Task.startOnMainActor Example

RootViewController.swift

import UIKit

class KPView: UIView {
  override init(frame: CGRect) {
    super.init(frame: frame)
  }

  @available(*, unavailable)
  required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }
}

class KPButton: KPView {
  var callback:  (@Sendable @MainActor () async throws -> Void)?

  var isEnabled = true
  private(set) var isLoading = false

  private var loadingTask: Task<Void, Never>?

  private let loadingIndicator: UIActivityIndicatorView = {
    let view = UIActivityIndicatorView(style: .medium)
    view.hidesWhenStopped = true
    return view
  }()

  override init(frame: CGRect) {
    super.init(frame: frame)
    layer.cornerRadius = 8
    backgroundColor = .red.withAlphaComponent(0.2)

    addSubview(loadingIndicator)
  }

  override func sizeThatFits(_ size: CGSize) -> CGSize {
    return CGSize(width: 200, height: 40)
  }

  override func layoutSubviews() {
    super.layoutSubviews()
    loadingIndicator.sizeToFit()
    loadingIndicator.center = CGPoint(x: bounds.midX, y: bounds.midY)
  }

  override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesEnded(touches, with: event)

    if
      isEnabled,
      !isLoading,
      let callback,
      touches.count == 1,
      let touch = touches.first,
      bounds.contains(touch.location(in: self))
    {
      isLoading = true

      // Change this to force flickering of the loadingIndicator because of the
      // startAnimating()/stopAnimating() calls in different RunLoop events.
      let showFlicker = true

      if showFlicker {
        loadingTask = Task { [weak self] in
          do {
            try await callback()
            self?.finishLoading()
          } catch {
            self?.finishLoading()
          }
        }
      } else {
        loadingTask = Task.startOnMainActor { [weak self] in
          do {
            try await callback()
            self?.finishLoading()
          } catch {
            self?.finishLoading()
          }
        }
      }

      loadingTask = Task.startOnMainActor { [weak self] in
        do {
          try await callback()
          self?.finishLoading()
        } catch {
          self?.finishLoading()
        }
      }

      if isLoading {
        loadingIndicator.startAnimating()
      } else {
        loadingTask = nil
      }
    }
  }

  private func finishLoading() {
    isLoading = false
    loadingIndicator.stopAnimating()
    loadingTask = nil
  }
}

class KPViewController: UIViewController {
  init() {
    super.init(nibName: nil, bundle: nil)
  }

  @available(*, unavailable)
  required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }
}

class RootViewController: KPViewController {
  private let button = KPButton()

  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .white
    view.addSubview(button)

    // Does not ficker if using Task.startOnMainActor
    button.callback = {
      print("loading stuff")
    }
    
    // Uncomment to show a loading indicator on the "button".
    // button.callback = {
    //   print("loading stuff")
    //   try await Task.sleep(for: .seconds(1))
    //   print("finished loading stuff")
    // }
  }

  override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    button.sizeToFit()
    button.center = view.center
  }
}