public virtual void RequestNavigation(NavigationRequest e)
{
if (_navGraph == null)
{
var navGraphNavigator =
(NavGraphNavigator)NavHost
.NavController
.NavigatorProvider
.GetNavigator(Java.Lang.Class.FromType(typeof(NavGraphNavigator)));
_navGraph = new NavGraph(navGraphNavigator);
var callbacks = new Callbacks(this);
NavHost.NavController.AddOnDestinationChangedListener(callbacks);
NavHost.ChildFragmentManager.RegisterFragmentLifecycleCallbacks(callbacks, false);
}
ApplyNavigationRequest(e);
}
/*
* The important thing to know going into reading this method is that it's not possible to
* modify the backstack. You can only push and pop to and from the top of the stack.
* So if a user uses an API like `RemovePage` or `InsertPage` we will typically ignore processing those natively
* unless it requires changes to the NavBar (i.e removing the first page with only 2 pages on the stack).
* Once the user performs an operation that changes the currently visible page then we process any stack changes
* that have occurred.
* Let's say the user has pages A,B,C,D on the stack
* If they remove Page B and Page C then we don't do anything. Then if the user pushes E onto the stack
* we just transform A,B,C,D into A,D,E.
* Natively that's a "pop" but we use the correct animation for a "push" so visually it looks like a push.
* This is also the reason why we aren't using the custom animation features on the navigation component itself.
* Because we might be natively popping but visually pushing.
*
* The Fragments that are on the stack also do not have a hard connection to the page they originally rendereded.
* Whenever a fragment is the "visible" fragment it just figures out what the current page is and displays that.
* Fragments are recreated everytime they are pushed on the stack but the handler renderer is not.
* It's just attached to a new fragment
* */
void ApplyNavigationRequest(NavigationRequest args)
{
if (IsNavigating)
{
// This should really never fire for the developer. Our xplat code should be handling waiting for navigation to
// complete before requesting another navigation from Core
// Maybe some day we'll put a navigation queue into Core? For now we won't
throw new InvalidOperationException("Previous Navigation Request is still Processing");
}
if (args.NavigationStack.Count == 0)
{
throw new InvalidOperationException("NavigationStack cannot be empty");
}
ActiveRequestedArgs = args;
IReadOnlyList <IView> newPageStack = args.NavigationStack;
bool animated = args.Animated;
var navController = NavHost.NavController;
var previousNavigationStack = NavigationStack;
var previousNavigationStackCount = previousNavigationStack.Count;
bool initialNavigation = NavigationStack.Count == 0;
// This updates the graphs public navigation stack property so it's outwardly correct
// But we've saved off the previous stack so we can correctly interpret navigation
UpdateNavigationStack(newPageStack);
// This indicates that this is the first navigation request so we need to initialize the graph
if (initialNavigation)
{
IsInitialNavigation = true;
Initialize(args.NavigationStack);
return;
}
// If the new stack isn't changing the visible page or the app bar then we just ignore
// the changes because there's no point to applying these to the native back stack
// We only apply changes when the currently visible page changes and/or the appbar
// will change (gain a back button)
if (newPageStack[newPageStack.Count - 1] == previousNavigationStack[previousNavigationStackCount - 1] &&
newPageStack.Count > 1 &&
previousNavigationStackCount > 1)
{
NavigationFinished(NavigationView);
return;
}
// The incoming fragment uses these variables to pick the correct animation for the current
// incoming navigation request
if (newPageStack[newPageStack.Count - 1] == previousNavigationStack[previousNavigationStackCount - 1])
{
IsPopping = null;
}
else
{
IsPopping = newPageStack.Count < previousNavigationStackCount;
}
IsAnimated = animated;
var iterator = NavHost.NavController.BackStack.Iterator();
var fragmentNavDestinations = new List <FragmentNavigator.Destination>();
while (iterator.HasNext)
{
if (iterator.Next() is NavBackStackEntry nbse &&
nbse.Destination is FragmentNavigator.Destination nvd)
{
fragmentNavDestinations.Add(nvd);
}
}
// Current BackStack has less entries then incoming new page stack
// This will add Back Stack Entries until the back stack and the new stack
// match up
if (fragmentNavDestinations.Count < newPageStack.Count)
{
for (int i = fragmentNavDestinations.Count; i < newPageStack.Count; i++)
{
var dest = AddFragmentDestination();
navController.Navigate(dest.Id);
}
}
// User just wants to replace the currently visible page but the number
// of items in the stack are still the same.
// In theory we could just prompt the currently active fragment to swap out the new PageView
// But this way we get an animation
else if (newPageStack.Count == fragmentNavDestinations.Count)
{
int lastFragId = fragmentNavDestinations[newPageStack.Count - 1].Id;
navController.PopBackStack();
navController.Navigate(lastFragId);
}
// Our back stack has more entries on it then
else
{
int popToId = fragmentNavDestinations[newPageStack.Count - 1].Id;
navController.PopBackStack(popToId, false);
}
// We only keep destinations around that are on the backstack
// This iterates over the new backstack and removes any destinations
// that are no longer apart of the back stack
var iterateNewStack = NavHost.NavController.BackStack.Iterator();
int startId = -1;
while (iterateNewStack.HasNext)
{
if (iterateNewStack.Next() is NavBackStackEntry nbse &&
nbse.Destination is FragmentNavigator.Destination nvd)
{
fragmentNavDestinations.Remove(nvd);
if (startId == -1)
{
startId = nvd.Id;
}
}
}
foreach (var activeDestinations in fragmentNavDestinations)
{
NavGraph.Remove(activeDestinations);
}
// If we end up removing the destination that was initially the StartDestination
// The Navigation Graph can get really confused
if (NavGraph.StartDestination != startId)
{
NavGraph.StartDestination = startId;
}
}