Implement a pull down gesture on a ScrollView to perform an action in SwiftUI. The idea is to monitor the vertical y offset of the ScrollView.

PullDownScrollViewGesture.swift

import SwiftUI

struct OverviewView: View { 
  private let threshold: CGFloat = 100.0
  @State private var showModal = false
  
  var body: some View {
    GeometryReader { geometry in
      ScrollView(showsIndicators: false) {
        VStack {
        }
        .anchorPreference(key: OffsetPreferenceKey.self, value: .top) { anchor in
            geometry[anchor].y
          }
        
      }
      .onPreferenceChange(OffsetPreferenceKey.self) { offset in
          if offset > threshold {
            showModal = true
          }
        }
      .sheet(isPresented: $showModal) {
          PresentedView()
      }              
    }
  }
}

private struct OffsetPreferenceKey: PreferenceKey {
  static var defaultValue: CGFloat = 0.0
  
  static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
    value = nextValue()
  }
}