PencilKit

Integrate drawing, sketching, and annotation into iOS and iPadOS apps using PencilKit (PKCanvasView, PKToolPicker, PKDrawing) and PaperKit. Targets Swift 6.3 / iOS 26+.

Contents

• Core PencilKit Architecture
• SwiftUI Integration Pattern
• Drawing Persistence and Data
• Input Policy and Gestures
• Route by Task
• Common Mistakes
• Review Checklist
• References

Core PencilKit Architecture

SwiftUI Integration Pattern

Bridge PKCanvasView into SwiftUI using UIViewRepresentable:

import SwiftUI
import PencilKit

struct CanvasViewRepresentable: UIViewRepresentable {
    @Binding var drawing: PKDrawing
    var tool: PKTool = PKInkingTool(.pen, color: .black, width: 5)
    var isRulerActive: Bool = false

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

    func updateUIView(_ uiView: PKCanvasView, context: Context) {
        if uiView.drawing != drawing {
            uiView.drawing = drawing
        }
        uiView.tool = tool
        uiView.isRulerActive = isRulerActive
    }

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

    class Coordinator: NSObject, PKCanvasViewDelegate {
        var parent: CanvasViewRepresentable
        init(_ parent: CanvasViewRepresentable) { self.parent = parent }

        func canvasViewDrawingDidChange(_ canvasView: PKCanvasView) {
            Task { @MainActor in parent.drawing = canvasView.drawing }
        }
    }
}

Drawing Persistence and Data

• Serialization: Serialize drawings using drawing.dataRepresentation(). Restore via PKDrawing(data:).
• Image Generation: Render drawings to raster images with drawing.image(from: canvasView.bounds, scale: canvasView.traitCollection.displayScale).
• Stroke Inspection: Iterate through drawing.strokes to inspect points, pressure, force, and azimuth.

Input Policy and Gestures

Configure canvasView.drawingPolicy:

• .default: Follows system preference (Apple Pencil only if configured in Settings).
• .anyInput: Allows finger drawing alongside Apple Pencil.
• .pencilOnly: Restricts drawing strictly to Apple Pencil; finger touches scroll the canvas.

Coordinating PKToolPicker: Attach the picker with toolPicker.setVisible(true, forFirstResponder: canvasView) and toolPicker.addObserver(canvasView). Ensure the canvas becomes first responder.

Route by Task

• For tracking tool picker changes, visibility, and frame obstruction, read Tool Picker Observer Pattern.
• For custom items in PKToolPicker (iOS 18+), read Custom Tool Picker Items.
• For stroke construction, comparison, and shape recognition, read Constructing Strokes Programmatically.
• For thumbnail generation and background image rendering, read Thumbnail Generation.
• For undo/redo coordination with UndoManager, read Undo/Redo Support.

Common Mistakes

• Forgetting to call canvasView.becomeFirstResponder() before showing PKToolPicker.
• Updating SwiftUI @Binding var drawing continuously during drawing gestures, causing hitching and feedback loops.
• Overriding finger scrolling gestures without setting drawingPolicy = .pencilOnly.
• Generating high-resolution raster images synchronously on the main thread from complex drawings.
• Assuming PKToolPicker floats automatically in SwiftUI without anchoring to a window or first responder.

Review Checklist

• PKCanvasView embedded in SwiftUI with a clean Coordinator
• drawingPolicy explicitly set (.anyInput, .pencilOnly, or .default)
• PKToolPicker tied to canvas first responder and visible state
• Drawing data serialized with dataRepresentation() and restored with error handling
• Raster image rendering performed asynchronously or off the main thread for large canvases
• Undo and redo actions integrated with the canvas's undoManager
• Dynamic Type and safe area insets respected when PKToolPicker obscures canvas regions

References

• PencilKit extended patterns and stroke inspection
• PencilKit documentation
• PKCanvasView
• PKToolPicker
• PKDrawing