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How to delete data in a .NET MAUI list

I am currently doing a project with the MVVM method in NET MAUI to add, modify and delete drivers.
I have a template that contains the name, first name and number of points of the driver.
Then I have two views each with a model view:
- One that represents the list of my drivers with the possibility to add a driver, to select a driver from the list by going to another page (PageListPilotViewModel).
- And another one which represents the selected driver in another page to be able to modify its data and the possibility of removing it. (ProfilePilotViewModel)
At the moment I can select, add the driver and modify the driver in the other page. But I can't delete the driver in the profile page.
Here is what I have done so far:
-> Models : Pilote Model
public class PiloteModel : INotifyPropertyChanged
{
private string _nom;
public string Nom
{
get { return _nom; }
set { _nom = value; OnPropertyChanged(); }
}
private string _prenom;
public string Prenom
{
get { return _prenom; }
set { _prenom = value; OnPropertyChanged(); }
}
private int _points;
public int Points
{
get { return _points; }
set { _points = value; OnPropertyChanged(); }
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
-> View : ProfilPilotePage
<vm:PageListPiloteViewModel></vm:PageListPiloteViewModel>
</ContentPage.BindingContext>
<VerticalStackLayout>
<StackLayout>
<Entry Text="{Binding Pilote.Nom, Mode=TwoWay}" Placeholder="{Binding Nom}"></Entry>
<Entry Text="{Binding Pilote.Prenom}" Placeholder="{Binding Pilote.Prenom}"></Entry>
<Entry Text="{Binding Pilote.Points}" Placeholder="{Binding Pilote.Points}"></Entry>
<Button Command="{Binding OnsupprimerPiloteCommand}">
</Button>
</StackLayout>
-> code behind the profilePilotPage view
public partial class ProfilPilotePage : ContentPage
{
private PageListPiloteViewModel _viewModel;
public ProfilPilotePage(PageListPiloteViewModel viewModel)
{
InitializeComponent();
_viewModel = viewModel;
_viewModel.SupprimerPiloteClicked += OnSupprimerPiloteClicked;
BindingContext = _viewModel;
}
private void OnSupprimerPiloteClicked(object sender, PiloteModel e)
{
_viewModel.ListePilotes.Remove(e);
}
-> model views : PageListPilotViewModel , to be able to delete also the driver in the list
public ICommand OnsupprimerPiloteCommand { get; set; }
public PageListPiloteViewModel()
{
ValiderCommand = new Command(AjouterPilote);
OnsupprimerPiloteCommand = new Command(OnSupprimerPiloteClicked);
SelectedPilote = new PiloteModel();
ListePilotes = new ObservableCollection<Models.PiloteModel>();
ListePilotes.Add(new Models.PiloteModel { Nom = "Fabio", Prenom = "Quartaro", Points = 215 });
}
private void OnSupprimerPiloteClicked()
{
SupprimerPiloteClicked?.Invoke(this, SelectedPilote);
}
->code behind the PageListPiloteView: with the error I encounter on the last :
await Navigation.PushAsync(new ProfilePilotPage{ BindingContext = viewModel }) :
CS7036 Error None of the specified arguments match the 'viewModel' mandatory parameter of 'ProfilePilotPage.ProfilePilotPage(PageListPilotViewModel)'
private async void SelectionnerPilote(object sender, SelectionChangedEventArgs e)
{
PiloteModel selectedPilote = (PiloteModel)((CollectionView)sender).SelectedItem;
ProfilPiloteViewModel viewModel = new ProfilPiloteViewModel();
viewModel.Pilote = selectedPilote;
await Navigation.PushAsync(new ProfilPilotePage{ BindingContext = viewModel });
}
}
Do you have any idea how to make the specified arguments mandatory please ?

You've mixed up constructor and initializer.
This line
await Navigation.PushAsync(new ProfilPilotePage{ BindingContext = viewModel });
should be
await Navigation.PushAsync(new ProfilPilotePage(viewModel));
The reason for this is that you're defining an argument in the signature of the ProfilPilotePage's constructor:
public ProfilPilotePage(PageListPiloteViewModel viewModel)
{
//...
}
Therefore, you must pass the ViewModel argument.

At first, you can try to use the ewerspej's solution or only add a default construction method without any parameter into the ProfilPilotePage to fix the error caused by await Navigation.PushAsync(new ProfilePilotPage{ BindingContext = viewModel }) . Such as:
public partial class ProfilPilotePage : ContentPage
{
private PageListPiloteViewModel _viewModel;
public ProfilPilotePage()
{
InitializeComponent();
}
public ProfilPilotePage(PageListPiloteViewModel viewModel)
{
InitializeComponent();
_viewModel = viewModel;
_viewModel.SupprimerPiloteClicked += OnSupprimerPiloteClicked;
BindingContext = _viewModel;
}
}
And then I saw you used both the mvvm and the code behind. You can remove the OnSupprimerPiloteClicked(object sender, PiloteModel e) in the page.cs and change the OnSupprimerPiloteClicked() in the view model. Such as:
private void OnSupprimerPiloteClicked()
{
ListePilotes.Remove(SelectedPilote);
}
Finally, I saw SelectedPilote = new PiloteModel(); in your viewmodel. Which item in the list did you want to delete? I think it should be the seleted item not a new PiloteModel().

UWP C# MVVM How To Access ViewModel from Other Page

I am tying to further understand MVVM with some example scenario. I have a rootpage with a 'maindisplay' textblock. I would like to display 'status' or 'scenarios' from activation of any form of UI eg. togglebutton on the 'maindisplay' textblock.
I am able to bind the the page navigation info in the rootpageviewmodel to the textblock. However, I am not able to achieve the result when displaying info from different page.
I have checked another post multiple-viewmodels-in-same-view & Accessing a property in one ViewModel from another it's quite similar but it didn't work.
Please help. Thanks.
While accessing the RootPageViewModel should retain the instance?
View
<TextBlock Text="{x:Bind RootViewModel.MainStatusContent, Mode=OneWay}"/>
RootPage.xaml.cs
public sealed partial class RootPage : Page
{
private static RootPage instance;
public RootPageViewModel RootViewModel { get; set; }
public RootPage()
{
RootViewModel = new RootPageViewModel();
this.InitializeComponent();
// Always use the cached page
this.NavigationCacheMode = NavigationCacheMode.Required;
}
public static RootPage Instance
{
get
{
if (instance == null)
{
instance = new RootPage();
}
return instance;
}
}
private void nvTopLevelNav_ItemInvoked(NavigationView sender, NavigationViewItemInvokedEventArgs args)
{
if (args.IsSettingsInvoked)
{
contentFrame.Navigate(typeof(SettingsPage));
RootViewModel.MainStatusContent = "Settings_Page";
}
else
{
var navItemTag = args.InvokedItemContainer.Tag.ToString();
RootViewModel.MainStatusContent = navItemTag;
switch (navItemTag)
{
case "Home_Page":
contentFrame.Navigate(typeof(HomePage));
break;
case "Message_Page":
contentFrame.Navigate(typeof(MessagePage));
break;
}
}
}
}
RootPage ViewModel:
public class RootPageViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private static RootPageViewModel instance = new RootPageViewModel();
public static RootPageViewModel Instance
{
get
{
if (instance == null)
instance = new RootPageViewModel();
return instance;
}
}
public RootPageViewModel()
{
}
private string _mainStatusContent;
public string MainStatusContent
{
get
{
return _mainStatusContent;
}
set
{
_mainStatusContent = value;
OnPropertyChanged();
}
}
protected void OnPropertyChanged([CallerMemberName] string name = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
MessagePage.xaml.cs - to access RootPage ViewModel
public sealed partial class MessagePage : Page
{
public MessagePageViewModel MessageViewModel { get; set; }
public MessagePage()
{
MessageViewModel = new MessagePageViewModel();
this.InitializeComponent();
// Always use the cached page
this.NavigationCacheMode = NavigationCacheMode.Required;
}
private void Message1_Checked(object sender, RoutedEventArgs e)
{
RootPageViewModel.Instance.MainStatusContent = "Message 1 Selected";
}
private void Message1_Unchecked(object sender, RoutedEventArgs e)
{
RootPageViewModel.Instance.MainStatusContent = "Message 1 De-Selected";
}
}
When I debug the value did write to the instance but did't update the TextBlock. Did I do anything wrong in my XAML binding?

UWP C# MVVM How To Access ViewModel from Other Page
The better way is make static variable for RootPage, but not make singleton instance for RootPage and RootPageViewModel.
For example:
public RootPage ()
{
this.InitializeComponent();
this.NavigationCacheMode = NavigationCacheMode.Required;
Instance = this;
RootViewModel = new RootPageViewModel();
}
public static RootPage Instance;
Usage
private void Message1_Checked(object sender, RoutedEventArgs e)
{
RootPage.Instance.RootViewModel.MainStatusContent = "Message 1 Selected";
}
private void Message1_Unchecked(object sender, RoutedEventArgs e)
{
RootPage.Instance.RootViewModel.MainStatusContent = "Message 1 De-Selected";
}

WPF combobox does not update when values change

I have WPF application with Combobox and Button. After I enter value into textbox and press button, the value from textbox should appear in updated list of combobox. I am trying to achieve this with MVVM and binding to combobox. Here is part of code from ViewModel.
public class ViewModel:INotifyPropertyChanged
{
DomainLogic dl = new DomainLogic();
public event PropertyChangedEventHandler PropertyChanged;
private ObservableCollection<string> expenseCategories = new ObservableCollection<string>();
public ObservableCollection<string> ExpenseCategories
{
get
{
return expenseCategories;
}
set
{
expenseCategories = value;
OnPropertyChanged("ExpenseCategories");
}
}
public ViewModel()
{
expenseCategories = new ObservableCollection<string>(dl.GetExpenseCategories());
}
private void OnPropertyChanged(string property)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(property));
}
}
Also I am using EF to access DB and DomainLogic class has a method to list all Expense Categories.
Here is code-behind from window:
DomainLogic dl = new DomainLogic();
public Window1()
{
InitializeComponent();
DataContext = new ViewModel();
}
private void button1_Click(object sender, RoutedEventArgs e)
{
if (!string.IsNullOrWhiteSpace(textBox.Text))
{
dl.CreateNewExpenseCategory(textBox.Text);
}
else
{
MessageBox.Show("Enter category!");
}
}
Here is also XAML:
<ComboBox x:Name="ExpCategory" HorizontalAlignment="Left" Margin="72,50,0,0" VerticalAlignment="Top" Width="130" ItemsSource="{Binding ExpenseCategories, UpdateSourceTrigger=PropertyChanged}" />
When I add the new category the combobox isn't updated. I'm new to the whole MVVM pattern and I think I'm missing something here.
//EDIT
public void CreateNewExpenseCategory(string name)
{
using (var context = new ExpenseEntities())
{
ExpenseCategory category = new ExpenseCategory() { CategoryName = name};
context.ExpenseCategory.Add(category);
context.SaveChanges();
}
}

The problem is that CollectionChanged event doesn't fire.
You are adding a new element inside your DataContext, but you aren't updating your local view of data.
Once you update the DataContext you should refresh your ObservableCollection or use Local.
Here how you could use Local:
public ViewModel()
{
expenseCategories = dl.GetExpenseCategories().Local;
}
So you can directly do:
expenseCategories.Add(new ExpenseCategory() {textBox.Text});
dl.GetContext().SaveChanges();
Or you have to update the ObservableCollection:
dl.CreateNewExpenseCategory(textBox.Text);
// Update your ViewModel ObservableCollection.
However i think that you should use a Command and not an Event so you can update the ObservableCollection directly inside the ViewModel.
Example:
using Prism.Commands;
//Other usings
public class ViewModel : INotifyPropertyChanged
{
// Your class methods and properties
public DelegateCommand<string> AddNewExpenseCategory
{
get
{
return new DelegateCommand<string>(Execute_AddNewExpenseCategory);
}
}
public void Execute_AddNewExpenseCategory(string param)
{
expenseCategories.Add(new ExpenseCategory() { param });
dl.GetContext().SaveChanges();
}

Update Listbox with item on navigation

When I navigate from a page to another I am passing an object of class and adding to list view. But the problem I am facing is that each time when the page is navigated the item is added in list box but the item that was previously added gets removed.
Can anyone help me to solve this issue and how I can retrieve the previous listbox item?
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
if ((App.Current as App).isThereAnyChange)
{
getitem();
changeUI();
}
}
private void changeUI()
{
//throw new NotImplementedException();
txt1.Text = "";
items.Add(item1);
Deployment.Current.Dispatcher.BeginInvoke(() =>
{
this.sniplist.ItemsSource = items;
});
}
private void getitem()
{
// throw new NotImplementedException();
item1 = PhoneApplicationService.Current.State["item"] as Item;
// Debug.WriteLine(item1.description);
}

Keep a static item in your ....xaml.cs file
static List<object> listSrc = new List<object>();
override OnNavigatedTo()
{
listSrc.Add("whatever you want");
sniplist.ItemsSource= listSrc;
}

Here a simple setup:
public class MainViewModel
{
public ObservableCollection<Item> Items { get; private set; }
public MainViewModel() { Items = new ObservableCollection<Item>(); }
}
in App.xaml.cs:
public partial class App : Application
{
private static MainViewModel viewModel = new MainViewModel();
public static MainViewModel ViewModel { get { return viewModel; } }
...
And then you can access that class from any page simply with App.ViewModel, and it will persist through page navigations.

Trouble binding to a textblock - Windows Phone - MVVM

I have spent a couple hours trying to figure out this ONE problem. Here's what is happening:
I am trying to bind a Title to my XAML file from my ViewModel. All of the code executes (I checked using breakpoints/watch), but the binding doesn't actually work. I am very new to development and especially MVVM, so I am having a hard time figuring this out. Relevant code:
App.Xaml.Cs
private static MainPageViewModel _mainPageViewModel = null;
public static MainPageViewModel MainPageViewModel
{
get
{
if (_mainPageViewModel == null)
{
_mainPageViewModel = new MainPageViewModel();
}
return _mainPageViewModel;
}
}
MainPageModel
public class MainPageModel : BaseModel
{
private string _pageTitle;
public string PageTitle
{
get { return _pageTitle; }
set
{
if (_pageTitle != value)
{
NotifyPropertyChanging();
_pageTitle = value;
NotifyPropertyChanged();
}
}
}
MainPageViewModel
private void LoadAll()
{
var page = new MainPageModel();
page.PageTitle = "title";
MainPageViewModel
public MainPageViewModel()
{
LoadAll();
}
MainPage.Xaml.Cs
public MainPage()
{
InitializeComponent();
DataContext = App.MainPageViewModel;
}
MainPage.Xaml
<Grid x:Name="LayoutRoot">
<phone:Panorama Title="{Binding PageTitle}">
Do I need a using statement in the Xaml too? I thought I just needed to set the data context in the MainPage.Xaml.Cs file.
I'm pretty sure I've posted all of the relevant code for this. Thanks everyone!!

The problem is here, in the view model class:
private void LoadAll()
{
var page = new MainPageModel();
page.PageTitle = "title";
All you've done here is create a local object "page" -- this will not be accessible anywhere outside the local scope. I suppose what you meant to do is make "page" a member of "MainPageViewModel":
public class MainPageViewModel
{
public MainPageModel Model { get; private set; }
private void LoadAll()
{
_page = new MainPageModel();
_page.PageTitle = "title";
}
}
This way, you'll be able to bind to the "PageTitle" property -- but remember, it's a nested property, so you'll need:
<phone:Panorama Title="{Binding Model.PageTitle}">