Supporting Xamarin.Forms Tabbed Page in Caliburn.Micro

One of the best features of Caliburn.Micro in my opinion is get away from “one view model per screen” and starting to compose view models together to form the screen.

The framework itself supplies conductors to help facilitate this, as well as the behaviour of View.Model as as “Template Selector” on steroids.

As a quick refresher what View.Model does when applied to ContentControl (or ContentView in Xamarin.Forms) is locate the view appropriate to the bound view model and inject it into the control (while binding the view and view model together).

For instance given the following xaml:

    <ContentControl cm:View.Model="{Binding ActiveItem}" />

The contents of the ContentControl will be the view appropriate for the type of ActiveItem. If for instance it was ProductsListViewModel then the contents would be ProductsListView. What’s really great is that as ActiveItem changes to am instance of another type then the contents wil update with the new view as well.

So how does this fit with TabbedPage in Xamarin.Forms?

Well it works pretty seamlessly with a few tricks.

First we’ll start with the view model that represents the tabbed page itself, I typically call this the shell. It inherits from Conductor<PageViewModel>.Collection.OneActive which essentially means that this view model conducts a collection of page view models where one is active at a time. This maps nicely to how a tab UI works.

public class ShellViewModel : Conductor<PageViewModel>.Collection.OneActive

In our shell view we have the TabbedPage itself, bound to the Items and ActiveItem of the shell view model. For the item template we have one gotcha. TabbedPage expects it’s contents to inherit from Page, so the root element of our data template but be ContentPage. Fortunately View.Model can handle this with no problem. We also make use of the display name property on Screen to supply the title for each tab.

    ItemsSource="{Binding Items}"
    SelectedItem="{Binding ActiveItem, Mode=TwoWay}">
            <ContentPage Title="{Binding DisplayName}" cm:View.Model="{Binding}" />

In our shell we can now add some pages:

protected override void OnInitialize()
    Items.Add(new ProductsListViewModel());
    Items.Add(new LocationsViewModel());
    Items.Add(new ContactViewModel());


As the tabs are selected our item template will locate the correct view for the above view models.

What’s really interesting about this is that our shell doesn’t have to care about how the pages below it look or work. It can focus on it’s single reponsiblity which is the tabs themselves and any behaviour conducting their flow.