How to Use the MVVM Architectural Pattern to Pass Data (both Push and Pop) Between Pages

Question

I'm writing an app where I need to perform CRUD operations on a list of people, called "Operators". Each Operator has a unique ID, a name, a phone number, an email address and an avatar.
With the help of @JessieZhang-MSFT , I am able to select a particular Operator from my SelectOperatorPage, then return this to my OperatorsPage.
From here, though, I wish to be able to Update the details of that Operator by pushing to a new OperatorInfoPage. This page should be able to be used for editing or deleting existing Operators or adding new Operators.

My problem is how to set Bindings so that data can be read from the OperatorInfoPage and the NewOperatorPage, and potentially how to delete the currently selected Operator.

The Pages, their code behinds and the Model and ViewModel classes are presented here...

OperatorModel.cs

using System;  
using System.Collections.Generic;  
using System.ComponentModel;  
using System.Runtime.CompilerServices;  
using System.Text;  
using SQLite;  
  
namespace HandsFreeNotes.Model  
{  
    //    public class OperatorModel  
    // The line above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
    public class OperatorModel: INotifyPropertyChanged  
    {  
        public int OperatorID { get; set; }  
        //        public string OperatorName { get; set; }  
        // The 1 line above changed to the 6 lines below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        string _operatorName;  
        public string OperatorName  
        {  
            set { SetProperty(ref _operatorName, value); }  
            get { return _operatorName; }  
        }  
  
        public string OperatorPhone { get; set; }  
        public string OperatorEmail { get; set; }  
        public string OperatorAvatar { get; set; }  
  
        // The 4 lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        public OperatorModel()  
        {  
  
        }  
  
        public OperatorModel(int operatorID, string operatorName, string operatorPhone, string operatorEmail, string operatorAvatar)  
        {  
            OperatorID = operatorID;  
            OperatorName = operatorName;  
            OperatorPhone = operatorPhone;  
            OperatorEmail = operatorEmail;  
            OperatorAvatar = operatorAvatar;  
        }  
        // All the lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        bool SetProperty(ref T storage, T value, [CallerMemberName] string propertyName = null)  
        {  
            if (Object.Equals(storage, value))  
                return false;  
  
            storage = value;  
            OnPropertyChanged(propertyName);  
            return true;  
        }  
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)  
        {  
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));  
        }  
  
        public event PropertyChangedEventHandler PropertyChanged;  
    }  
  
  
}

OperatorViewModel.cs

using HandsFreeNotes.Model;  
using System;  
using System.Collections.Generic;  
using System.ComponentModel;  
using System.Text;  
using System.Threading.Tasks;  
using Xamarin.Forms;  
  
namespace HandsFreeNotes.ViewModel  
{  
    public class OperatorViewModel : INotifyPropertyChanged  
    {  
        private string _HFNName;  
  
        public string HFNName  
        {  
            get  
            {  
                return _HFNName;  
            }  
              
            set  
            {  
                _HFNName = value;  
                RaisePropertyChanged("HFNName");  
            }  
        }  
  
        public List OperatorList;  
        public OperatorModel SelectedOperator;  
  
        public async Task GetOperators()  
        {  
            if (OperatorList == null)  
            {  
                OperatorList = new List();  
                OperatorList.Add(new OperatorModel(1, "Grime", "+61419659866", "apps@grime.net", "gs300.png"));  
                OperatorList.Add(new OperatorModel(2, "Brookey", "+61419659866", "brookey@grime.net", "gmb.jpg"));  
                OperatorList.Add(new OperatorModel(3, "CC", "+61422192456", "shaunachick@gmail.com", "cc.jpg"));  
                OperatorList.Add(new OperatorModel(4, "Monte", "+61412595878", "monte@aussieweb.com.au", "monte2a.jpg"));  
                OperatorList.Add(new OperatorModel(5, "Jeff", "+61409998197", "jeff@content2convert.com.au", "jeff.jpg"));  
            }  
  
            return true;  
        }  
  
        public event PropertyChangedEventHandler PropertyChanged;  
  
        protected void RaisePropertyChanged(string prop)  
        {  
            if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(prop)); }  
        }  
  
        public bool UpdateOperator(OperatorModel opm)  
        {  
            SelectedOperator.OperatorName = opm.OperatorName;  
            SelectedOperator.OperatorPhone = opm.OperatorPhone;  
            SelectedOperator.OperatorEmail = opm.OperatorEmail;  
            SelectedOperator.OperatorAvatar = opm.OperatorAvatar;  
  
            return true;  
        }  
    }  
}

OperatorsPage.xaml

OperatorsPage.xaml.cs

using HandsFreeNotes.ViewModel;  
using HandsFreeNotes.Model;  
using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
using System.Threading.Tasks;  
  
using Xamarin.Forms;  
using Xamarin.Forms.Xaml;  
  
namespace HandsFreeNotes.View  
{  
    [XamlCompilation(XamlCompilationOptions.Compile)]  
  
    public partial class OperatorsPage : ContentPage  
    {  
        OperatorViewModel ovm;  
        OperatorModel opm;  
  
        // public OperatorsPage(OperatorViewModel _ovm)  
        // The line above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        public OperatorsPage()  
        {  
            // add a bit of padding to cater to the "notch" on the iPhone.  
            if (Device.RuntimePlatform == Device.iOS)  
            {  
                Padding = new Thickness(0, 40, 0, 0);  
            }  
  
            InitializeComponent();  
  
            // ovm = _ovm;  
            // opm = ovm.SelectedOperator;  
            // The 2 lines above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
            opm = new OperatorModel();  
  
            BindingContext = opm;  
  
        }  
  
        private async void BackButton_Clicked(object sender, EventArgs e)  
        {  
            await Navigation.PopModalAsync();  
            // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        }  
  
        private async void AddOperatorButton_Clicked(object sender, EventArgs e)  
        {  
            await Navigation.PushModalAsync(new NewOperatorPage());  
            // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
  
        }  
  
        private async void SelectOperatorButton_Clicked(object sender, EventArgs e)  
        {  
            // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
            SelectOperatorPage page = new SelectOperatorPage();  
            page.ReturnValue += delegate (object s, OperatorModel operatorModel)  
            {  
                BackCall(s, operatorModel);  
            };  
  
            await Navigation.PushModalAsync(page);  
        }  
        // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        private void BackCall(object s, OperatorModel model)  
        {  
            opm.OperatorName = model.OperatorName;  
        }  
  
        private async void UpdateOperatorButton_Clicked(object sender, EventArgs e)  
        {  
            //var operatorInfoPage = new OperatorInfoPage();  
            //operatorInfoPage.BindingContext = ovm;  
            //await Navigation.PushModalAsync(operatorInfoPage);  
  
        }  
    }  
}

SelectOperatorPage.xaml

SelectOperatorPage.xaml.cs

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
using System.Threading.Tasks;  
  
using Xamarin.Forms;  
using Xamarin.Forms.Xaml;  
using HandsFreeNotes.ViewModel;  
using HandsFreeNotes.Model;  
  
namespace HandsFreeNotes.View  
{  
    [XamlCompilation(XamlCompilationOptions.Compile)]  
    public partial class SelectOperatorPage : ContentPage  
    {  
        OperatorViewModel ovm;  
  
        // Added the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        public EventHandler ReturnValue;  
  
        public SelectOperatorPage()  
        {  
            // add a bit of padding to cater to the "notch" on the iPhone.  
            if (Device.RuntimePlatform == Device.iOS)  
            {  
                Padding = new Thickness(0, 40, 0, 0);  
            }  
  
            InitializeComponent();  
  
            ovm = new OperatorViewModel();  
  
            BindingContext = ovm;  
        }  
  
        protected override async void OnAppearing()  
        {  
            base.OnAppearing();  
            await ovm.GetOperators();  
            OperatorListView.ItemsSource = ovm.OperatorList;  
        }  
  
        private async void BackButton_Clicked(object sender, EventArgs e)  
        {  
            await Navigation.PopModalAsync();  
        }  
  
        private async void OperatorListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)  
        {  
            if (e.SelectedItem != null)  
            {  
                // Typecast the e.SelectedItem object to an OperatorModel  
  
                // ovm.SelectedOperator = (OperatorModel)e.SelectedItem;  
                // await DisplayAlert("Alert", "Selected Operator: " + ovm.SelectedOperator.OperatorName, "Exit");  
  
                // The 2 lines above were changed to the 7 below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
  
                OperatorModel operatorModel = (OperatorModel)e.SelectedItem;  
                ovm.SelectedOperator = operatorModel;  
                EventHandler handler = ReturnValue;  
                if (handler != null)  
                {  
                    handler(this, operatorModel);  
                }  
  
  
                await Navigation.PopModalAsync();  
            }  
        }  
    }  
}

OperatorInfoPage.xaml

xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="HandsFreeNotes.View.OperatorInfoPage">

Accepted Answer

That leaves Edit/Update Operator. Currently, the updateOperatorButton_Clicked does nothing...

Please copy following code in UpdateOperatorButton_Clicked method of OperatorsPage.xaml.cs

private async void UpdateOperatorButton_Clicked(object sender, EventArgs e)
        {

            if (SelectModel != null)
            {

                  var operatorInfoPage = new OperatorInfoPage(SelectModel);
                operatorInfoPage.ReturnValue += delegate (object s, OperatorModel operatorModel)
                {
                    BackCall(s, operatorModel);
                };
                await Navigation.PushModalAsync(operatorInfoPage);
            }
            else
            {
                await DisplayAlert("info", "please select item", "OK");
            }

        }

Here is code about OperatorInfoPage.xaml

Here is code about OperatorInfoPage.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace App94
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class OperatorInfoPage : ContentPage
    {
        // OperatorViewModel ovm;
        // OperatorModel opm;

        // public OperatorModel opm1 { get; set; }

        OperatorModel _opm1 ;
        public OperatorModel opm1
        {
            get
            {
                return _opm1;
            }

            set
            {
                if (_opm1 != value)
                {
                    _opm1 = value;
                    OnPropertyChanged("opm1");

                }
            }

        }
        int SelectItemIndex;

        public EventHandler ReturnValue;

        public OperatorInfoPage(OperatorModel opm)
        {
            // add a bit of padding to cater to the "notch" on the iPhone.
            if (Device.RuntimePlatform == Device.iOS)
            {
                Padding = new Thickness(0, 40, 0, 0);
            }

            InitializeComponent();
            SelectItemIndex= OperatorViewModel.OperatorList.IndexOf(opm);

            //  ovm = _ovm;
            // opm = ovm.SelectedOperator;
            this.opm1 = opm;
            BindingContext = this;
        }

        private async void BackButton_Clicked(object sender, EventArgs e)
        {
            await Navigation.PopModalAsync();
        }

        private async void OKButton_Clicked(object sender, EventArgs e)
        {
            OperatorViewModel.OperatorList[SelectItemIndex].OperatorName = opm1.OperatorName;
            OperatorViewModel.OperatorList[SelectItemIndex].OperatorPhone = opm1.OperatorPhone;
            OperatorViewModel.OperatorList[SelectItemIndex].OperatorEmail = opm1.OperatorEmail;
            OperatorViewModel.OperatorList[SelectItemIndex].OperatorAvatar = opm1.OperatorAvatar;
            // OperatorViewModel.OperatorList.Add(opm1);

            OperatorModel operatorModel = opm1;

            EventHandler handler = ReturnValue;
            if (handler != null)
            {
                handler(this, operatorModel);
            }


            await Navigation.PopModalAsync();
        }

        private async void CancelButton_Clicked(object sender, EventArgs e)
        {
            await Navigation.PopModalAsync();
        }
    }
}

Answer

Hello,

Welcome to our Microsoft Q&A platform!

If do not change lots of code to achieve Delete or Add function, a straightforward way is adding static value to the OperatorList, I change the type of OperatorList from list to ObservableCollection for updating dramatically. Here is edited OperatorViewModel.

   public class OperatorViewModel : INotifyPropertyChanged  
       {  
           private string _HFNName;  
     
           public string HFNName  
           {  
               get  
               {  
                   return _HFNName;  
               }  
     
               set  
               {  
                   _HFNName = value;  
                   RaisePropertyChanged("HFNName");  
               }  
           }  
     
           public OperatorViewModel()  
           {  
               GetOperators();  
           }  
     
           public static ObservableCollection OperatorList;  
           public OperatorModel SelectedOperator;  
     
           public async Task GetOperators()  
           {  
               if (OperatorList == null)  
               {  
                   OperatorList = new ObservableCollection();  
                   OperatorList.Add(new OperatorModel(1, "Grime", "+61419659866", "apps@grime.net", "gs300.png"));  
                   OperatorList.Add(new OperatorModel(2, "Brookey", "+61419659866", "brookey@grime.net", "gmb.jpg"));  
                   OperatorList.Add(new OperatorModel(3, "CC", "+61422192456", "shaunachick@gmail.com", "cc.jpg"));  
                   OperatorList.Add(new OperatorModel(4, "Monte", "+61412595878", "monte@aussieweb.com.au", "monte2a.jpg"));  
                   OperatorList.Add(new OperatorModel(5, "Jeff", "+61409998197", "jeff@content2convert.com.au", "jeff.jpg"));  
               }  
     
               return true;  
           }  
     
           public event PropertyChangedEventHandler PropertyChanged;  
     
           protected void RaisePropertyChanged(string prop)  
           {  
               if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(prop)); }  
           }  
     
           public bool UpdateOperator(OperatorModel opm)  
           {  
               SelectedOperator.OperatorName = opm.OperatorName;  
               SelectedOperator.OperatorPhone = opm.OperatorPhone;  
               SelectedOperator.OperatorEmail = opm.OperatorEmail;  
               SelectedOperator.OperatorAvatar = opm.OperatorAvatar;  
     
               return true;  
           }  
       }

Then I achieve Remove function.

   private async void DeleteOperatorButton_Clicked(object sender, EventArgs e)  
           {  
               if (SelectModel != null)  
               {  
     
                   OperatorViewModel.OperatorList.Remove(SelectModel);  
                   SelectModel = null;  
                   SelectedOperatorEntry.Text = "";  
               }  
               else  
               {  
                 await  DisplayAlert("info","please select item","OK");  
               }  
           }

Here is all of code about the OperatorsPage.xaml.cs

   using System;  
   using System.Collections.Generic;  
   using System.Linq;  
   using System.Text;  
   using System.Threading.Tasks;  
     
   using Xamarin.Forms;  
   using Xamarin.Forms.Xaml;  
     
   namespace App94  
   {  
       [XamlCompilation(XamlCompilationOptions.Compile)]  
       public partial class OperatorsPage : ContentPage  
       {  
           OperatorViewModel ovm;  
           OperatorModel opm;  
     
           // public OperatorsPage(OperatorViewModel _ovm)  
           // The line above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
           public OperatorsPage()  
           {  
               // add a bit of padding to cater to the "notch" on the iPhone.  
               if (Device.RuntimePlatform == Device.iOS)  
               {  
                   Padding = new Thickness(0, 40, 0, 0);  
               }  
     
               InitializeComponent();  
     
               // ovm = _ovm;  
               // opm = ovm.SelectedOperator;  
               // The 2 lines above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
               opm = new OperatorModel();  
     
               BindingContext = opm;  
     
           }  
     
           private async void BackButton_Clicked(object sender, EventArgs e)  
           {  
               await Navigation.PopModalAsync();  
               // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
           }  
     
           private async void AddOperatorButton_Clicked(object sender, EventArgs e)  
           {  
               await Navigation.PushModalAsync(new NewOperatorPage());  
               // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
     
           }  
     
           private async void SelectOperatorButton_Clicked(object sender, EventArgs e)  
           {  
               // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
               SelectOperatorPage page = new SelectOperatorPage();  
               page.ReturnValue += delegate (object s, OperatorModel operatorModel)  
               {  
                   BackCall(s, operatorModel);  
               };  
     
               await Navigation.PushModalAsync(page);  
           }  
           // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
     
           OperatorModel SelectModel;  
           private void BackCall(object s, OperatorModel model)  
           {  
               SelectModel = model;  
               opm.OperatorName = model.OperatorName;  
           }  
     
            
     
           private async void DeleteOperatorButton_Clicked(object sender, EventArgs e)  
           {  
               if (SelectModel != null)  
               {  
     
                   OperatorViewModel.OperatorList.Remove(SelectModel);  
                   SelectModel = null;  
                   SelectedOperatorEntry.Text = "";  
               }  
               else  
               {  
                 await  DisplayAlert("info","please select item","OK");  
               }  
           }  
     
           private async void UpdateOperatorButton_Clicked(object sender, EventArgs e)  
           {  
               //var operatorInfoPage = new OperatorInfoPage();  
               //operatorInfoPage.BindingContext = ovm;  
               //await Navigation.PushModalAsync(operatorInfoPage);  
               if (opm != null)  
               {  
     
                   //  var operatorInfoPage = new OperatorInfoPage(opm);  
                   //operatorInfoPage.BindingContext = ovm;  
                   //  await Navigation.PushModalAsync(operatorInfoPage);  
     
               }  
     
     
           }  
       }  
   }

I achieve add function as well, here is NewOperatorPage.xaml

Here is NewOperatorPage.xaml.cs

   public partial class NewOperatorPage : ContentPage  
       {  
           public NewOperatorPage()  
           {  
               InitializeComponent();  
           }  
     
           private void Button_Clicked(object sender, EventArgs e)  
           {  
               var name=OperatorName.Text;  
               var Phone= OperatorPhone.Text;  
               var Email= OperatorEmail.Text;  
               var Avatar =  OperatorAvatar.Text;  
               OperatorViewModel.OperatorList.Add(new OperatorModel(OperatorViewModel.OperatorList.Count,name,Phone,Email,Avatar));  
               Navigation.PopModalAsync();  
           }  
       }

Best Regards,

Leon Lu

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Answer

@Leon Lu (Shanghai Wicresoft Co,.Ltd.) - It is close. I need to show you more than "reply" allows, so I'll show it here. Your solution compiles but only Add Operator works. Modifiy is not selectable and Delete asks to select an Operator with no means of doing that. I shall show you here the SecectOperator.xaml that I am using:

...and the code-behind, SelectOperator.xaml.cs:

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
using System.Threading.Tasks;  
  
using Xamarin.Forms;  
using Xamarin.Forms.Xaml;  
using HandsFreeNotes.ViewModel;  
using HandsFreeNotes.Model;  
  
namespace HandsFreeNotes.View  
{  
    [XamlCompilation(XamlCompilationOptions.Compile)]  
    public partial class SelectOperatorPage : ContentPage  
    {  
        OperatorViewModel ovm;  
  
        // Added the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
        public EventHandler ReturnValue;  
  
        public SelectOperatorPage()  
        {  
            // add a bit of padding to cater to the "notch" on the iPhone.  
            if (Device.RuntimePlatform == Device.iOS)  
            {  
                Padding = new Thickness(0, 40, 0, 0);  
            }  
  
            InitializeComponent();  
  
            ovm = new OperatorViewModel();  
  
            BindingContext = ovm;  
        }  
  
        protected override async void OnAppearing()  
        {  
            base.OnAppearing();  
            await ovm.GetOperators();  
            OperatorListView.ItemsSource = OperatorViewModel.OperatorList;  
        }  
  
        private async void BackButton_Clicked(object sender, EventArgs e)  
        {  
            await Navigation.PopModalAsync();  
        }  
  
        private async void OperatorListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)  
        {  
            if (e.SelectedItem != null)  
            {  
                // Typecast the e.SelectedItem object to an OperatorModel  
  
                // ovm.SelectedOperator = (OperatorModel)e.SelectedItem;  
                // await DisplayAlert("Alert", "Selected Operator: " + ovm.SelectedOperator.OperatorName, "Exit");  
  
                // The 2 lines above were changed to the 7 below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021  
  
                OperatorModel operatorModel = (OperatorModel)e.SelectedItem;  
                ovm.SelectedOperator = operatorModel;  
                EventHandler handler = ReturnValue;  
                if (handler != null)  
                {  
                    handler(this, operatorModel);  
                }  
  
  
                await Navigation.PopModalAsync();  
            }  
        }  
    }  
}

Thanks for your persistence.

Answer

Please copy my OperatorsPage.xaml.cs code in your demo. I make some changes in the BackCall method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace App94
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class OperatorsPage : ContentPage
    {
        OperatorViewModel ovm;
        OperatorModel opm;

        // public OperatorsPage(OperatorViewModel _ovm)
        // The line above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021
        public OperatorsPage()
        {
            // add a bit of padding to cater to the "notch" on the iPhone.
            if (Device.RuntimePlatform == Device.iOS)
            {
                Padding = new Thickness(0, 40, 0, 0);
            }

            InitializeComponent();

            // ovm = _ovm;
            // opm = ovm.SelectedOperator;
            // The 2 lines above changed to the line below at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021
            opm = new OperatorModel();

            BindingContext = opm;

        }

        private async void BackButton_Clicked(object sender, EventArgs e)
        {
            await Navigation.PopModalAsync();
            // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021
        }

        private async void AddOperatorButton_Clicked(object sender, EventArgs e)
        {
            await Navigation.PushModalAsync(new NewOperatorPage());
            // The line above originally commented out at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021

        }

        private async void SelectOperatorButton_Clicked(object sender, EventArgs e)
        {
            // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021
            SelectOperatorPage page = new SelectOperatorPage();
            page.ReturnValue += delegate (object s, OperatorModel operatorModel)
            {
                BackCall(s, operatorModel);
            };

            await Navigation.PushModalAsync(page);
        }
        // The lines below added at the behest of JessieZhang-2116 (Microsoft Q&A) 01/05/2021

        OperatorModel SelectModel;
        private void BackCall(object s, OperatorModel model)
        {
            SelectModel = model;
            opm.OperatorName = model.OperatorName;
        }



        private async void DeleteOperatorButton_Clicked(object sender, EventArgs e)
        {
            if (SelectModel != null)
            {

                OperatorViewModel.OperatorList.Remove(SelectModel);
                SelectModel = null;
                SelectedOperatorEntry.Text = "";
            }
            else
            {
              await  DisplayAlert("info","please select item","OK");
            }
        }

        private async void UpdateOperatorButton_Clicked(object sender, EventArgs e)
        {
            //var operatorInfoPage = new OperatorInfoPage();
            //operatorInfoPage.BindingContext = ovm;
            //await Navigation.PushModalAsync(operatorInfoPage);
            if (opm != null)
            {

                //  var operatorInfoPage = new OperatorInfoPage(opm);
                //operatorInfoPage.BindingContext = ovm;
                //  await Navigation.PushModalAsync(operatorInfoPage);

            }


        }
    }
}

How to Use the MVVM Architectural Pattern to Pass Data (both Push and Pop) Between Pages

3 additional answers