I have two pages: MainPage and FilterPage(modal page). with their respective Viewmodels: MainViewModel and FilterViewModel.

In MainPage I have a listview that's populated with data from an API. The data is passed to the FilterPage where it is filtered by some specific criteria. In the end a new list is created which is assigned to the binded variable of the listview. What I noticed is that after the modal page closes the listview's items arent updated. What is the proper way to do this?

Model:

public class Multilist
    {
        public string Title { get; set; }    
        public string Date { get; set; } 
        public string Status { get; set; }
        public string Customer { get; set; }

    }

MainViewModel:

public class MainViewModel : INotifyPropertyChanged 
{

private IList<Multilist> mainList = new List<Multilist>();
        public IList<Multilist> MainList
        {
            get => mainList;
            set
            {
                if (value == mainList)
                    return;
                mainList = value;
                OnPropertyChanged();
            }
        }


#region INotifyPropertyChanged
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var changed = PropertyChanged;
            if (changed == null)
                return;

            changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion



public MainViewModel(INavigation navigation)
        {
            this._navigation = navigation; 

            Task.Run(async () => await GetData());

            GotoFilterPageCommand = new AsyncCommand(GotoFilterPage);            

        }

private async Task GetData()
        {
            
            //Gets data from API
            MainList = data;

        }

private async Task GotoFilterPage()
        {
            await this._navigation.PushModalAsync(new FilterPage(MainList.ToList()), true);
            
        }


}

FilterViewModel:

public class FilterViewModel : INotifyPropertyChange
    {

public List<Multilist> OldList { get; set; }

private IList<Multilist> mainList = new List<Multilist>();
        public IList<Multilist> MainList
        {
            get => mainList;
            set
            {
                if (value == mainList)
                    return;
                mainList = value;
                OnPropertyChanged();
            }
        }


#region INotifyPropertyChanged
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var changed = PropertyChanged;
            if (changed == null)
                return;

            changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion


        public FilterViewModel(List<Multilist> oldlist)
        {
            Oldlist = oldlist;
            SomeCommand = new AsyncCommand(SomeTask);


        }


private async Task SomeTask()
        {
            // Some code here
            CreateNewList(OldList);
        }

private async Task CreateNewList(List<Multilist> oldlist)
        {
            //Some code here --> newMainList
            
            pageA.MainList = newMainList;
            await App.Current.MainPage.Navigation.PopModalAsync();
        }



}

The listview in MainPage:

<ListView x:Name="TestListView"
                  ItemsSource="{Binding MainList}"                     
                  Grid.Row="4" Grid.ColumnSpan="3"
                  HasUnevenRows="True"
                  >
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <StackLayout Padding="0,0,0,1">
                                <Grid VerticalOptions="Fill" Padding="10">

                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="60"></RowDefinition>

                                    </Grid.RowDefinitions>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="*"></ColumnDefinition>
                                        <ColumnDefinition Width="*"></ColumnDefinition>
                                        <ColumnDefinition Width="*"></ColumnDefinition>
                                        <ColumnDefinition Width="*"></ColumnDefinition>

                                    </Grid.ColumnDefinitions>
                                    <Label Text="{Binding Title}" HorizontalTextAlignment="Start" VerticalTextAlignment="Center" Grid.Row="0" Grid.Column="0" />
                                    <Label Text="{Binding Date}" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" Grid.Row="0" Grid.Column="1" />
                                    <Label Text="{Binding Customer}" HorizontalTextAlignment="Start" VerticalTextAlignment="Center" Grid.Row="0" Grid.Column="2" />
                                    <Label Text="{Binding Status}" HorizontalTextAlignment="End" VerticalTextAlignment="Center" Grid.Row="0" Grid.Column="3" />

                                </Grid>
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>

CodePudding user response：

You could use Singleton to make a global instance for both MainViewModel and FilterViewModel.

I make a simple example for your reference.

Model:

 public class Person
{
    public string Name { get; set; }
    public string FirstName { get; set; }
    public int Age { get; set; }
}

ViewModel:

  public class PersonViewModel
{
    #region Singleton Pattern
    private PersonViewModel()
    {
        Persons = new ObservableCollection<Person>()
        {
            new Person(){ Name="A"},
            new Person(){ Name="A2"},
            new Person(){ Name="A3"},
            new Person(){ Name="A4"},
        };
    }
    public static PersonViewModel Instance { get; } = new PersonViewModel();
    #endregion
    private ObservableCollection<Person> _person;
    public ObservableCollection<Person> Persons
    {
        get { return _person; }
        set { _person = value; }
    }
} 

Page24: //MainPage

 <StackLayout>
        <ListView ItemsSource="{Binding MainList}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <StackLayout>
                            <Label Text="{Binding Name}"></Label>                               
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
        <Button Text="Navigate To FilterPage" Clicked="Button_Clicked"> 
     </Button>
    </StackLayout>

Page24 Code behind:

public Page24()
    {
        InitializeComponent();
        this.BindingContext = new Page24ViewModel();
    }

    private void Button_Clicked(object sender, EventArgs e)
    {
        Navigation.PushAsync(new FilterPage());
    }

Page24ViewModel://MainViewModel

public class Page24ViewModel : INotifyPropertyChanged
{
    private PersonViewModel _personViewModel;

    public Page24ViewModel()
    {
        _personViewModel = PersonViewModel.Instance;

    }
    private ObservableCollection<Person> mainList;
    public ObservableCollection<Person> MainList
    {
        get { return _personViewModel.Persons; }
        set
        {
            mainList = value;
            OnPropertyChanged();
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

FilterPage:

<ContentPage.Content>
    <StackLayout>
        <StackLayout>
            <Label Text="Name:"></Label>
            <Entry x:Name="entry"></Entry>
        </StackLayout>
        <StackLayout Orientation="Horizontal">
            <Button x:Name="btn_Add" Text="Add" Clicked="btn_Add_Clicked"></Button>
            <!--<Button x:Name="btn_Delete" Text="Delete" Clicked="btn_Delete_Clicked"></Button>-->
        </StackLayout>
        <ListView ItemsSource="{Binding MainList}">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <Label Text="{Binding Name}"></Label>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout>
</ContentPage.Content>

FilterPage code behind:

 private PersonViewModel _personViewModel;
    public FilterPage()//modal page
    {
        InitializeComponent();
        this.BindingContext = new FilterViewModel();
    }

    private void btn_Add_Clicked(object sender, EventArgs e)
    {
        _personViewModel = PersonViewModel.Instance;
        _personViewModel.Persons.Add(new Person() { Name = entry.Text });
    }

FilterViewModel:

 public class FilterViewModel : INotifyPropertyChanged
{
    private PersonViewModel _personViewModel;

    public FilterViewModel()
    {
        _personViewModel = PersonViewModel.Instance;

    }
    private ObservableCollection<Person> newMainList;
    public ObservableCollection<Person> MainList
    {
        get { return _personViewModel.Persons; }
        set
        {
            newMainList = value;
            OnPropertyChanged();
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}