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This recipe shows how to zoom an image in SwiftUI using the pinch/magnify gesture. The image is zoomed in or out at the midpoint between the fingers, supports dragging and double tap to zoom in or reset. The end result looks like this:

preview

The recipe goes as follows:

  1. Use a custom UIVIew with UIPinchGestureRecognizer. This allows you to track the anchor point, which is the point midway between the fingers. The fact that we're zooming in based on the anchor point will result in an offset of the zoomed view.
  2. Wrap that custom view via UIViewRepresentable, and allow it to publish its current scale factor, anchor point and offset.
  3. Put the wrapped View as an overlay to the content of ViewModifier to capture the gesture, and use its bindings to modify the scaleEffect and offset.
  4. Optionally, add a TapGesture to support double taps for quickly zooming in or resetting.
// Constrains a value between the limits
func clamp(_ value: CGFloat, _ minValue: CGFloat, _ maxValue: CGFloat) -> CGFloat {
  min(maxValue, max(minValue, value))
}

// UIView that relies on UIPinchGestureRecognizer to detect scale, anchor point and offset
class ZoomableView: UIView {
  let minScale: CGFloat
  let maxScale: CGFloat
  let scaleChange: (CGFloat) -> Void
  let anchorChange: (UnitPoint) -> Void
  let offsetChange: (CGSize) -> Void

  private var scale: CGFloat = 1 {
    didSet {
      scaleChange(scale)
    }
  }
  private var anchor: UnitPoint = .center {
    didSet {
      anchorChange(anchor)
    }
  }
  private var offset: CGSize = .zero {
    didSet {
      offsetChange(offset)
    }
  }

  private var isPinching: Bool = false
  private var startLocation: CGPoint = .zero
  private var location: CGPoint = .zero
  private var numberOfTouches: Int = 0

  init(minScale: CGFloat,
       maxScale: CGFloat,
       scaleChange: @escaping (CGFloat) -> Void,
       anchorChange: @escaping (UnitPoint) -> Void,
       offsetChange: @escaping (CGSize) -> Void) {
    self.minScale = minScale
    self.maxScale = maxScale
    self.scaleChange = scaleChange
    self.anchorChange = anchorChange
    self.offsetChange = offsetChange
    super.init(frame: .zero)
    let pinchGesture = UIPinchGestureRecognizer(target: self, action: #selector(pinch(gesture:)))
    pinchGesture.cancelsTouchesInView = false
    addGestureRecognizer(pinchGesture)
  }

  required init?(coder: NSCoder) {
    fatalError()
  }

  @objc private func pinch(gesture: UIPinchGestureRecognizer) {
    switch gesture.state {
    case .began:
      isPinching = true
      startLocation = gesture.location(in: self)
      anchor = UnitPoint(x: startLocation.x / bounds.width, y: startLocation.y / bounds.height)
      numberOfTouches = gesture.numberOfTouches
    case .changed:
      if gesture.numberOfTouches != numberOfTouches {
        let newLocation = gesture.location(in: self)
        let jumpDifference = CGSize(width: newLocation.x - location.x, height: newLocation.y - location.y)
        startLocation = CGPoint(x: startLocation.x + jumpDifference.width, y: startLocation.y + jumpDifference.height)
        numberOfTouches = gesture.numberOfTouches
      }
      scale = clamp(gesture.scale, minScale, maxScale)
      location = gesture.location(in: self)
      offset = CGSize(width: location.x - startLocation.x, height: location.y - startLocation.y)
    case .possible, .cancelled, .failed:
      isPinching = false
      scale = 1.0
      anchor = .center
      offset = .zero
    case .ended:
      isPinching = false
    @unknown default:
      break
    }
  }
}

// Wraps ZoomableView and exposes it to SwiftUI
struct ZoomableOverlay: UIViewRepresentable {
  @Binding var scale: CGFloat
  @Binding var anchor: UnitPoint
  @Binding var offset: CGSize
  let minScale: CGFloat
  let maxScale: CGFloat

  func makeUIView(context: Context) -> ZoomableView {
    let uiView = ZoomableView(minScale: minScale,
                              maxScale: maxScale,
                              scaleChange: { scale = $0 },
                              anchorChange: { anchor = $0 },
                              offsetChange: { offset = $0 })
    return uiView
  }

  func updateUIView(_ uiView: ZoomableView, context: Context) { }
}

// Applies ZoomableOverlay to intercept gestures and apply scale, 
// anchor point and offset
struct Zoomable: ViewModifier {
  @Binding var scale: CGFloat
  @State private var anchor: UnitPoint = .center
  @State private var offset: CGSize = .zero
  let minScale: CGFloat
  let maxScale: CGFloat

  init(scale: Binding<CGFloat>,
       minScale: CGFloat,
       maxScale: CGFloat) {
    _scale = scale
    self.minScale = minScale
    self.maxScale = maxScale
  }

  func body(content: Content) -> some View {
    content
      .scaleEffect(scale, anchor: anchor)
      .offset(offset)
      .animation(.spring()) // looks more natural
      .overlay(ZoomableOverlay(scale: $scale,
                               anchor: $anchor,
                               offset: $offset,
                               minScale: minScale,
                               maxScale: maxScale))
      .gesture(TapGesture(count: 2).onEnded {
        if scale != 1 { // reset the scale
          scale = clamp(1, minScale, maxScale)
          anchor = .center
          offset = .zero
        } else { // quick zoom
          scale = clamp(2, minScale, maxScale)
        }
      })
  }
}

extension View {
  func zoomable(scale: Binding<CGFloat>,
                minScale: CGFloat = 0.5,
                maxScale: CGFloat = 3) -> some View {
    modifier(Zoomable(scale: scale, minScale: minScale, maxScale: maxScale))
  }
}

Finally, here's some sample usage:

struct ZoomableTest: View {
  @State private var scale: CGFloat = 1

  var body: some View {
    VStack {
      Spacer()
      Image("icon")
        .resizable()
        .scaledToFit()
        .zoomable(scale: $scale)
      Spacer()
      HStack {
        Button("Reset") {
          scale = 1
        }
        Spacer()
        Text("Zoom: \(String(format: "%.02f", scale * 100) )%")
      }
      .padding()
    }
  }
}

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