从GridView访问ItemsSource

Question

I have a button, When it's clicked it populates my Datagrid. The code is written within the .xaml.cs file, which I believe breaks the MVVM rule but it's just a temporary situation. I know it's not ideal for MVVM.

Calculate.xaml.cs

public void PopulateGrid(object sender, RoutedEventArgs e)
{
    BindableCollection<Payments> PaymentCollection = new BindableCollection<Payments>
    ....
    Datagrid.ItemsSource = PaymentCollection
    ....
}

My question is if there's a way to read the Datagrids ItemsSource From the ViewModel.

What I've Tried

LoansViewModel

public BindableCollection<Payments> paymentCollection {get; set;}

Calculate.xaml

<telerik:RadGridView ItemsSource="{Binding paymentCollection, Mode=TwoWay}" ... />

The collection paymentCollection Doesn't Update after calculate is clicked.

Answer 1

Your xaml code looks like it should work provided the DataContext of your grid is set to your ViewModel instance where your paymentCollection property is declared.

Once your binding is set, it calls the get on the paymentCollection property. If your collection property object is not reassigned any further, and you add and remove elements from it, and it notifies on those changes via INotifyCollectionChanged, it will work. This is how ObservableCollection works and used most commonly for such scenarios.

However, if when you calculate, you re-assign your paymentCollection property with a new instance, your grid will not update, because you now have an entirely different collection. In that case you will need to notify the view that the paymentCollection property itself has changed. In which case you should implement it as a notification property:

    private BindableCollection<Payments>_paymentCollection;
    public BindableCollection<Payments> paymentCollection {
        get { return _paymentCollection; }
        set {
            _paymentCollection = value;
            OnPropertyChanged("paymentCollection");
        }
    }

    protected void OnPropertyChanged(string name) {
        PropertyChangedEventHandler handler = PropertyChanged;
        if(handler != null) {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }

Answer 2

Just do this the correct MVVM way. Get rid of your PopulateGrid method in the .xaml.cs file and eliminate setting the Click property in your xaml. Instead bind the command property of the button to an ICommand property in your ViewModel the same way you are binding the ItemsSource of the RadGridView. You will need an implementation of ICommand to use and MVVM Lights RelayCommand is one option for that.

Here is the code for the ICommand:

private ICommand _populateGridCommand;
public ICommand PopulateGridCommand
{
   get
   {
      if (_populateGridCommand == null)
      {
         _populateGridCommand = new RelayCommand(() => PopulateGrid());
      }
      return _populateGridCommand;
   }
}

public void PopulateGrid()
{
   PaymentCollection.Clear();
   //load data and then add to the collection
}

UPDATE

To do this in code behind, you'll need to access the ViewModel and work on the collection from it. I don't like this but it should work.

public void PopulateGrid(object sender, RoutedEventArgs e)
{
   var loansVM = DataGrid.DataContext as LoansViewModel;
   loansVM.paymentsCollection.Clear();
   var newData = //load data
   foreach (var data in newData)
      loansVM.paymentsCollection.Add(data);
}