Xamarin Select All in ListView

Issue

I'm looking to have a Select All checkbox that will update all the other checkboxes in the listview when it's selected or deselected but I can't find a way to make them update. I've tried a foreach statement, as well as a for statement in the ViewModel and to run the task when the Select All checkbox is changed, but they don't seem to update the UI. Any help is appreaciated!

view model:

 public class SyncViewModel : BaseViewModel
    {
        public ObservableCollection<Company> CompaniesCollection { get; set; }
        public ObservableCollection<WellGroup> WellGroupCollection { get; set; }
        public ObservableCollection<Well> WellsCollection { get; set; }

        public SyncViewModel()
        {
            Title = "Sync";
            CompaniesCollection = new ObservableCollection<Company>();
            WellGroupCollection = new ObservableCollection<WellGroup>();
            WellsCollection = new ObservableCollection<Well>();
        }

        public async Task InitializeData()
        {

            var wellDataStore = new WellDataStore();

            var companies = await wellDataStore.GetAllGroups();
            if (companies != null)
            {
                CompaniesCollection.Clear();

                foreach (var company in companies)
                {
                    CompaniesCollection.Add(company);
                }
            }

        }
        
        public async Task SyncData()
        {

            IsBusy = true;

            // load and process data
            IsBusy = false;
            
        } 
    }
}

xaml:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:viewModel="clr-namespace:SiteVisits.ViewModels" xmlns:model="clr-namespace:SiteVisits.Models" 
             x:DataType="viewModel:SyncViewModel"
             x:Class="SiteVisits.Views.Sync">

    <ContentPage.ToolbarItems>
        <ToolbarItem Text="Sync" Clicked="Sync_Clicked" />
    </ContentPage.ToolbarItems>
    <StackLayout>
        <ActivityIndicator 
            IsVisible="{Binding IsBusy}" 
            IsRunning="{Binding IsBusy}" />
        <CheckBox x:Name="SelectAll" Color="Blue" CheckedChanged="CheckAll"  />
        <Label Text="Select All" FontSize="Large" VerticalOptions="Center"/>
        <ListView x:Name="Companies"
                ItemsSource="{Binding CompaniesCollection}"
                SelectionMode="Single"
                HasUnevenRows="True"
                ItemTapped="Companies_Selection">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <StackLayout Padding="10" x:DataType="model:Company">
                            <CheckBox x:Name="Name" Color="Blue" IsChecked="{Binding IsChecked}" />
                            <Label Text="{Binding Name}" 
                                        FontSize="Large"
                                        VerticalOptions="Center"/>
                        </StackLayout>

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

xaml.cs:

  SyncViewModel viewModel;
        public Sync(SyncViewModel viewModel)
        {
            InitializeComponent();

            this.viewModel = viewModel;
            BindingContext = this.viewModel;
        }
       
        protected override async void OnAppearing()
        {
            await viewModel.InitializeData();
        }

        async private void Sync_Clicked(object sender, EventArgs e)
        {
            await viewModel.SyncData(); 
        }

        private void Companies_Selection(object sender, ItemTappedEventArgs e)
        {

            if (e.Item != null)
            {
                var company = e.Item as Company;
                company.IsChecked = !company.IsChecked;
            }
        }

Solution

Since you are using MVVM, I would use a binding for the checkbox

    <CheckBox x:Name="SelectAll" Color="Blue" IsChecked="{Binding AllChecked}"  />

So then you will need this bool property, something like this. So when the value changes, it updates all the collection

    private bool _allChecked;
    public bool AllChecked { get => _allChecked;
        set
        {
            if (value != _allChecked)
            {
                UpdateCompaniesCollection(value);
                OnPropertyChanged(nameof(AllChecked));
            }
        }
    }

And then you will need the method to update the collection. One way would be

    void UpdateCompaniesCollection(bool newValue)
    {
        for(int i = 0; i < CompaniesCollection.Count; i++)
        {
            var tempCompany = CompaniesCollection[i];
            tempCompany.IsChecked = newValue;
            CompaniesCollection[i] = tempCompany;
        }
    }

That should do the trick. This only will trigger the change of the elements inside the collection when the Checkbox is checked. But if you want to also the the other way round (if a item in unchecked, then deselect the allCheck), that would be more complicated.

Answered By - Juan Sturla