Feature/Pinch to zoom (#319)

Sources/UI/SwiftUI/View Modifiers/PinchToZoomModifier.swift

@@ -0,0 +1,109 @@
+//
+//  PinchToZoomModifier.swift
+//  PovioKit
+//
+//  Created by Yll Fejziu on 13/05/2024.
+//  Copyright © 2024 Povio Inc. All rights reserved.
+//
+
+import SwiftUI
+
+/// https://stackoverflow.com/a/59878898
+@available(iOS 15.0, *)
+public struct PinchToZoomModifier: ViewModifier {
+  @StateObject private var viewModel = PinchZoomViewModel()
+
+  public func body(content: Content) -> some View {
+    content
+      .scaleEffect(viewModel.scale, anchor: viewModel.anchor)
+      .offset(viewModel.offset)
+      .animation(viewModel.isPinching ? .none : .spring(), value: viewModel.scale)
+      .overlay(
+        PinchZoom(viewModel: viewModel)
+      )
+  }
+}
+
+@available(iOS 15.0, *)
+public extension View {
+  func pinchToZoom() -> some View {
+    self.modifier(PinchToZoomModifier())
+  }
+}
+
+// MARK: - Private
+private class PinchZoomViewModel: ObservableObject {
+  @Published var scale: CGFloat = 1.0
+  @Published var anchor: UnitPoint = .center
+  @Published var offset: CGSize = .zero
+  @Published var isPinching: Bool = false
+  var startLocation: CGPoint = .zero
+  var location: CGPoint = .zero
+  var numberOfTouches: Int = 0
+}
+
+private class PinchZoomView: UIView {
+  var viewModel: PinchZoomViewModel
+
+  init(viewModel: PinchZoomViewModel) {
+    self.viewModel = viewModel
+    super.init(frame: .zero)
+
+    let pinchGesture = UIPinchGestureRecognizer(target: self, action: #selector(pinch(gesture:)))
+    pinchGesture.cancelsTouchesInView = false
+    addGestureRecognizer(pinchGesture)
+  }
+
+  @available(*, unavailable)
+  required init?(coder: NSCoder) {
+    fatalError("init(coder:) has not been implemented")
+  }
+
+  @objc private func pinch(gesture: UIPinchGestureRecognizer) {
+    switch gesture.state {
+    case .began:
+      viewModel.isPinching = true
+      viewModel.startLocation = gesture.location(in: self)
+      viewModel.anchor = UnitPoint(x: viewModel.startLocation.x / bounds.width,
+                                   y: viewModel.startLocation.y / bounds.height)
+      viewModel.numberOfTouches = gesture.numberOfTouches
+    case .changed:
+      if gesture.numberOfTouches != viewModel.numberOfTouches {
+        // If the number of fingers being used changes, the start location needs to be adjusted to avoid jumping.
+        let newLocation = gesture.location(in: self)
+        let jumpDifference = CGSize(width: newLocation.x - viewModel.location.x,
+                                    height: newLocation.y - viewModel.location.y)
+        viewModel.startLocation = CGPoint(
+          x: viewModel.startLocation.x + jumpDifference.width,
+          y: viewModel.startLocation.y + jumpDifference.height
+        )
+        viewModel.numberOfTouches = gesture.numberOfTouches
+      }
+      viewModel.scale = gesture.scale
+      viewModel.location = gesture.location(in: self)
+      viewModel.offset = CGSize(
+        width: viewModel.location.x - viewModel.startLocation.x,
+        height: viewModel.location.y - viewModel.startLocation.y
+      )
+    case .ended, .cancelled, .failed:
+      viewModel.isPinching = false
+      viewModel.scale = 1.0
+      viewModel.anchor = .center
+      viewModel.offset = .zero
+    default:
+      break
+    }
+  }
+}
+
+private struct PinchZoom: UIViewRepresentable {
+  @ObservedObject var viewModel: PinchZoomViewModel
+
+  func makeUIView(context: Context) -> PinchZoomView {
+    .init(viewModel: viewModel)
+  }
+
+  func updateUIView(_ uiView: PinchZoomView, context: Context) {
+    // state is automatically updated via ViewModel binding
+  }
+}