NullReference when ShowViewModel is called for MvxTabBarViewController - xamarin

Question

I created a MvxTabBarViewController and when I'm trying open that using ShowViewModel method, a NullReferenceException occurs. The ViewModel property is always null and the exception occurs if I try instanciate that.
There is my MvxTabViewController
[Register("ProjectDetailsView")]
public class ProjectDetailsView : MvxTabBarViewController<ProjectDetailsViewModel>
{
private JVMenuPopoverViewController _menuController;
public override void ViewDidLoad()
{
base.ViewDidLoad();
if (ViewModel == null)
{
ViewModel = new ProjectDetailsViewModel();
}
CriaAbas();
CriaMenu();
}
private void CriaMenu()
{
var itensMenu = new List<JVMenuItem>();
var selectFavoriteProjectsItem = new JVMenuActionItem
{
Title = "Select Favorite Projects",
Command = () =>
{
var favoriteProjectsView = this.CreateViewControllerFor<FavoriteProjectsViewModel>() as UIViewController;
NavigationController.PushViewController(favoriteProjectsView, true);
}
};
itensMenu.Add(selectFavoriteProjectsItem);
var logoutItem = new JVMenuActionItem()
{
Title = "Logout",
Command = ViewModel.LogoutCommand.Execute
};
itensMenu.Add(logoutItem);
_menuController = new JVMenuPopoverViewController(itensMenu);
var menuButton = new UIBarButtonItem();
menuButton.Clicked += (sender, args) =>
{
_menuController.ShowMenuFromController(this);
};
menuButton.Image = UIImage.FromBundle("images/ic_menu_white_36pt.png");
NavigationItem.RightBarButtonItem = menuButton;
}
private void CriaAbas()
{
var abas = new List<UIViewController>();
var controller = new UINavigationController();
var informationView = this.CreateViewControllerFor(ViewModel.ProjectInformationViewModel) as UIViewController;
informationView.TabBarItem = new UITabBarItem("Information", UIImage.FromBundle("images/ic_info_outline.png"), 0);
informationView.Title = "Information";
controller.PushViewController(informationView, false);
abas.Add(informationView);
controller = new UINavigationController();
var milestonesView = this.CreateViewControllerFor(ViewModel.ProjectMilestonesViewModel) as UIViewController;
milestonesView.TabBarItem = new UITabBarItem("Milestones", UIImage.FromBundle("images/ic_assistant_photo_48pt.png"), 0);
milestonesView.Title = "Milestones";
controller.PushViewController(milestonesView, false);
abas.Add(milestonesView);
ViewControllers = abas.ToArray();
SelectedViewController = ViewControllers[0];
}
}
I'm trying to open this view calling ShowViewModel();
What I'm doing wrong?
there is a part of my stack trace:
mvx: Diagnostic: 72,18 Showing ViewModel ProjectDetailsViewModel
iOSNavigation: Diagnostic: 72,18 Navigate requested
Unhandled Exception:
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
at MvvmCross.iOS.Views.MvxTabBarViewController.set_DataContext (System.Object value) [0x00001] in C:\vcs\git\MvvmCross\MvvmCross\iOS\iOS\Views\MvxTabBarViewController.cs:38
at MvvmCross.iOS.Views.MvxTabBarViewController.set_ViewModel (IMvxViewModel value) [0x00001] in C:\vcs\git\MvvmCross\MvvmCross\iOS\iOS\Views\MvxTabBarViewController.cs:44
at MvvmCross.iOS.Views.MvxTabBarViewController`1[TViewModel].set_ViewModel (MvvmCross.iOS.Views.TViewModel value) [0x00001] in C:\vcs\git\MvvmCross\MvvmCross\iOS\iOS\Views\MvxTabBarViewController.cs:68
at PROSPERI_EPMFast.iOS.Views.ProjectDetailsView.ViewDidLoad () [0x00016] in C:\Projetos Vinicius\PROSPERI_EPMFast\PROSPERI_EPMFast.iOS\Views\ProjectDetailsView.cs:22
at (wrapper managed-to-native) ObjCRuntime.Messaging:IntPtr_objc_msgSendSuper (intptr,intptr)
at UIKit.UITabBarController..ctor () [0x0003b] in /Users/builder/data/lanes/3412/3cf8aaed/source/maccore/src/build/ios/native/UIKit/UITabBarController.g.cs:54
at MvvmCross.Platform.iOS.Views.MvxEventSourceTabBarController..ctor () [0x00000] in C:\vcs\git\MvvmCross\MvvmCross\Platform\iOS\Views\MvxEventSourceTabBarController.cs:20
at MvvmCross.iOS.Views.MvxTabBarViewController..ctor () [0x00000] in C:\vcs\git\MvvmCross\MvvmCross\iOS\iOS\Views\MvxTabBarViewController.cs:20
at MvvmCross.iOS.Views.MvxTabBarViewController`1[TViewModel]..ctor () [0x00000] in C:\vcs\git\MvvmCross\MvvmCross\iOS\iOS\Views\MvxTabBarViewController.cs:56
at PROSPERI_EPMFast.iOS.Views.ProjectDetailsView..ctor () <0x1b486900 + 0x0002b> in <filename unknown>:0
at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (System.Reflection.MonoCMethod,object,object[],System.Exception&)
at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00002] in /Users/builder/data/lanes/3412/3cf8aaed/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:644

Answer 1

For the sake of completion: https://stackoverflow.com/a/30283563/6143949
I'm guessing here the problem here will be specific to the way that
TabBarViewController is constructed.
ViewDidLoad is a virtual method and it is called the first time the
View is accessed.
In the case of TabBarViewController this happens during the iOS base
View constructor - i.e. it occurs before the class itself has had its
constructor called.
The only way around this I've found is to add a check against the
situation in ViewDidLoad, and to make a second call to ViewDidLoad
during the class constructor.
You can see this in action N-25 -
https://github.com/MvvmCross/NPlus1DaysOfMvvmCross/blob/976ede3aafd3a7c6e06717ee48a9a45f08eedcd0/N-25-Tabbed/Tabbed.Touch/Views/FirstView.cs#L17
Something like:
public class MainView : MvxTabBarViewController
{
private bool _constructed;
public MainView()
{
_constructed = true;
// need this additional call to ViewDidLoad because UIkit creates the view before the C# hierarchy has been constructed
ViewDidLoad();
}
public override void ViewDidLoad()
{
if (!_constructed)
return;
base.ViewDidLoad();
var vm = (MainViewModel)this.ViewModel;
if (vm == null)
return;
}
}
Solution provided by Stuart (https://stackoverflow.com/users/373321/stuart)