Widget build(BuildContext context) {
  Widget navigator;
    if (_navigator != null) {
    navigator = Navigator(
      key: _navigator,
      // If window.defaultRouteName isn't '/', we should assume it was set
      // intentionally via `setInitialRoute`, and should override whatever
      // is in [widget.initialRoute].
      initialRoute: WidgetsBinding.instance.window.defaultRouteName != Navigator.defaultRouteName
          ? WidgetsBinding.instance.window.defaultRouteName
          : widget.initialRoute ?? WidgetsBinding.instance.window.defaultRouteName,
      onGenerateRoute: _onGenerateRoute,
      onGenerateInitialRoutes: widget.onGenerateInitialRoutes == null
        ? Navigator.defaultGenerateInitialRoutes
        : (NavigatorState navigator, String initialRouteName) {
          return widget.onGenerateInitialRoutes(initialRouteName);
        },
      onUnknownRoute: _onUnknownRoute,
      observers: widget.navigatorObservers,
    );
  }
  ...
}

void initState() {
  super.initState();
  insertAll(widget.initialEntries);
}

Widget build(BuildContext context) {
  // These lists are filled backwards. For the offstage children that
  // does not matter since they aren't rendered, but for the onstage
  // children we reverse the list below before adding it to the tree.
  final List<Widget> onstageChildren = <Widget>[];
  final List<Widget> offstageChildren = <Widget>[];
  bool onstage = true;
  for (int i = _entries.length - 1; i >= 0; i -= 1) {
    final OverlayEntry entry = _entries[i];
    if (onstage) {
      onstageChildren.add(_OverlayEntry(entry));
      if (entry.opaque)
        onstage = false;
    } else if (entry.maintainState) {
      offstageChildren.add(TickerMode(enabled: false, child: _OverlayEntry(entry)));
    }
  }
  return _Theatre(
    onstage: Stack(
      fit: StackFit.expand,
      children: onstageChildren.reversed.toList(growable: false),
    ),
    offstage: offstageChildren,
  );
}

Route<T> _routeNamed<T>(String name, { @required Object arguments, bool allowNull = false }) {
  if (allowNull && widget.onGenerateRoute == null)
    return null;
  final RouteSettings settings = RouteSettings(
    name: name,
    arguments: arguments,
  );
  Route<T> route = widget.onGenerateRoute(settings) as Route<T>;
  if (route == null && !allowNull) {
    route = widget.onUnknownRoute(settings) as Route<T>;
  }
  return route;
}

Route<dynamic> _onGenerateRoute(RouteSettings settings) {
  final String name = settings.name;
  final WidgetBuilder pageContentBuilder = name == Navigator.defaultRouteName && widget.home != null
      ? (BuildContext context) => widget.home
      : widget.routes[name];
  if (pageContentBuilder != null) {
    final Route<dynamic> route = widget.pageRouteBuilder<dynamic>(
      settings,
      pageContentBuilder,
    );
    return route;
  }
  if (widget.onGenerateRoute != null)
    return widget.onGenerateRoute(settings);
  return null;
}

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
  assert(_debugLocked && !_debugUpdatingPage);
  // Clean up the list, sending updates to the routes that changed. Notably,
  // we don't send the didChangePrevious/didChangeNext updates to those that
  // did not change at this point, because we're not yet sure exactly what the
  // routes will be at the end of the day (some might get disposed).
  int index = _history.length - 1;
  _RouteEntry next;
  _RouteEntry entry = _history[index];
  _RouteEntry previous = index > 0 ? _history[index - 1] : null;
  bool canRemoveOrAdd = false; // Whether there is a fully opaque route on top to silently remove or add route underneath.
  Route<dynamic> poppedRoute; // The route that should trigger didPopNext on the top active route.
  bool seenTopActiveRoute = false; // Whether we've seen the route that would get didPopNext.
  final List<_RouteEntry> toBeDisposed = <_RouteEntry>[];
  while (index >= 0) {
    switch (entry.currentState) {
        // ...
    }
    index -= 1;
    next = entry;
    entry = previous;
    previous = index > 0 ? _history[index - 1] : null;
  }
  // Now that the list is clean, send the didChangeNext/didChangePrevious
  // notifications.
  _flushRouteAnnouncement();
  // Announces route name changes.
  final _RouteEntry lastEntry = _history.lastWhere(_RouteEntry.isPresentPredicate, orElse: () => null);
  final String routeName = lastEntry?.route?.settings?.name;
  if (routeName != _lastAnnouncedRouteName) {
    RouteNotificationMessages.maybeNotifyRouteChange(routeName, _lastAnnouncedRouteName);
    _lastAnnouncedRouteName = routeName;
  }
  // Lastly, removes the overlay entries of all marked entries and disposes
  // them.
  for (final _RouteEntry entry in toBeDisposed) {
    for (final OverlayEntry overlayEntry in entry.route.overlayEntries)
      overlayEntry.remove();
    entry.dispose();
  }
  if (rearrangeOverlay)
    overlay?.rearrange(_allRouteOverlayEntries);
}

void _flushRouteAnnouncement() {
  int index = _history.length - 1;
  while (index >= 0) {
    final _RouteEntry entry = _history[index];
    if (!entry.suitableForAnnouncement) {
      index -= 1;
      continue;
    }
    final _RouteEntry next = _getRouteAfter(index + 1, _RouteEntry.suitableForTransitionAnimationPredicate);
    if (next?.route != entry.lastAnnouncedNextRoute) {
      if (entry.shouldAnnounceChangeToNext(next?.route)) {
        entry.route.didChangeNext(next?.route);
      }
      entry.lastAnnouncedNextRoute = next?.route;
    }
    final _RouteEntry previous = _getRouteBefore(index - 1, _RouteEntry.suitableForTransitionAnimationPredicate);
    if (previous?.route != entry.lastAnnouncedPreviousRoute) {
      entry.route.didChangePrevious(previous?.route);
      entry.lastAnnouncedPreviousRoute = previous?.route;
    }
    index -= 1;
  }
}

enum _RouteLifecycle {
  staging, // we will wait for transition delegate to decide what to do with this route.
  //
  // routes that are present:
  //
  add, // we'll want to run install, didAdd, etc; a route created by onGenerateInitialRoutes or by the initial widget.pages
  adding, // we'll want to run install, didAdd, etc; a route created by onGenerateInitialRoutes or by the initial widget.pages
  // routes that are ready for transition.
  push, // we'll want to run install, didPush, etc; a route added via push() and friends
  pushReplace, // we'll want to run install, didPush, etc; a route added via pushReplace() and friends
  pushing, // we're waiting for the future from didPush to complete
  replace, // we'll want to run install, didReplace, etc; a route added via replace() and friends
  idle, // route is being harmless
  //
  // routes that are not present:
  //
  // routes that should be included in route announcement and should still listen to transition changes.
  pop, // we'll want to call didPop
  remove, // we'll want to run didReplace/didRemove etc
  // routes should not be included in route announcement but should still listen to transition changes.
  popping, // we're waiting for the route to call finalizeRoute to switch to dispose
  removing, // we are waiting for subsequent routes to be done animating, then will switch to dispose
  // routes that are completely removed from the navigator and overlay.
  dispose, // we will dispose the route momentarily
  disposed, // we have disposed the route
}

void initState() {
  super.initState();
  for (final NavigatorObserver observer in widget.observers) {
    assert(observer.navigator == null);
    observer._navigator = this;
  }
  String initialRoute = widget.initialRoute;
  if (widget.pages.isNotEmpty) {
    _history.addAll(
      widget.pages.map((Page<dynamic> page) => _RouteEntry(
        page.createRoute(context),
        initialState: _RouteLifecycle.add,
      ))
    );
  } else {
    // If there is no page provided, we will need to provide default route
    // to initialize the navigator.
    initialRoute = initialRoute ?? Navigator.defaultRouteName;
  }
  if (initialRoute != null) {
    _history.addAll(
      widget.onGenerateInitialRoutes(
        this,
        widget.initialRoute ?? Navigator.defaultRouteName
      ).map((Route<dynamic> route) =>
        _RouteEntry(
          route,
          initialState: _RouteLifecycle.add,
        ),
      ),
    );
  }
  _flushHistoryUpdates();
}

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
  // ...
  while (index >= 0) {
    switch (entry.currentState) {
      case _RouteLifecycle.add:
        assert(rearrangeOverlay);
        entry.handleAdd(
          navigator: this,
        );
        assert(entry.currentState == _RouteLifecycle.adding);
        continue;
      case _RouteLifecycle.adding:
        if (canRemoveOrAdd || next == null) {
          entry.didAdd(
            navigator: this,
            previous: previous?.route,
            previousPresent: _getRouteBefore(index - 1, _RouteEntry.isPresentPredicate)?.route,
            isNewFirst: next == null
          );
          assert(entry.currentState == _RouteLifecycle.idle);
          continue;
        }
        break;
      case _RouteLifecycle.idle:
        if (!seenTopActiveRoute && poppedRoute != null)
          entry.handleDidPopNext(poppedRoute);
        seenTopActiveRoute = true;
        // This route is idle, so we are allowed to remove subsequent (earlier)
        // routes that are waiting to be removed silently:
        canRemoveOrAdd = true;
        break;
        // ...
    }
    index -= 1;
    next = entry;
    entry = previous;
    previous = index > 0 ? _history[index - 1] : null;
  }
  // ...
}

void handleAdd({ @required NavigatorState navigator}) {
  assert(currentState == _RouteLifecycle.add);
  assert(navigator != null);
  assert(navigator._debugLocked);
  assert(route._navigator == null);
  route._navigator = navigator;
  route.install();
  assert(route.overlayEntries.isNotEmpty);
  currentState = _RouteLifecycle.adding;
}

Iterable<OverlayEntry> createOverlayEntries() sync* {
  yield _modalBarrier = OverlayEntry(builder: _buildModalBarrier);
  yield OverlayEntry(builder: _buildModalScope, maintainState: maintainState);
}

Widget _buildModalBarrier(BuildContext context) {
  Widget barrier;
  if (barrierColor != null && !offstage) { // changedInternalState is called if these update
    assert(barrierColor != _kTransparent);
    final Animation<Color> color = animation.drive(
      ColorTween(
        begin: _kTransparent,
        end: barrierColor, // changedInternalState is called if this updates
      ).chain(_easeCurveTween),
    );
    barrier = AnimatedModalBarrier(
      color: color,
      dismissible: barrierDismissible, // changedInternalState is called if this updates
      semanticsLabel: barrierLabel, // changedInternalState is called if this updates
      barrierSemanticsDismissible: semanticsDismissible,
    );
  } else {
    barrier = ModalBarrier(
      dismissible: barrierDismissible, // changedInternalState is called if this updates
      semanticsLabel: barrierLabel, // changedInternalState is called if this updates
      barrierSemanticsDismissible: semanticsDismissible,
    );
  }
  return IgnorePointer(
    ignoring: animation.status == AnimationStatus.reverse || // changedInternalState is called when this updates
              animation.status == AnimationStatus.dismissed, // dismissed is possible when doing a manual pop gesture
    child: barrier,
  );
}

Widget _buildModalScope(BuildContext context) {
  return _modalScopeCache ??= _ModalScope<T>(
    key: _scopeKey,
    route: this,
    // _ModalScope calls buildTransitions() and buildChild(), defined above
  );
}

Widget build(BuildContext context) {
  return _ModalScopeStatus(
    route: widget.route,
    isCurrent: widget.route.isCurrent, // _routeSetState is called if this updates
    canPop: widget.route.canPop, // _routeSetState is called if this updates
    child: Offstage(
      offstage: widget.route.offstage, // _routeSetState is called if this updates
      child: PageStorage(
        bucket: widget.route._storageBucket, // immutable
        child: FocusScope(
          node: focusScopeNode, // immutable
          child: RepaintBoundary(
            child: AnimatedBuilder(
              animation: _listenable, // immutable
              builder: (BuildContext context, Widget child) {
                return widget.route.buildTransitions(
                  context,
                  widget.route.animation,
                  widget.route.secondaryAnimation,
                  IgnorePointer(
                    ignoring: widget.route.animation?.status == AnimationStatus.reverse,
                    child: child,
                  ),
                );
              },
              child: _page ??= RepaintBoundary(
                key: widget.route._subtreeKey, // immutable
                child: Builder(
                  builder: (BuildContext context) {
                    return widget.route.buildPage(
                      context,
                      widget.route.animation,
                      widget.route.secondaryAnimation,
                    );
                  },
                ),
              ),
            ),
          ),
        ),
      ),
    ),
  );
}

void didAdd({ @required NavigatorState navigator, @required bool isNewFirst, @required Route<dynamic> previous, @required Route<dynamic> previousPresent }) {
  route.didAdd();
  currentState = _RouteLifecycle.idle;
  if (isNewFirst) {
    route.didChangeNext(null);
  }
  for (final NavigatorObserver observer in navigator.widget.observers)
    observer.didPush(route, previousPresent);
}

Future<T> push<T extends Object>(Route<T> route) {
  assert(!_debugLocked);
  assert(() {
    _debugLocked = true;
    return true;
  }());
  assert(route != null);
  assert(route._navigator == null);
  _history.add(_RouteEntry(route, initialState: _RouteLifecycle.push));
  _flushHistoryUpdates();
  assert(() {
    _debugLocked = false;
    return true;
  }());
  _afterNavigation(route);
  return route.popped;
}

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
  // ...
  while (index >= 0) {
    switch (entry.currentState) {
      // ...
      case _RouteLifecycle.push:
      case _RouteLifecycle.pushReplace:
      case _RouteLifecycle.replace:
        assert(rearrangeOverlay);
        entry.handlePush(
          navigator: this,
          previous: previous?.route,
          previousPresent: _getRouteBefore(index - 1, _RouteEntry.isPresentPredicate)?.route,
          isNewFirst: next == null,
        );
        assert(entry.currentState != _RouteLifecycle.push);
        assert(entry.currentState != _RouteLifecycle.pushReplace);
        assert(entry.currentState != _RouteLifecycle.replace);
        if (entry.currentState == _RouteLifecycle.idle) {
          continue;
        }
        break;
      case _RouteLifecycle.pushing: // Will exit this state when animation completes.
        if (!seenTopActiveRoute && poppedRoute != null)
          entry.handleDidPopNext(poppedRoute);
        seenTopActiveRoute = true;
        break;
      case _RouteLifecycle.idle:
        if (!seenTopActiveRoute && poppedRoute != null)
          entry.handleDidPopNext(poppedRoute);
        seenTopActiveRoute = true;
        // This route is idle, so we are allowed to remove subsequent (earlier)
        // routes that are waiting to be removed silently:
        canRemoveOrAdd = true;
        break;
      // ...
    }
    index -= 1;
    next = entry;
    entry = previous;
    previous = index > 0 ? _history[index - 1] : null;
  }
  // ...
}

void pop<T extends Object>([ T result ]) {
  assert(!_debugLocked);
  assert(() {
    _debugLocked = true;
    return true;
  }());
  final _RouteEntry entry = _history.lastWhere(_RouteEntry.isPresentPredicate);
  if (entry.hasPage) {
    if (widget.onPopPage(entry.route, result))
      entry.currentState = _RouteLifecycle.pop;
  } else {
    entry.pop<T>(result);
  }
  if (entry.currentState == _RouteLifecycle.pop) {
    // Flush the history if the route actually wants to be popped (the pop
    // wasn't handled internally).
    _flushHistoryUpdates(rearrangeOverlay: false);
    assert(entry.route._popCompleter.isCompleted);
  }
  assert(() {
    _debugLocked = false;
    return true;
  }());
  _afterNavigation<dynamic>(entry.route);
}

bool didPop(T result) {
  final bool returnValue = super.didPop(result);
  assert(returnValue);
  if (finishedWhenPopped)
    navigator.finalizeRoute(this);
  return returnValue;
}

void finalizeRoute(Route<dynamic> route) {
  // FinalizeRoute may have been called while we were already locked as a
  // responds to route.didPop(). Make sure to leave in the state we were in
  // before the call.
  bool wasDebugLocked;
  assert(() { wasDebugLocked = _debugLocked; _debugLocked = true; return true; }());
  assert(_history.where(_RouteEntry.isRoutePredicate(route)).length == 1);
  final _RouteEntry entry =  _history.firstWhere(_RouteEntry.isRoutePredicate(route));
  if (entry.doingPop) {
    // We were called synchronously from Route.didPop(), but didn't process
    // the pop yet. Let's do that now before finalizing.
    entry.currentState = _RouteLifecycle.pop;
    _flushHistoryUpdates(rearrangeOverlay: false);
  }
  assert(entry.currentState != _RouteLifecycle.pop);
  entry.finalize();
  _flushHistoryUpdates(rearrangeOverlay: false);
  assert(() { _debugLocked = wasDebugLocked; return true; }());
}

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
  // ...
  while (index >= 0) {
    switch (entry.currentState) {
      // ...
      case _RouteLifecycle.pop:
        if (!seenTopActiveRoute) {
          if (poppedRoute != null)
            entry.handleDidPopNext(poppedRoute);
          poppedRoute = entry.route;
        }
        entry.handlePop(
          navigator: this,
          previousPresent: _getRouteBefore(index, _RouteEntry.willBePresentPredicate)?.route,
        );
        assert(entry.currentState == _RouteLifecycle.popping);
        canRemoveOrAdd = true;
        break;
      case _RouteLifecycle.popping:
        // Will exit this state when animation completes.
        break;
      case _RouteLifecycle.dispose:
        // Delay disposal until didChangeNext/didChangePrevious have been sent.
        toBeDisposed.add(_history.removeAt(index));
        entry = next;
        break;
      case _RouteLifecycle.disposed:
      case _RouteLifecycle.staging:
        assert(false);
        break;
    }
    index -= 1;
    next = entry;
    entry = previous;
    previous = index > 0 ? _history[index - 1] : null;
  }
  // ...
}

void _flushHistoryUpdates({bool rearrangeOverlay = true}) {
  // ...
  while (index >= 0) {
    switch (entry.currentState) {
        // ...
      case _RouteLifecycle.remove:
        if (!seenTopActiveRoute) {
          if (poppedRoute != null)
            entry.route.didPopNext(poppedRoute);
          poppedRoute = null;
        }
        entry.handleRemoval(
          navigator: this,
          previousPresent: _getRouteBefore(index, _RouteEntry.willBePresentPredicate)?.route,
        );
        assert(entry.currentState == _RouteLifecycle.removing);
        continue;
      case _RouteLifecycle.removing:
        if (!canRemoveOrAdd && next != null) {
          // We aren't allowed to remove this route yet.
          break;
        }
        entry.currentState = _RouteLifecycle.dispose;
        continue;
      case _RouteLifecycle.dispose:
        // Delay disposal until didChangeNext/didChangePrevious have been sent.
        toBeDisposed.add(_history.removeAt(index));
        entry = next;
        break;
      case _RouteLifecycle.disposed:
      case _RouteLifecycle.staging:
        assert(false);
        break;
    }
    index -= 1;
    next = entry;
    entry = previous;
    previous = index > 0 ? _history[index - 1] : null;
  }
  // ...
}