Pull-to-Refresh: SwiftUI, UIKit-Backed, Actually-Usable

RefreshableScrollView.swift

//
//  RefreshableScrollView.swift
//  --
//
//  Created by Bradley on 3/24/21.
//

import Combine
import SwiftUI
import UIKit

struct RefreshableScrollOptions {
  var refreshControlTintColor: UIColor? = nil
}

fileprivate struct RefreshableScrollViewRepresentable<Content: View>: UIViewControllerRepresentable {
  @Binding var isRefreshing: Bool
  let options: RefreshableScrollOptions
  let content: () -> Content
  
  @inlinable init(
    isRefreshing: Binding<Bool>,
    options: RefreshableScrollOptions = RefreshableScrollOptions(),
    @ViewBuilder content: @escaping () -> Content
  ) {
    self._isRefreshing = isRefreshing
    self.options = options
    self.content = content
  }
  
  func makeCoordinator() -> Coordinator {
    Coordinator(self)
  }

  func makeUIViewController(
    context: Context
  ) -> UIScrollViewViewController<Content> {
    // Create scroll view passed to UIScrollViewViewController. We do so here so
    // that this view representable may manage the scroll view's refresh
    // control.
    let scrollView = UIScrollView(frame: .zero)
    
    // Create scroll view's wrapping view controller.
    let scrollViewController = UIScrollViewViewController<Content>(
      scrollView: scrollView
    )

    // Create refresh control. Note that we could customize this in the future.
    let refreshControl = UIRefreshControl()
    refreshControl.tintColor = options.refreshControlTintColor
    
    // Add delegate to watch for when the user scrolls.
    refreshControl.addTarget(
      context.coordinator,
      action: #selector(Coordinator.didPullToRefresh),
      for: .valueChanged
    )

    // Attach the refresh control to the scroll view.
    scrollView.refreshControl = refreshControl
    
    return scrollViewController
  }

  func updateUIViewController(
    _ scrollViewController: UIScrollViewViewController<Content>,
    context: Context
  ) {
    // Complete refreshing. This just works.
    if !self.isRefreshing {
      scrollViewController.scrollView.refreshControl?.endRefreshing()
    }
    
    // Update scroll view contents.
    scrollViewController.updateContent(content: content())
  }
  
  class Coordinator: NSObject {
    var parent: RefreshableScrollViewRepresentable
    
    init(_ parent: RefreshableScrollViewRepresentable) {
      self.parent = parent
    }
    
    @objc func didPullToRefresh() {
      parent._isRefreshing.wrappedValue = true
    }
  }
}

fileprivate class UIScrollViewViewController<Content: View>: UIViewController {
  var hostingController: UIHostingController<Content?> =
    UIHostingController(rootView: nil)
  
  var scrollView: UIScrollView
  
  init(
    scrollView: UIScrollView
  ) {
    self.scrollView = scrollView
    
    super.init(nibName: nil, bundle: nil)
  }
  
  required init?(coder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
  
  override func viewDidLoad() {
    super.viewDidLoad()
    
    self.view.addSubview(self.scrollView)
    self.pinEdges(of: self.scrollView, to: self.view)
    
    // Make hosting controller's background transparent. This will allow
    // SwiftUI to handle view styling without interference.
    hostingController.view.backgroundColor = UIColor(white: 1.0, alpha: 0.0)
    
    // Begin adding content view to scroll view.
    hostingController.willMove(toParent: self)
    addChild(hostingController)

    // Set content view within scroll view.
    scrollView.addSubview(hostingController.view)
    pinEdges(of: hostingController.view, to: scrollView)

    // Finalize adding content view to scroll view.
    hostingController.didMove(toParent: self)
  }

  func updateContent(content: Content) {
    hostingController.rootView = content
  }

  func pinEdges(of viewA: UIView, to viewB: UIView) {
    viewA.translatesAutoresizingMaskIntoConstraints = false
    viewB.addConstraints([
      viewA.leadingAnchor.constraint(equalTo: viewB.leadingAnchor),
      viewA.trailingAnchor.constraint(equalTo: viewB.trailingAnchor),
      viewA.topAnchor.constraint(equalTo: viewB.topAnchor),
      viewA.bottomAnchor.constraint(equalTo: viewB.bottomAnchor)
    ])
  }
}

// SwiftUI wrapper that will also handle necessary GeometryReader required to
// fit the SwiftUI contents (`content`) within the UIKit UIScrollView
// appropriately.
struct RefreshableScrollView<Content: View>: View {
  @Binding var isRefreshing: Bool
  let options: RefreshableScrollOptions
  let content: () -> Content
  
  init(
    isRefreshing: Binding<Bool>,
    options: RefreshableScrollOptions = RefreshableScrollOptions(),
    @ViewBuilder content: @escaping () -> Content
  ) {
    self._isRefreshing = isRefreshing
    self.options = options
    self.content = content
  }
  
  var body: some View {
    GeometryReader { geometry in
      RefreshableScrollViewRepresentable(
        isRefreshing: $isRefreshing,
        options: options
      ) {
        ZStack {
          content()
        }
        .frame(
          width: geometry.size.width,
          alignment: .top
        )
      }
    }
  }
}

// Example Usage and Preview Content for xCode.
fileprivate struct PreviewView: View {
  @State var isRefreshing = false
  @State var now = Date()
  @State var rowCount = 1
  
  var body: some View {
    // Wrapped in NavigationView as example, as a NavigationView causes many
    // bugs in other solutions found.
    NavigationView {
      VStack {
        RefreshableScrollView(
          isRefreshing: $isRefreshing,
          options: RefreshableScrollOptions(
            refreshControlTintColor: .white
          )
        ) {
          VStack(spacing: 10.0) {
            ForEach(0..<rowCount, id: \.self) {
              Text("\(Calendar.current.date(byAdding: .hour, value: $0, to: now)!)")
                .frame(
                  minWidth: 0.0,
                  maxWidth: .infinity,
                  minHeight: 60.0,
                  maxHeight: 60.0,
                  alignment: .center
                )
                .background(Color.purple)
            }
            .padding(
              EdgeInsets(
                top: 5.0,
                leading: 10.0,
                bottom: 5.0,
                trailing: 10.0
              )
            )
          }
        }
      }
      .onChange(of: isRefreshing) { newIsRefreshing in
        if newIsRefreshing {
          DispatchQueue.main.asyncAfter(deadline: .now() + 2.0) {
            self.isRefreshing = false
            self.now = Date()
            self.rowCount += 1
          }
        }
      }
      .frame(
        maxWidth: .infinity,
        maxHeight: 400.0,
        alignment: .center
      )
      .background(Color.green400)
    }
  }
}

struct RefreshableScrollView_Previews: PreviewProvider {
  static var previews: some View {
    PreviewView()
      .previewDevice(PreviewDevice(rawValue: "iPhone 12 Pro Max"))
      .previewDisplayName("iPhone 12 Pro Max")
  }
}

How would customization of the pull to refresh view work? (changing size of indicator, scroll threshold)

Not working properly, this is preventing list cells from being reused, all cells are loaded in advance. Poor performance.

great work