private void DidShowViewController(UINavigationController navigationController, UIViewController viewController, bool animated)
{
TraceViewControllers(nameof(DidShowViewController), viewController);
if (!(viewController is PageViewController pageViewController))
{
// When the ViewController isn't a PageViewController, it means it doesn't have anything to do with the Frame.
// It's possibly a modal ViewController.
// We just ignore it.
return;
}
var lastRequest = pageViewController.AssociatedRequests.LastOrDefault();
if (lastRequest != null)
{
// Mark the request as handled by the NavigationController.
lastRequest.WasHandledByController = true;
if (lastRequest == _controllerToFrameRequest)
{
_frameToControllerRequests.Remove(_controllerToFrameRequest);
_controllerToFrameRequest = null;
}
else
{
if (TryGetLast(_frameToControllerRequests, out var frameRequest))
{
if (NavigationRequest.Correlates(lastRequest, frameRequest))
{
// Now that the NavigationController handled the frame request, we remove it from the list.
_frameToControllerRequests.RemoveLast();
}
else
{
// Note for the future: We might be able to improve this by reseting the Frame to the NavigationController's content.
// However, this case doesn't seem to really happen.
// Something bad happened. We clear the request queue to try to recover.
_frameToControllerRequests.Clear();
this.Log().Error($"Can't process DidShowViewController because the last request doesn't match the current request.");
}
}
else
{
// It's possible that the NavigationController is the source of this event. When that's the case, the list of Frame requests is possibly empty.
}
}
}
else
{
this.Log().Error($"Can't process DidShowViewController because the current PageViewController's AssociatedRequests list is empty.");
}
}
/// <summary>
/// This is called on <see cref="Frame.Navigating"/>.
/// We use this handler to cancel the navigation when the request conflicts with the <see cref="NavigationController"/>.
/// </summary>
private void OnFrameNavigating(object sender, NavigatingCancelEventArgs e)
{
if (e.Cancel)
{
// If something cancelled the navigation, we simply ignore the event.
return;
}
var frameRequest = new NavigationRequest(_frame, e);
if (_controllerToFrameRequest != null)
{
// We get here when the UINavigationController initiated a navigation (like a back swipe) that is being executed by the Frame.
if (NavigationRequest.Correlates(frameRequest, _controllerToFrameRequest))
{
// We queue the request so that we can handle it in OnFrameNavigated and ignore it in OnFrameBackStackChanged.
_frameToControllerRequests.AddFirst(_controllerToFrameRequest);
}
else
{
// When the Frame's request doesn't matche the UINavigationController's request. We cancel the Frame's request.
// Ex: The UINavigationController is doing a native back, but the Frame wants to go forward.
// This sequencing can happen when you press back during an ViewModel operation that usually ends with a navigation.
e.Cancel = true;
if (this.Log().IsEnabled(LogLevel.Debug))
{
this.Log().Debug("Cancelled frame navigating request because a native navigation is in progress.");
}
}
}
else
{
// We queue the request so that we can handle it in OnFrameNavigated and ignore it in OnFrameBackStackChanged.
_frameToControllerRequests.AddFirst(frameRequest);
}
}
/// <summary>
/// This is called on <see cref="Frame.NavigationStopped"/>.
/// We use this handler to remove requests cancelled by <see cref="NavigatingCancelEventArgs.Cancel"/>.
/// </summary>
private void OnFrameNavigationStopped(object sender, NavigationEventArgs e)
{
var request = new NavigationRequest(_frame, e);
if (TryGetFirst(_frameToControllerRequests, out var frameToControllerRequest) && NavigationRequest.Correlates(request, frameToControllerRequest))
{
if (this.Log().IsEnabled(LogLevel.Debug))
{
this.Log().Debug("Aborted navigation request because the Frame.Navigating event was cancelled.");
}
_frameToControllerRequests.RemoveFirst();
}
else
{
// We shouldn't get here because the frame events are synchronous.
this.Log().Error($"Can't process OnFrameNavigationStopped because the request in queue doesn't match the current request.");
}
}
/// <summary>
/// This is called on <see cref="Frame.Navigated"/>.
/// We use this handler to create requets for the <see cref="NavigationController"/>.
/// </summary>
private void OnFrameNavigated(object sender, NavigationEventArgs e)
{
// We create a request object from the current state. We only use this object to correlate it with existing requests.
var request = new NavigationRequest(_frame, e);
if (TryGetFirst(_frameToControllerRequests, out var frameRequest) && NavigationRequest.Correlates(request, frameRequest))
{
// Mark the request as handled by the frame because we're in the Navigated handler.
frameRequest.WasHandledByFrame = true;
if (frameRequest == _controllerToFrameRequest)
{
// If the request is the one created by the NavigationController, we don't have to do anything at this point.
// The DidShowViewController method will simply remove it from the list once it gets called.
}
else
{
// Get the page from the event args.
var page = e.Content as Page;
// Use that page to get the native ViewController.
var viewController = page.FindViewController() ?? new PageViewController(page);
// If that ViewController is a PageViewController, we add the request to its list.
(viewController as PageViewController)?.AssociatedRequests.Add(frameRequest);
// We get the isAnimated flag from the transition info.
var isAnimated = GetIsAnimated(frameRequest.TransitionInfo);
switch (frameRequest.NavigationMode)
{
case NavigationMode.Back:
if (this.Log().IsEnabled(LogLevel.Debug))
{
this.Log().Debug("Poping ViewController to replicate Frame's back navigation.");
}
NavigationController.PopViewController(isAnimated);
break;
case NavigationMode.Forward:
case NavigationMode.New:
if (this.Log().IsEnabled(LogLevel.Debug))
{
this.Log().Debug($"Pushing ViewController ({page.GetType().Name}) to replicate Frame's forward navigation.");
}
NavigationController.PushViewController(viewController, isAnimated);
break;
case NavigationMode.Refresh:
default:
// Refresh currently doesn't have an effect.
break;
}
}
}
else
{
// We shouldn't get here because the frame events are synchronous.
if (frameRequest == null)
{
this.Log().Error($"Can't process OnFrameNavigated because the request queue is empty.");
}
else
{
this.Log().Error($"Can't process OnFrameNavigated because the request in queue doesn't match the current request.");
}
}
}