ObjectDataProvider in MVVM not necessary?

Question

I have created a simple MVVM, with only three classes CashFlowView, CashFlowViewModel, CashFlowModel.

I use an infragistic's 9.1 XamDataPresenter (or xamDataGrid).

    <igDP:XamDataPresenter Name="xamDataPresenter1" DataSource="{Binding Source={StaticResource CashFlowData}}">

    <ObjectDataProvider x:Key="CashFlowData" ObjectType="{x:Type ViewModel:CashflowViewModel}" MethodName="GetCashFlows" />

Inside my ViewModel:

public ObservableCollection<CashflowModel> GetCashFlows()
        {
            return new ObservableCollection<CashflowModel>() { ... };
        }

ViewModel is connected to View by this:

this.DataContext = new CashflowViewModel();

As long as I connect the grid to the ObjectDataProvider its perfectly running fine. But I wished I could just connect to a property within my ViewModel instead.

According to Infragistics all I have to do is this:

<igDP:XamDataGrid DataSource="{Binding Path=ViewModelCollection}"/>

But in this case it seems I need to bind to a collection of another ViewModel to represent my rows inside the grid. And thats where I get confused.

I tried this and it doesnt work:

<igDP:XamDataPresenter Name="xamDataPresenter1" DataSource="{Binding Path=CashFlows}">

Inside the ViewModel:

public ObservableCollection<CashflowDataGridViewModel> CashFlows
        {
            get
            {
                return new ObservableCollection<CashflowDataGridViewModel>();
            }
        }

But how do I create my second ViewModel (CashflowDataGridViewModel) ?

I tried adding this proprty within this second ViewModel:

public CashflowModel CashFlow
        {
            get
            {
                return new CashflowModel() {...};
            }
        }

But all I get shown on my view is "Cashflow" column header without any of the underlying headers of the actual cashflowModel class.

Answer 1

To be able to bind the View to properties on the ViewModel, the DataContext needs to be set to an instance of your ViewModel. What I commonly do is to include the following line in the constructor of the code-behind for my View:

this.DataContext = new SomeAwesomeViewModel();

You can also set the DataContext for containers if you want different groups of controls to use different ViewModels (e.g., Grid.DataContext, StackPanel.DataContext, etc.).

Once you have the DataContext set, you should be able to bind to the properties of that ViewModel.

Update

Here's a bit of sample code to get you going.

public class CashFlowViewModel
{
    public ObservableCollection<FlowViewModel> DataGridData
    {
        get...
    }
}

That's the property which should provide the data for the DataGrid. Now, here's what the FlowViewModel class could look like.

public class FlowViewModel
{
    decimal flowAmount;
    public decimal FlowAmount
    {
        get { return flowAmount; }
        set
        {
            if(flowAmount == value)
                return;

            flowAmount = value;
            NotifyPropertyChanged("FlowAmount");
        }
    }
    .
    .
    .

    private void NotifyPropertyChanged(string propertyName)
    {
        if(PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}