RemovingTransitionContext.swift

import UIKit
import FluidPortal

/// A context object to interact with container view controller for transitions.
@MainActor
public final class RemovingTransitionContext: TransitionContext {

  public enum CompletionEvent: Sendable {
    /// Transition has been finished (no interruption was in there)
    case succeeded
    /// Transition has been interrupted
    case interrupted
    /// Transition has been cancelled in interaction
    case cancelled
  }

  public private(set) var isCompleted: Bool = false

  /// A view controller for removing
  public let fromViewController: UIViewController

  /// A view controller that displays after removed
  public let toViewController: UIViewController?

  private let onAnimationCompleted: (RemovingTransitionContext) -> Void
  private let onRequestedDisplayOnTop:
    (DisplaySource) -> FluidStackController.DisplayingOnTopSubscription

  private var hasNotifiedCompletion: Bool = false
  private var callbacks: [(CompletionEvent) -> Void] = []

  init(
    contentView: FluidStackController.StackingPlatterView,
    fromViewController: UIViewController,
    toViewController: UIViewController?,
    onAnimationCompleted: @escaping (RemovingTransitionContext) -> Void,
    onRequestedDisplayOnTop: @escaping (DisplaySource) -> FluidStackController
      .DisplayingOnTopSubscription
  ) {
    self.fromViewController = fromViewController
    self.toViewController = toViewController
    self.onAnimationCompleted = onAnimationCompleted
    self.onRequestedDisplayOnTop = onRequestedDisplayOnTop
    super.init(contentView: contentView)
  }

  /**
   Notifies controller transition has been completed.
   */
  public func notifyAnimationCompleted() {
    assert(Thread.isMainThread)
    guard isInvalidated == false, isCompleted == false else { return }
    isCompleted = true
    onAnimationCompleted(self)
  }

  public func notifyCancelled() {
    assert(Thread.isMainThread)
    guard isInvalidated == false, isCompleted == false else { return }
    isInvalidated = true
    hasNotifiedCompletion = true
    callbacks.forEach { $0(.cancelled) }
    onAnimationCompleted(self)
  }

  /**
   Makes toViewController disabled in user-interaction until finish transition.
   */
  public func disableUserInteractionUntileFinish() {

    func run(viewController: UIViewController) {

      // TODO: there is re-entrancy issue in running transitions simultaneously.

      viewController.view.isUserInteractionEnabled = false

      addCompletionEventHandler { [weak viewController] _ in
        viewController?.view.isUserInteractionEnabled = true
      }
    }

    toViewController.map {
      run(viewController: $0)
    }

    run(viewController: fromViewController)

  }

  public func requestDisplayOnTop(_ source: DisplaySource)
    -> FluidStackController.DisplayingOnTopSubscription
  {
    onRequestedDisplayOnTop(source)
  }

  /// Marks as this current transition has been outdated.
  /// Another transition's started by owner.
  /// Triggers ``addCompletionEventHandler(_:)`` with ``TransitionContext/CompletionEvent/interrupted``
  override func invalidate() {
    assert(Thread.isMainThread)
    isInvalidated = true
    guard hasNotifiedCompletion == false else { return }
    hasNotifiedCompletion = true
    callbacks.forEach { $0(.interrupted) }
  }

  /**
   Adds closure that handles completion events (``CompletionEvent``)

   Use-cases:
   - Trigger to clean up transient resources for this transition.
   */
  public func addCompletionEventHandler(_ closure: @escaping (CompletionEvent) -> Void) {
    assert(Thread.isMainThread)
    callbacks.append(closure)
  }

  /**
   Triggers ``addCompletionEventHandler(_:)`` with ``TransitionContext/CompletionEvent/succeeded``
   */
  func transitionSucceeded() {
    guard hasNotifiedCompletion == false else { return }
    hasNotifiedCompletion = true
    callbacks.forEach { $0(.succeeded) }
  }

  deinit {

  }

}