Some autocomplete stuff...

MvxAutoCompleteTextViewConstraintTargetBinding.cs

using System;
using System.Reflection;
using Cirrious.MvvmCross.Binding.Android.Views;
using Cirrious.MvvmCross.Binding.Bindings.Target;
using Cirrious.MvvmCross.Binding.Interfaces;
using Cirrious.MvvmCross.Interfaces.Platform.Diagnostics;

namespace Cirrious.MvvmCross.Binding.Android.Target
{
    public class MvxAutoCompleteTextViewConstraintTargetBinding : MvxPropertyInfoTargetBinding<MvxBindableAutoCompleteTextView>
    {
        public MvxAutoCompleteTextViewConstraintTargetBinding(object target, PropertyInfo targetPropertyInfo)
            : base(target, targetPropertyInfo)
        {
            var autoComplete = View;
            if (autoComplete == null)
            {
                MvxBindingTrace.Trace(MvxTraceLevel.Error, "Error - AutoComplete is null in MvxAutoCompleteTextViewConstraintTargetBinding");
            }
            else
            {
                autoComplete.ConstraintChanged += AutoCompleteOnConstraintChanged;
            }
        }

        private void AutoCompleteOnConstraintChanged(object sender, EventArgs eventArgs)
        {
            FireValueChanged(View.Constraint);
        }

        public override MvxBindingMode DefaultMode
        {
            get
            {
                return MvxBindingMode.OneWayToSource;
            }
        }

        protected override void Dispose(bool isDisposing)
        {
            base.Dispose(isDisposing);
            if (isDisposing)
            {
                var autoComplete = View;
                if (autoComplete != null)
                {
                    autoComplete.ConstraintChanged -= AutoCompleteOnConstraintChanged;
                }
            }
        }
    }
}

MvxBindableAutoCompleteTextView.cs

#region Copyright
// <copyright file="MvxBindableAutoCompleteTextView.cs" company="Cirrious">
// (c) Copyright Cirrious. http://www.cirrious.com
// This source is subject to the Microsoft Public License (Ms-PL)
// Please see license.txt on http://opensource.org/licenses/ms-pl.html
// All other rights reserved.
// </copyright>
// 
// Project Lead - Stuart Lodge, Cirrious. http://www.cirrious.com
#endregion

using System;
using System.Collections;
using Android.Content;
using Android.Util;
using Android.Widget;

namespace Cirrious.MvvmCross.Binding.Android.Views
{
    public class MvxBindableAutoCompleteTextView
        : AutoCompleteTextView
    {
        public MvxBindableAutoCompleteTextView(Context context, IAttributeSet attrs)
            : this(context, attrs, new MvxFilteringBindableListAdapter(context))
        {
        }

        public MvxBindableAutoCompleteTextView(Context context, IAttributeSet attrs, MvxFilteringBindableListAdapter adapter)
            : base(context, attrs)
        {
            var itemTemplateId = MvxBindableListViewHelpers.ReadTemplatePath(context, attrs);
            adapter.ItemTemplateId = itemTemplateId;
            Adapter = adapter;
        }

        public new MvxFilteringBindableListAdapter Adapter
        {
            get { return base.Adapter as MvxFilteringBindableListAdapter; }
            set
            {
                var existing = Adapter;
                if (existing == value)
                    return;

                if (existing != null)
                    existing.ConstraintChanged -= AdapterOnConstraintChanged;

                if (existing != null && value != null)
                {
                    value.ItemsSource = existing.ItemsSource;
                    value.ItemTemplateId = existing.ItemTemplateId;
                }

                if (value != null)
                    value.ConstraintChanged += AdapterOnConstraintChanged;

                base.Adapter = value;
            }
        }

        private void AdapterOnConstraintChanged(object sender, EventArgs eventArgs)
        {
            FireConstraintChanged();
        }

        public IList ItemsSource
        {
            get { return Adapter.ItemsSource; }
            set { Adapter.ItemsSource = value; }
        }

        public int ItemTemplateId
        {
            get { return Adapter.ItemTemplateId; }
            set { Adapter.ItemTemplateId = value; }
        }

        public string Constraint
        {
            get { return Adapter.Constraint; }
        }

        public event EventHandler ConstraintChanged;

        private void FireConstraintChanged()
        {
            var handler = ConstraintChanged;
            if (handler != null)
            {
                handler(this, EventArgs.Empty);
            }
        }
    }
}

MvxFilteringBindableListAdapter.cs

using System;
using Android.App;
using Android.Content;
using Android.Widget;
using Java.Lang;

namespace Cirrious.MvvmCross.Binding.Android.Views
{
    public class MvxFilteringBindableListAdapter
        : MvxBindableListAdapter, IFilterable
    { 
        private class MyFilter : Filter
        {
            private readonly MvxFilteringBindableListAdapter _owner;

            public MyFilter(MvxFilteringBindableListAdapter owner)
            {
                _owner = owner;
            }

            #region Overrides of Filter

            protected override FilterResults PerformFiltering(ICharSequence constraint)
            {
                if (constraint != null)
                {
                    _owner.Constraint = constraint.ToString();
                }

                return new FilterResults();
            }

            protected override void PublishResults(ICharSequence constraint, FilterResults results)
            {
                // ignored
            }

            #endregion
        }

        private string _constraint;

        public event EventHandler ConstraintChanged;

        public string Constraint
        {
            get { return _constraint; }
            private set
            {
                if (_constraint == value)
                    return;
                _constraint = value; 
                FireConstraintChanged();
            }
        }

        private void FireConstraintChanged()
        {
            var activity = Context as Activity;

            if (activity == null)
                return;

            activity.RunOnUiThread(() =>
                                       {
                                           var handler = ConstraintChanged;
                                           if (handler != null)
                                               handler(this, EventArgs.Empty);
                                       });
        }

        public MvxFilteringBindableListAdapter(Context context) : base(context)
        {
            Filter = new MyFilter(this);
        }

        #region Implementation of IFilterable

        public Filter Filter { get; set; }

        #endregion
    }
}

ViewModelSnippet

        private string _s1;
        public string S1
        {
            get { return _s1; }
            private set { _s1 = value; FirePropertyChanged("S1"); }
        }

        private string _s2;
        public string S2
        {
            get { return _s2; }
            private set { _s2 = value; FirePropertyChanged("S2"); BindTarget.Clear();BindTarget.Add(S2); BindTarget.Add(S2 + "_2");BindTarget.Add(S2 + "_3"); }
        }

        public ObservableCollection<string> BindTarget { get; set; }

xml snippet...

      <Mvx.MvxBindableAutoCompleteTextView
         android:layout_width="fill_parent"
         android:layout_height="wrap_content"
         local:MvxBind="{'Text':{'Path':'S1','Mode':'TwoWay'},'ItemsSource':{'Path':'BindTarget'},'Constraint':{'Path':'S2','Mode':'OneWayToSource'}}"
        />