XfxComboBox sample

EditAppointmentPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:material="clr-namespace:SuaveControls.MaterialForms;assembly=SuaveControls.MaterialForms"
             xmlns:xfx="clr-namespace:Xfx;assembly=Xfx.Controls"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Class="DentalAssistantXF.Views.EditAppointmentPage"
             Title="{Binding Title}">

    <ContentPage.ToolbarItems>
        <ToolbarItem Icon="ic_check_white_24dp" Command="{Binding SaveAppointmentCommand}" />
    </ContentPage.ToolbarItems>

    <StackLayout>

        <StackLayout Style="{StaticResource Strip}">
            <Label Text="Appointment information" />
        </StackLayout>
        
        <StackLayout Padding="15,0">
            <xfx:XfxComboBox Placeholder="Search for a patient..."
                             Threshold="3"
                             Text="{Binding FilterText, Mode=TwoWay}"
                             ItemsSource="{Binding Patients}"
                             SelectedItem="{Binding SelectedPatient, Mode=TwoWay}"
                             SortingAlgorithm="{Binding SortingAlgorithm}" />
            
            <DatePicker Date="{Binding Appointment.AppointmentDate}" Format="dd/MM/yyyy" />
            
            <TimePicker Time="{Binding Appointment.AppointmentTime}" Format="HH:mm" />
            
            <material:MaterialEntry Text="{Binding Appointment.Subject}" Placeholder="Reason..." />
        </StackLayout>

    </StackLayout>

</ContentPage>

EditAppointmentViewModel.cs

using DentalAssistantXF.Models;
using DentalAssistantXF.Services;
using DentalAssistantXF.Utils;
using Prism.Commands;
using Prism.Mvvm;
using Prism.Navigation;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using Xamarin.Forms;

namespace DentalAssistantXF.ViewModels
{
    public class EditAppointmentPageViewModel : BindableBase, INavigatingAware
    {
        private readonly IDatabaseService _databaseService;
        private Appointment _appointment;
        private Patient _selectedPatient;
        private Patient _tempPatient;
        private string _title;
        private string _filterText;

        public EditAppointmentPageViewModel(IDatabaseService databaseService)
        {
            _databaseService = databaseService;

            SaveAppointmentCommand = new DelegateCommand(SaveAppointment);
            Patients = new ObservableCollection<Patient>();
        }        

        public Appointment Appointment
        {
            get { return _appointment; }
            set { SetProperty(ref _appointment, value); }
        }

        public Patient SelectedPatient
        {
            get { return _selectedPatient; }
            set { SetProperty(ref _selectedPatient, value); }
        }

        public string Title
        {
            get { return _title; }
            set { SetProperty(ref _title, value); }
        }
        
        public string FilterText
        {
            get { return _filterText; }
            set { SetProperty(ref _filterText, value); }
        }
        
        public Func<string, ICollection<string>, ICollection<string>> SortingAlgorithm { get; } = (text, values) => values
		              .Where(x => x.ToLower().StartsWith(text.ToLower()))
		              .OrderBy(x => x)
		              .ToList();

        public void OnNavigatingTo(NavigationParameters parameters)
        {
            _tempPatient = null;

            if (parameters != null)
            {
                Appointment = (Appointment)parameters["Appointment"];
                Title = Appointment.Id < 1 ? "Add new appointment" : "Edit appointment";                

                if (parameters["Patient"] != null)
                {
                    _tempPatient = (Patient)parameters["Patient"];
                }

                GetPatientsAsync();
            }            
        }        

        public ObservableCollection<Patient> Patients { get; }

        public DelegateCommand SaveAppointmentCommand { get; }

        private async void GetPatientsAsync()
        {
            var patients = await _databaseService.DentalAssistantDB.GetPatientsAsync();
            Patients.Clear();
            patients.ToList().ForEach(Patients.Add);
            if (_tempPatient != null)
            {
                SelectedPatient = Patients.Where(p => p.Id == _tempPatient.Id).FirstOrDefault();
            }
        }

        private async void SaveAppointment()
        {
            if(SelectedPatient == null)
            {
                HelperFunctions.ShowToastMessage(ToastMessageType.Error, "You have to select a patient first");
                return;
            }

            Appointment.PatientId = SelectedPatient.Id;
            try
            {
                if (await _databaseService.DentalAssistantDB.SaveAppointmentAsync(Appointment) > 0)
                {
                    HelperFunctions.ShowToastMessage(ToastMessageType.Success, "Appointment saved successfully");
                    MessagingCenter.Send(this, Constants.OnAddOrEditAppointmentMsg);
                    MessagingCenter.Send(this, Constants.OnDashboardDataChangeMsg);
                }
            }
            catch (Exception e)
            {

            }
        }
    }
}