In PrismForms we got the problem, that the NavigationStack is empty after navigating to a new page. That means after using the hardware back-button on the SecondPage, the app is closed. Although the back-arrow in the header on Android isnt shown. If looking closely you can see the back-arrow for a short moment after the page is switched. I guess thats before the NavigationStack gets cleared.
To the first page we navigate with the following command in OnInitialized() in our App.xaml.cs which derives from PrismApplication.
NavigationService.NavigateAsync("NavigationPage/StartPage");
(If only Navigating to „StartPage“ here, the Stack doesnt get cleared.)
That has do to with PageNavigationService.ProcessNavigationForNavigationPage(...) calling
bool clearNavStack = GetClearNavigationPageNavigationStack(currentPage); and PageNavigationService.ProcessNavigationForContentPage(...) not.
From the StartPage to the next we navigate with NavigateAsync("SecondPage")“. Here the described behaviour appears.
For navigation we use a class which wraps the Prism NavigationService. We hold him as a property and get him via Unity in our constructor:
this.PrismNavigation = prismNavigation ?? throw new ArgumentNullException(nameof(prismNavigation));
The methods „NavigateAsync“ and „GoBackAsync“, etc. we just pass through.
This way we want to seperate our ViewModel-Project from references to XamarinForms to later be able to use the same ViewModels for for example a WPF-GUI.
Why is the stack beeing cleared by our own NavigationService? If we register the original Prism NavigationService in App.xaml.cs instead, navigating back works as expected again. We found the point in the framework and could avoid the clearing with a drity hack, but that’s against the navigation-logic implemented in PrismForms, but we don’t understand how to do it the correct way.
Every help appreciated!
We edited a few things to get it working after finding some interesting information by Brian Lagunas in the forlast-post here:
https://github.com/PrismLibrary/Prism/issues/591
Although the topic was about something else, it led to improvements for overwriting the Navigation Service.
Remember that in your viewModels the Navigation Service must be named "navigationService" by convention. Also we switched from just holding the Prism Navigation Service as a parameter to deriving from it as suggested in the link above.
public class MyNavigationService : UnityPageNavigationService
Related
I have an app with four main pages, switched through a tab bar (no "back" button).
One page has a lot of content (ScrollView) and takes quite a few seconds until it's rendered. I handle that by showing a "loading" overlay while the work is done. But for that specific page I'd like to keep the view alive, so that when the user switches to another page and comes back later, the page is ready without loading everything again.
I'm not sure how to do that in MvvmCross, though.
I did read the documentation and from what I understood the View Presenter would be the right way to do it, since the docs say:
"Another kind of presentation changes your app can request through
hints includes clearing / modifying the BackStack, changing a root
while maintaining the existent views, … possibilities are really
endless. Once again your app is king here!"
I guess I would need to create a custom MvxPresentationHint for that, but I don't quite get it :(
How or rather where would I access and store/load the View?
I'm generally still quite unfamiliar with MvvmCross (how it works under the hood) and especially customization of Mvx classes, even though I've been using it for a while.
Any explanation and preferably code examples beyond what's written in the documentation would be extremely appreciated!
It isn't meaningful to attempt to "store" a view in MVVM. The XF view is a representation of what will be created with native (e.g. "Android" or "iOS") widgets. Creating and measuring/laying out those native widgets is what is slow. MVVM View Presenter won't speed up that logic.
Instead of "store", you need "keep alive":
For a ContentPage called MyPage, when you create it, store it in a static variable. Then re-use that variable. If you never need more than one of these, you can store it in the class itself.
Modify the "code behind", MyPage.xaml.cs:
public partial class MyPage : ContentPage
{
// Singleton Pattern.
private static MyPage _it;
public static MyPage It {
get {
if (_it == null)
_it = new MyPage();
return _it;
}
}
// "private", because calling this directly defeats the purpose. Instead, use `MyPage.It`.
private MyPage()
{
InitializeComponent();
}
}
To create it, whereever you would put:
new MyPage()
instead put this:
MyPage.It
For instance, you might do PushAsync(MyPage.It);
This will always return the SAME INSTANCE of MyPage. So once it has been created, it keeps its state.
IMPORTANT: Note that the constructor is only called ONCE. Any code that needs to be done each time the page appears, put in override .. OnAppearing() method.
LIMITATION: Views "expect" to be part of the visual hierarchy when they are manipulated. If you attempt to alter the page or its view model while it is not on the screen, you may encounter problems. Those are beyond the scope of this answer - create a new StackOverflow question with the details of any problem you encounter.
I have an issue where several of my pages are using the SfBackdropPage control from Syncfusion. This control requires the page to have a base of SfBackdropPage and not the usual ContentPage otherwise it just won't work.
To make MVVMCross work, the page needs have a base such as MvxContentPage
You see where this is going ?
I opened a ticket with Syncfusion to see if they could work around this issue but basically they just said its not possible. So since I needed to have that control as part of the page I had to leave the base as it was.
How do I setup MVVMCross to work with these pages without having the page inherit from MvxContentPage ?
I've used the following in the setup.cs in the Android project:
protected override IMvxViewsContainer InitializeViewLookup(IDictionary<Type, Type> viewModelViewLookup)
{
viewModelViewLookup.Add(typeof(SitesViewModel), typeof(SitesView));
return base.InitializeViewLookup(viewModelViewLookup);
}
I can navigate to the page but I get a null reference exception due to the binding context not been set. I've tried to set this in the XAML but it requires a parameterless constructor but that's not possible as the view model uses Dependency Injection as it relies on these object been passed in.
Can anyone give any pointers, I'm still learning MVVMCross so hopefully I've just missed something.
XF: 4.8.0.1687
MVVMCross: 7.1.2
UPDATE 1:
I've tried to find something in the MVVMCross source to see how it does the setup for the binding context, but I've not found anything. So for now, I'm manually creating the binding context in the page code behind and using Mvx.IocProvider.Resolve<> to pass the required interfaces.
I'm not sure if this is the best to workaround this issue but it works. Maybe there is a better way ?
UPDATE 2:
It seems that update 1 route is no good as the doing this creates a new instance of the VM which is to be expected but an instance already exists which is created by the MVVMCross framework. The question is how do I get the VM instance from the code behind ? The only way I have found so far is to use IMvxOverridePresentationAttribute and the cast the request to MvxViewModelInstanceRequest which then allows access to the VM instance which I assign to the binding context. I may be better off creating another question as to the best approach for this method.
I am developing a Xamarin.Forms app with Prism in which I'd like to incorporate an onboarding page (more specific a top user benefits page, following the Material Design guidelines).
The app itself is structured around a NavigationPage to which I navigate with
NavigationService.NavigateAsync("/NavigationRootPage/MainPage")
in my App(). On the first start I'd like to show the onboarding page instead and I am having quite some issues getting it right.
My first approach was to navigate to MainPage with
_navigationService.NavigateAsync("/NavigationRootPage/MainPage")
from my viewmodel, when the user clicks the Get Started! button. This kind of worked by is also kind of ugly, since this absolute navigation will destroy the oboarding page immediately and not animate the transition. Furthermore at the moment the oboarding page is destroyed, the MainPage will be built, which will take a small, but noticeable, amount of time. In effect the user will notice the MainPage being built up, which does not look smooth at all.
My second approach was to navigate in a relative fashion from my viewmodel.
_navigationService.NavigateAsync("NavigationRootPage/MainPage")
This works way smoother, with the transition animation and after the animation is done, the MainPage is already ready to go. But again there is a major drawback. You can navigate back to the onbaording page, which we dont't want neither. And - to my knowledge - there is no (clean) way to remove the page from the navigation stack, too. I tried calling PageUtility.DestroyPage on my onboarding page, but this only worsened things, since it seemed to keep the page, but destroy the viewmodel, resulting in my onboarding page being shown without data when pushing the back button.
My third approach did not work at all, although it seemed promising to me. In my App() I pushed my navigation page with the main page then then my onboarding page modal
NavigationService.NavigateAsync("/NavigationRootPage/MainPage");
NavigationService.NavigateAsync("OnboardingPage", useModalNavigation: true);
and then in my viewmodel
_navigationService.GoBackAsync(useModalNavigation: true)
but this approach
showed the header of the NavigationPage although the onbaording page was supposed to be shown as a modal
refused to GoBackAsync - when calling this method, nothing happens
waiting for the first call to NavigateAsync did not change anything either.
Some other things I've tried
Implemented INavigationOptions in my viewmodel with ClearNavigationStackOnNavigation being true
Tried setting Application.MainPage after my NavigationPage was shown, but to no avail
I would have believed that this was king of a common requirement when programming an app. Is there something I've missed? Or will I have to live with one of those drawbacks?
I think you are overthinking this. If you want to show your Onborading page first then just navigate to it first.
`NavigateAsync("NavigationPage/OnboardingPage");
Or if you want to have the MainPage in the stack, then start the app with a deeplink
`NavigateAsync("NavigationPage/MainPage/OnboardingPage");
If you don't want to show the navigation header, just hide it for the onboarding page.
Don't use an absolute navigation unless you want to completely reset the navigation stack (equivalent to MainPage = new MainPage()).
I have found a solution, which is not the most beautiful one (it is actually quite ugly) but working.
In my MainPages viewmodel I implemented INavigatingAware and navigated to my OnboardingPage modally
public async void OnNavigatingTo(NavigationParameters parameters)
{
await _navigationService.NavigateAsync("LandingPage", useModalNavigation: true);
}
However, removing the moal page with
this._navigationService.GoBackAsync(useModalNavigation: true);
does not work as expected. The modal is not removed (although it should work this way from looking at the code, but I did not manage to debug with Re# generated PDBs). Hence I had to go the ugly (non-MVVM) way and create an event handler for Button.Click and remove the modal manually
private void Button_OnClicked(object sender, EventArgs e)
{
this.Navigation.PopModalAsync();
}
This will return to my MainPage smoothly with an animation and my MainPageis already up and running without being built up while already visible.
Still, I'd appreciate an answer on how to do it the Prism way.
I have a problem.
My app is a tab bar controller and its first view controller is a split view controller.
This seems to be not ok for Apple because documents say a split voew controller must be the root, so perhaps that is the reason of my problems.
The problem is that sometimes, willHideViewController from UISplitViewControllerDelegate is not called, so, for this reason, the upper/left button sometimes is not created, which is anoying.
I realised, to reproduce this error, try several times this:
-Landscape mode.
-Select a tab different to split view controller tap.
-Move the iPad to portrait in that tab.
-Go to the split view controller tab, and sometimes, willHideViewController is not called so you will not see the upper button. However if I rotate my iPad to landscape and after that to portrait, it's fixed.
I tried to force manually several rotations to work around this problem, but no luck.
I still have to try any split view controller clone class from github or similar.
Do you have idea what's going on or any work around?
Here I show you two examples working properly.
Thanks a lot for your help.
Where do you set the splitViewControllers delegate? Perhaps you can set the delegate when you load the tab. It sounds like you set it only when you have rotated once?
Otherwise, see this example
Here they have the TableViewController be the delegate of the splitviewcontroller. Perhaps you could do the same with the TabBarController?
I've concluded that this can't be done in any way that I consider 'sufficiently' legitimate. It's possible to get frustratingly close, but the issue of having the willShow..., willHide disseminated to the split view controllers under each tab remains.
The solution that seems most likely to work is,
https://github.com/grgcombs/IntelligentSplitViewController/blob/master/IntelligentSplitViewController.m
Though this code is undoubtedly clever, it's a bit too 'side door' for me. I suspect (but don't know) that just invoking the delegate methods is not sufficient. Surely the UISplitViewController itself needs to change it's internal state as well as calling the delegate methods? This method 'just' invokes the delegate methods when there's an orientation change.
So... I've decided on a more legitimate solution, which is to use the new method introduced in iOS 5.
- (BOOL) splitViewController:(UISplitViewController *)svc
shouldHideViewController:(UIViewController *)vc
inOrientation:(UIInterfaceOrientation)orientation
{
return NO;
}
So, the master menu is never hidden, and therefore the problem of managing the popover doesn't arise.
Of course, this is still not totally 'legit' as it still includes UISplitViewControllers that are not at the top level (the UITabViewController is at the top level, and the split views are on each tab)
Good luck with whichever solution you choose.
I'll update this reply when I've confirmed Apple will approve an app using this solution.
I'm programming in Flex Builder Burrito for an mobile application.
I'm trying to get a variable from navigator.PopView()
and i found the following site: adobe View and ViewNavigator
On that page is written that you can get to an returnedObject:
The ViewNavigator will save this object internally, and the new view can access it from with the navigator.returnedObject property.
The problem is when I want to acces the returnedObject flash builder doesn't seem to find that even the package isn't found.
I've found my problem on an other site.
There it's plain simple explained.
Do the override public function createReturnObject():Object.
On the page where it needs to be send back.
Then on the popped view, you can acces it by returnedObject.
On same page next lines are
The property is a ViewReturnObject
which contains the object that was
returned
and the context in which the removed view was pushed (See Setting
the View Context).
ViewNavigator.poppedViewReturnedObject
is guaranteed to be set by the time
the new view
receives the add event and will be destroyed after the view receives its
viewActivate
event.
and also a NOTE
Note, the return object is only stored when a view is popped of
the navigation stack
or replaced through the use of the pop and replace navigation
operations (e.g.,
replaceView, popView, etc...). It will be cleared after the new view
receives its
ViewNavigatorEvent.VIEW_ACTIVATE event.
I think should try function to get popuped view poppedViewReturnedObject of ViewNavigator
Its description is also on same page
public function get poppedViewReturnedObject():ViewReturnObject
Hopes that help