How do you do proper binding and updating of Xamarin.Forms ListView?

Question

Using the MVVM pattern, I have a Model, ViewModel and View, which contains a ListView. The ListView is bound to a member of the ViewModel that is an ObservableCollection of the Model class. I can get the binding for the initial display to work and can update properties on the Model class for the appropriate row upon acting on the view, but I cannot get the view to refresh, pulling data from the Model class in the ObservableCollection. The ListView class does not contain method to invalidate or force a refresh, which would address my issue. How do I get the ListView to refresh after updating data on the Model?

Here is a simple example of what I am trying to do: Each row contains a Button and Label. Upon clicking a button, I can update the label, which will be reflected on screen. What I need to do is update the Model, which in turn should force an update of the view. However, I cannot get this to work. In an actual application, updating of the Model will be in a business logic layer, not in the view, after which I need to force refresh of the ListView.

Example Code:

using System;
using System.Collections.ObjectModel;
using Xamarin.Forms;

namespace ListViewTest
{   
    public class Model
    {
        public static int ids = 0;
        public Model(string count) 
        {
            Id = +1;
            Count = count;
        }
        public int Id { get; set; }
        public string Count { get; set; }
    }

    public class ModelList : ObservableCollection<Model>
    {
    }

    public class ViewModel
    {
        ModelList list = new ModelList();
        public ModelList ViewModelList
        {
            get { return list; }
            set 
            { 
                list = value; 
            }
        }
    }

    public partial class MainPage : ContentPage
    {   
        public ViewModel viewModel;

        public class DataCell : ViewCell
        {
            public DataCell()
            {
                var Btn = new Button();
                Btn.Text = "Click";
                var Data = new Label();
                Data.SetBinding(Label.TextProperty,"Count");
                Btn.Clicked += (object sender, EventArgs e) => 
                {
                    Model model = (Model)(((Button)sender).Parent.BindingContext);
                    int count = Convert.ToInt32(model.Count);
                    count++;
                    model.Count = count.ToString();

                    // Need to refresh ListView from data source here... How???
                };

                StackLayout s = new StackLayout();
                s.Orientation = StackOrientation.Horizontal;
                s.Children.Add(Btn);
                s.Children.Add(Data);
                this.View = s;
            }
        }

        public MainPage ()
        {
            viewModel = new ViewModel();
            viewModel.ViewModelList.Add(new Model("0"));
            viewModel.ViewModelList.Add(new Model("0"));
            InitializeComponent();
        }

        public void InitializeComponent()
        {
            ListView listView = new ListView
            {
                ItemsSource = viewModel.ViewModelList,
                ItemTemplate = new DataTemplate(() =>
                    {
                        return new DataCell();
                    })
            };
            Content = listView;
        }
    }
}

Solution 1

i think your model needs to implement INotifyPropertyChanged. So the UI knows that a value in model has changed

something like this :

public class Model : INotifyPropertyChanged
{
    // boiler-plate
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    // props
    private string _count;
    public string Count 
    {
        get { return _count; }
        set { SetField(ref _count, value, "Count"); }
    }
}

Solution 2

Remove the "set" method from your ViewModelList property. Whenever you use an ObservableCollection, just change the items in that one collection. Never replace it with another ObservableCollection.