Mvvm Cross Sharing Parameters Between Tabs

How we Could Share Data Between Tabs in runtime?

If simple navigation parameters at MvvmCross is not enough. And we need to share some data after collecting from the database or after some user modifications. We should just add behavior for our root tabbed page. 😉

How we can do that?

Here we have true freedom about the way. But as the example I will show you two ways:

1. More Generic with Interfaces
2. Strongly Typed for special solution.

More Generic Way:

Here we will pass parameters by IParametrizedViewModel interface where have only one field:

public interface IParametrizedViewModel
{
    ObservableCollection<User> UserCollection { get; set; }
}

All our view models for tabs are implement it.

Solution:

1. Creating new class at your Xamarin.Forms project inherited from Behavior<MvxTabbedPage>
2. Create two local fields: one for our bindable object(AssociatedObject) and second for last view model(_currentViewModel)
3. Override OnAttachedTo and OnDetachingFrom
4. There add subscription to CurrentPageChanged event on bindable
5. Assign bindable into our AssociatedObject field.
6. At our function assigned to CurrentPageChanged event we should:
   1. Check if AssociatedObject.CurrentPage is IMvxPage(base interface for all MvvmCross pages)(p)
   2. Check if p.ViewModel is IParametrizedViewModel(view model that we need)(newViewModel)
   3. Assign data from_currentViewModel into newViewModel
   4. Rewrite reference of _currentViewModel into newViewModel
7. Attach new behavior at HomePage.xaml:

<views:MvxTabbedPage ...>
<TabbedPage.Behaviors>
        <behavior:ParametrizedTabBehavior/>
    </TabbedPage.Behaviors>
</views:MvxTabbedPage>

Strongly Typed Way:

Here we want to cover only one specific scenario - passing Secret User.

Property SecretUser is only at DetailsViewModel and at ThirdDetailsViewModel, it's missed at SecondDetailsViewModel:

private User _secretUser;
public User SecretUser
{
    get => _secretUser;
    set => SetProperty(ref _secretUser, value);
}

Here we also can add one more interface and passing values only for view models that implement it. But here we want to pass that property from DetailsViewModel to ThirdViewModel, because the last one can't generate or modify it.

Solution:

1. Creating new class at your Xamarin.Forms project inherited from Behavior<MvxTabbedPage<HomeViewModel>>
2. Create one local fields for our bindable object(AssociatedObject)
3. Override OnAttachedTo and OnDetachingFrom
4. There add subscription to CurrentPageChanged event on bindable
5. Assign bindable into our AssociatedObject field.
6. At our function assigned to CurrentPageChanged event we should:
   1. Check if AssociatedObject.CurrentPage is IMvxPage(base interface for all MvvmCross pages)(p)
   2. Call function OnSelectedTabChanged from our HomeViewModel and pass like an argument page view model p.ViewModel
7. At HomeViewModel inside our function OnSelectedTabChanged we should:
   1. Check if _previousViewModel is DetailsViewModel and selectedViewModel is ThirdDetailsViewModel.
   2. Then just assign SecretUser from DetailsViewModel into ThirdDetailsViewModel
8. Attach new behavior at HomePage.xaml:

<views:MvxTabbedPage ...>
<TabbedPage.Behaviors>
        <behavior:ParametrizedTabBehavior/>
        <behavior:HomeSecretBehavior/>
    </TabbedPage.Behaviors>
</views:MvxTabbedPage>

Example in GIF: