FocusBehavior.cs

using System;
using System.Runtime.CompilerServices;

namespace app.Behaviors {
    public class FocusBehavior : Behavior<View> {
        private View currentView;

        public static BindableProperty IsFocusedProperty = BindableProperty.Create(nameof(IsFocused), typeof(bool), typeof(FocusBehavior));

        public bool IsFocused {
            get => (bool)GetValue(IsFocusedProperty);
            set => SetValue(IsFocusedProperty, value);
        }

        public FocusBehavior() {
        }

        protected override void OnAttachedTo(View bindable) {
            base.OnAttachedTo(bindable);

            currentView = bindable;
            currentView.Unfocused += CurrentView_Unfocused;
        }

        private void CurrentView_Unfocused(object sender, FocusEventArgs e) {
            IsFocused = false;
        }

        protected override void OnDetachingFrom(View bindable) {
            base.OnDetachingFrom(bindable);

            currentView.Unfocused -= CurrentView_Unfocused;
            currentView = null;
        }

        protected override void OnPropertyChanged([CallerMemberName] string propertyName = null) {
            base.OnPropertyChanged(propertyName);

            if (propertyName == nameof(IsFocused) && IsFocused && currentView != null) {
                currentView.Focus();
            }
        }
    }
}