Using the new Android's BottomNavigationView with MvvmCross 6

bottom_navigation.axml

<!-- Resources/layout/bottom_navigation.axml -->
<!-- "android:titleCondensed" is used as tab ID -->
<?xml version="1.0" encoding="UTF-8" ?>
<menu xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/tab_first"
        android:enabled="true"
        android:icon="@drawable/tab_first"
        android:title="First"
        android:titleCondensed="tab_first"
        app:showAsAction="always" />
    <item
        android:id="@+id/tab_second"
        android:enabled="true"
        android:icon="@drawable/tab_second"
        android:title="Second"
        android:titleCondensed="tab_second"
        app:showAsAction="always" />
    <item
        android:id="@+id/tab_third"
        android:enabled="true"
        android:icon="@drawable/tab_third"
        android:title="Third"
        android:titleCondensed="tab_third"
        app:showAsAction="always" />
</menu>

MvxBottomNavigationItemChangedBinding.cs

// a custom binding for Droid to get "app:MvxBind" in root_view working

using System;
using Android.Support.Design.Widget;
using MvvmCross.Binding;
using MvvmCross.Commands;
using MvvmCross.Platforms.Android.Binding.Target;

namespace YourApp.Droid.Bindings
{
    public class MvxBottomNavigationItemChangedBinding : MvxAndroidTargetBinding
    {
        readonly BottomNavigationView _bottomNav;
        IMvxCommand _command;

        public override MvxBindingMode DefaultMode => MvxBindingMode.TwoWay;

        public override Type TargetType => typeof(MvxCommand);

        public MvxBottomNavigationItemChangedBinding(BottomNavigationView bottomNav) : base(bottomNav)
        {
            _bottomNav = bottomNav;
            _bottomNav.NavigationItemSelected += OnNavigationItemSelected;
        }

        public override void SetValue(object value)
        {
            _command = (IMvxCommand)value;
        }

        protected override void SetValueImpl(object target, object value)
        {

        }

        void OnNavigationItemSelected(object sender, BottomNavigationView.NavigationItemSelectedEventArgs e)
        {
            
            if (_command != null)
                _command.Execute(e.Item.TitleCondensedFormatted.ToString());
        }

        protected override void Dispose(bool isDisposing)
        {
            if (isDisposing)
                _bottomNav.NavigationItemSelected -= OnNavigationItemSelected;

            base.Dispose(isDisposing);
        }
    }
}

root_view.axml

<!-- Resources/layout/root_view.axml -->
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_main"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:minWidth="25px"
    android:minHeight="25px">
    <FrameLayout
        android:id="@+id/tabs_container_frame"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/bottom_navigation"
        android:background="@android:color/holo_purple" />
    <android.support.design.widget.BottomNavigationView
        android:id="@+id/bottom_navigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="@android:color/white"
        app:itemIconTint="@color/bottom_navigation_colors"
        app:itemTextColor="@color/bottom_navigation_colors"
        app:menu="@menu/bottom_navigation"
        app:MvxBind="BottomNavigationSelectedBindingKey BottomNavigationItemSelectedCommand" />
</RelativeLayout>

RootView.cs

// the root view of your Droid hierarchy, it uses fragments for tabs

using Android.App;
using Android.OS;
using Android.Support.Design.Widget;
using MvvmCross.Droid.Support.V7.AppCompat; // don't forget to install this package from NuGet
using MvvmCross.Platforms.Android.Presenters.Attributes;
using YourApp.Core.ViewModels; // the namespace where your RootViewModel is

namespace YourApp.Droid
{
    [Activity(Label = "YourAppName")]
    [MvxActivityPresentation]
    public class RootView : MvxAppCompatActivity<RootViewModel>
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            SetContentView(Resource.Layout.root_view);

            var bottomNavigation = FindViewById<BottomNavigationView>(Resource.Id.bottom_navigation);
            ViewModel.BottomNavigationItemSelectedCommand.Execute(ViewModel.Tabs[0].TabId);
        }
    }
}

RootViewModel.cs

// the root view model in your Core project

using MvvmCross;
using MvvmCross.Commands;
using MvvmCross.Navigation;
using MvvmCross.ViewModels;
using System.Collections.Generic;

namespace YourApp.Core.ViewModels
{
    public class RootViewModel : MvxViewModel
    {
        public IMvxCommand<string> BottomNavigationItemSelectedCommand { get; private set; }

        List<TabViewModel> _tabs;
        public List<TabViewModel> Tabs
        {
            get => _tabs;
            set => SetProperty(ref _tabs, value);
        }

        readonly IMvxNavigationService _navigationService;

        public RootViewModel(IMvxNavigationService navigationService)
        {
            _navigationService = navigationService;
            BottomNavigationItemSelectedCommand = new MvxCommand<string>(BottomNavigationItemSelected);
            
            var tabs = new List<TabViewModel>
            {
                Mvx.IoCConstruct<FirstViewModel>(),
                Mvx.IoCConstruct<SecondViewModel>(),
                Mvx.IoCConstruct<ThirdViewModel>(),
            };

            Tabs = tabs;
        }

        // Android-only, not used on iOS
        void BottomNavigationItemSelected(string tabId)
        {
            foreach (var item in Tabs)
            {
                if (tabId == item.TabId)
                {
                    _navigationService.Navigate(item);
                    break;
                }
            }
        }
	}
}

Setup.cs

// you need a custom MvxAndroidSetup to register the custom binding

using Android.Support.Design.Widget;
using YourApp.Core;
using YourApp.Droid.Bindings;
using MvvmCross.Binding.Bindings.Target.Construction;
using MvvmCross.Droid.Support.V7.AppCompat;
using MvvmCross.Platforms.Android.Core;

namespace YourApp.Droid
{
    
    public class Setup : MvxAndroidSetup<App>
    {
        protected override void FillTargetFactories(IMvxTargetBindingFactoryRegistry registry)
		{
            MvxAppCompatSetupHelper.FillTargetFactories(registry);
            base.FillTargetFactories(registry);
            registry.RegisterCustomBindingFactory<BottomNavigationView>("BottomNavigationSelectedBindingKey",
                                                                        view => new MvxBottomNavigationItemChangedBinding(view));
		}
	}
}

TabViewModel.cs

// subclass this with your tab view models and set the IDs equal to "android:titleCondensed" in bottom_navigation.axml

using MvvmCross;
using MvvmCross.ViewModels;

namespace YourApp.Core.ViewModels
{
    public class TabViewModel : MvxViewModel
    {
        public string TabName { get; protected set; }
        public string TabId { get; protected set; }
    }
}