit and create a PKCanvasView.

import PencilKit

Platform availability: iOS 13+, iPadOS 13+, Mac Catalyst 13.1+, visionOS 1.0+.

PKCanvasView Basics

PKCanvasView is a UIScrollView subclass that captures Apple Pencil and finger input and renders strokes.

import PencilKit
import UIKit

class DrawingViewController: UIViewController, PKCanvasViewDelegate {
    let canvasView = PKCanvasView()

    override func viewDidLoad() {
        super.viewDidLoad()
        canvasView.delegate = self
        canvasView.drawingPolicy = .anyInput
        canvasView.tool = PKInkingTool(.pen, color: .black, width: 5)
        canvasView.frame = view.bounds
        canvasView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.addSubview(canvasView)
    }

    func canvasViewDrawingDidChange(_ canvasView: PKCanvasView) {
        // Drawing changed -- save or process
    }
}

Drawing Policies

PolicyBehavior
.defaultApple Pencil draws; finger scrolls
.anyInputBoth pencil and finger draw
.pencilOnlyOnly Apple Pencil draws; finger always scrolls
canvasView.drawingPolicy = .pencilOnly

Configuring the Canvas

// Set a large drawing area (scrollable)
canvasView.contentSize = CGSize(width: 2000, height: 3000)

// Enable/disable the ruler
canvasView.isRulerActive = true

// Set the current tool programmatically
canvasView.tool = PKInkingTool(.pencil, color: .blue, width: 3)
canvasView.tool = PKEraserTool(.vector)

PKToolPicker

PKToolPicker displays a floating palette of drawing tools. The canvas automatically adopts the selected tool.

class DrawingViewController: UIViewController {
    let canvasView = PKCanvasView()
    let toolPicker = PKToolPicker()

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        toolPicker.setVisible(true, forFirstResponder: canvasView)
        toolPicker.addObserver(canvasView)
        canvasView.becomeFirstResponder()
    }
}

Custom Tool Picker Items

Create a tool picker with specific tools.

let toolPicker = PKToolPicker(toolItems: [
    PKToolPickerInkingItem(type: .pen, color: .black),
    PKToolPickerInkingItem(type: .pencil, color: .gray),
    PKToolPickerInkingItem(type: .marker, color: .yellow),
    PKToolPickerEraserItem(type: .vector),
    PKToolPickerLassoItem(),
    PKToolPickerRulerItem()
])

Ink Types

TypeDescription
.penSmooth, pressure-sensitive pen
.pencilTextured pencil with tilt shading
.markerSemi-transparent highlighter
.monolineUniform-width pen
.fountainPenVariable-width calligraphy pen
.watercolorBlendable watercolor brush
.crayonTextured crayon

PKDrawing Serialization

PKDrawing is a value type (struct) that holds all stroke data. Serialize it to Data for persistence.

// Save
func saveDrawing(_ drawing: PKDrawing) throws {
    let data = drawing.dataRepresentation()
    try data.write(to: fileURL)
}

// Load
func loadDrawing() throws -> PKDrawing {
    let data = try Data(contentsOf: fileURL)
    return try PKDrawing(data: data)
}

Combining Drawings

var drawing1 = PKDrawing()
let drawing2 = PKDrawing()
drawing1.append(drawing2)

// Non-mutating
let combined = drawing1.appending(drawing2)

Transforming Drawings

let scaled = drawing.transformed(using: CGAffineTransform(scaleX: 2, y: 2))
let translated = drawing.transformed(using: CGAffineTransform(translationX: 100, y: 0))

Exporting to Image

Generate a UIImage from a drawing.

func exportImage(from drawing: PKDrawing, scale: CGFloat = 2.0) -> UIImage {
    drawing.image(from: drawing.bounds, scale: scale)
}

// Export a specific region
let region = CGRect(x: 0, y: 0, width: 500, height: 500)
let scale = UITraitCollection.current.displayScale
let croppedImage = drawing.image(from: region, scale: scale)

Stroke Inspection

Access individual strokes, their ink, and control points.

for stroke in drawing.strokes {
    let ink = stroke.ink
    print("Ink type: \(ink.inkType), color: \(ink.color)")
    print("Bounds: \(stroke.renderBounds)")

    // Access path points
    let path = stroke.path
    print("Points: \(path.count), created: \(path.creationDate)")

    // Interpolate along the path
    for point in path.interpolatedPoints(by: .distance(10)) {
        print("Location: \(point.location), force: \(point.force)")
    }
}

Constructing Strokes Programmatically

let points = [
    PKStrokePoint(location: CGPoint(x: 0, y: 0), timeOffset: 0,
                  size: CGSize(width: 5, height: 5), opacity: 1,
                  force: 0.5, azimuth: 0, altitude: .pi / 2),
    PKStrokePoint(location: CGPoint(x: 100, y: 100), timeOffset: 0.1,
                  size: CGSize(width: 5, height: 5), opacity: 1,
                  force: 0.5, azimuth: 0, altitude: .pi / 2)
]
let path = PKStrokePath(controlPoints: points, creationDate: Date())
let stroke = PKStroke(ink: PKInk(.pen, color: .black), path: path,
                      transform: .identity, mask: nil)
let drawing = PKDrawing(strokes: [stroke])

SwiftUI Integration

Wrap PKCanvasView in a UIViewRepresentable for SwiftUI.

import SwiftUI
import PencilKit

struct CanvasView: UIViewRepresentable {
    @Binding var drawing: PKDrawing
    @Binding var toolPickerVisible: Bool

    func makeUIView(context: Context) -> PKCanvasView {
        let canvas = PKCanvasView()
        canvas.delegate = context.coordinator
        canvas.drawingPolicy = .anyInput
        canvas.drawing = drawing
        return canvas
    }

    func updateUIView(_ canvas: PKCanvasView, context: Context) {
        if canvas.drawing != drawing {
            canvas.drawing = drawing
        }
        let toolPicker = context.coordinator.toolPicker
        toolPicker.setVisible(toolPickerVisible, forFirstResponder: canvas)
        if toolPickerVisible { canvas.becomeFirstResponder() }
    }

    func makeCoordinator() -> Coordinator { Coordinator(self) }

    class Coordinator: NSObject, PKCanvasViewDelegate {
        let parent: CanvasView
        let toolPicker = PKToolPicker()

        init(_ parent: CanvasView) {
            self.parent = parent
            super.init()
        }

        func canvasViewDrawingDidChange(_ canvasView: PKCanvasView) {
            parent.drawing = canvasView.drawing
        }
    }
}

Usage in SwiftUI

struct DrawingScreen: View {
    @State private var drawing = PKDrawing()
    @State private var showToolPicker = true

    var body: some View {
        CanvasView(drawing: $drawing, toolPickerVisible: $showToolPicker)
            .ignoresSafeArea()
    }
}

PaperKit Relationship

PaperKit (iOS 26+) extends PencilKit with a complete markup experience including shapes, text boxes, images, stickers, and loupes. Use PaperKit when you need more than freeform drawing.

CapabilityPencilKitPaperKit
Freeform drawingYesYes
Shapes & linesNoYes
Text boxesNoYes
Images & stickersNoYes
Markup toolbarNoYes
Data modelPKDrawingPaperMarkup

PaperKit uses PencilKit under the hood -- PaperMarkupViewController accepts PKTool for its drawingTool property and PaperMarkup can append a PKDrawing.

Common Mistakes

DON'T: Forget to call becomeFirstResponder for the tool picker

The tool picker only appears when its associated responder is first responder.

// WRONG: Tool picker never shows
toolPicker.setVisible(true, forFirstResponder: canvasView)

// CORRECT: Also become first responder
toolPicker.setVisible(true, forFirstResponder: canvasView)
canvasView.becomeFirstResponder()

DON'T: Create multiple tool pickers for the same canvas

One PKToolPicker per canvas. Creating extras causes visual conflicts.

// WRONG
func viewDidAppear(_ animated: Bool) {
    let picker = PKToolPicker()  // New picker every appearance
    picker.setVisible(true, forFirstResponder: canvasView)
}

// CORRECT: Store picker as a property
let toolPicker = PKToolPicker()

DON'T: Ignore content version for backward compatibility

Newer ink types crash on older OS versions. Set maximumSupportedContentVersion if you need backward-compatible drawings.

// WRONG: Saves a drawing with .watercolor, crashes on iOS 16
canvasView.tool = PKInkingTool(.watercolor, color: .blue)

// CORRECT: Limit content version for compatibility
canvasView.maximumSupportedContentVersion = .version2

DON'T: Compare drawings by data representation

PKDrawing data is not deterministic; the same visual drawing can produce different bytes. Use equality operators instead.

// WRONG
if drawing1.dataRepresentation() == drawing2.dataRepresentation() { }

// CORRECT
if drawing1 == drawing2 { }

Review Checklist

  • PKCanvasView.drawingPolicy set appropriately (.default for Pencil-primary apps)
  • PKToolPicker stored as a property, not recreated each appearance
  • canvasView.becomeFirstResponder() called to show the tool picker
  • Drawing serialized via dataRepresentation() and loaded via PKDrawing(data:)
  • canvasViewDrawingDidChange delegate method used to track changes
  • maximumSupportedContentVersion set if backward compatibility needed
  • Exported images use appropriate scale factor for the device
  • SwiftUI wrapper avoids infinite update loops by checking drawing != binding
  • Tool picker observer added before becoming first responder
  • Drawing bounds checked before image export (empty drawings have .zero bounds)

References

  • Extended PencilKit patterns (advanced strokes, Scribble, delegate): references/pencilkit-patterns.md
  • PencilKit framework
  • PKCanvasView
  • PKDrawing
  • PKToolPicker
  • PKInkingTool
  • PKStroke
  • Drawing with PencilKit
  • Configuring the PencilKit tool picker

skills

pencilkit

SKILL.md

CHANGELOG.md

README.md

tile.json