Listen ScrollView contentOffset and set it

[phuhk]

hi there, I'm late for the party. As title, can we using this library to do something like this? Listen ScrollView contentOffset and set it. Sorry if I asking question at wrong place.

[lucastimeless]

In iOS 14 you can use: ScrollViewReader to set the content offset.

In iOS 14 for listening to the contentOffset you could do something like this (although with the current version you will need: PR #56 or another fix for issue #55)

import SwiftUI
import Introspect
import Combine

struct ScrollViewDidScrollViewModifier: ViewModifier {
    
    class ViewModel: ObservableObject {
        var contentOffsetSubscription: AnyCancellable?
    }

    @StateObject var viewModel = ViewModel()
    var didScroll: (CGPoint) -> Void
    
    func body(content: Content) -> some View {
        content
        .introspectScrollView { scrollView in
            if viewModel.contentOffsetSubscription == nil {
                viewModel.contentOffsetSubscription = scrollView.publisher(for: \.contentOffset)
                .sink { contentOffset in
                    didScroll(contentOffset)
                }
            }
        }
    }
}

extension ScrollView {
    func didScroll(_ didScroll: @escaping (CGPoint) -> Void) -> some View {
        self.modifier(ScrollViewDidScrollViewModifier(didScroll: didScroll))
    }
}

struct ContentView: View {
    var body: some View {
        ScrollView {
            ForEach(1..<21) { _ in 
                Text("Hello, world!")
                    .padding()
            }
        }
        .didScroll { contentOffset in
            print(contentOffset)
        }
    }
}

[JannThomas]

This is not an Introspect Issue, but @lucastimeless has also provided a nice solution. That said I will close this issue.

[chrysb]

if viewModel.contentOffsetSubscription == nil {
                viewModel.contentOffsetSubscription = scrollView.publisher(for: \.contentOffset)
                .sink { contentOffset in
                    didScroll(contentOffset)
                }
            }

Please be careful with this solution, as sinking the publisher within the view creates a retain and your view (and related viewmodels) will not be de-allocated.

Here's the fix:

import Foundation
import SwiftUI
import Combine

struct ScrollViewDidScrollViewModifier: ViewModifier {
  class ViewModel: ObservableObject {
    @Published var contentOffset: CGPoint = .zero
    
    var contentOffsetSubscription: AnyCancellable?
    
    func subscribe(scrollView: UIScrollView) {
      contentOffsetSubscription = scrollView.publisher(for: \.contentOffset).sink { [weak self] contentOffset in
        self?.contentOffset = contentOffset
      }
    }
  }

  @StateObject var viewModel = ViewModel()
  var didScroll: (CGPoint) -> Void
  
  func body(content: Content) -> some View {
    content
      .introspectScrollView { scrollView in
        if viewModel.contentOffsetSubscription == nil {
          viewModel.subscribe(scrollView: scrollView)
        }
      }
      .onReceive(viewModel.$contentOffset) { contentOffset in
        didScroll(contentOffset)
      }
  }
}

extension View {
  func didScroll(_ didScroll: @escaping (CGPoint) -> Void) -> some View {
    self.modifier(ScrollViewDidScrollViewModifier(didScroll: didScroll))
  }
}

[hyouuu]

@lucastimeless thanks for the answer! However it keeps publishing values and the CPU usage is at 100% - is there a way to only listen to value changes?