WPF MVVM Light DataGrid and separate Add Item View

Question

I am learning WPF and MVVM and I am trying to make a program which has a DataGrid and a button which opens another form by using a command from where you can add an item to the DataGrid.

The problem is that I am not sure how I should implement this with the viewmodels. I have 1 viewmodel which is for my DataGrid form which works correctly and is using a repository to retrieve data from Entity Framework.

Is it possible to add an object from the Add Form and have it appear in the DataGrid in the other form automatically when I press the add button or do I need to do some refresh action on the DataGrid? I am using observable collections in my viewmodel and I have implemented the OnPropertyChange functionality in the collection parameter in the view model.

As far as I can understand when I put something in a repository, every view model which gets data from it should be refreshed... though I am not sure if I should write some message code for that to work.

I am pretty confused about this and I hope that someone can shed some light here... Thanks in advance. Tell me if I have missed to mention something and I will add it:)

Edit: This is how I register my models:

SimpleIoc.Default.Register<ParentsListViewModel>();
SimpleIoc.Default.Register<EditParentViewModel>();

And this is how I register my repository:

SimpleIoc.Default.Register<IParentsRepository, ParentsRepository>();

And this is how I retrieve the instances of the ViewModels:

ParentsListViewModel parentsListViewModelInstance =  ServiceLocator.Current.GetInstance<ParentsListViewModel>();
EditParentViewModel editParentViewModelInstance = ServiceLocator.Current.GetInstance<EditParentViewModel>();

The ParentsListViewModel is my DataGrid ViewModel and the EditParentViewModel is the one is use for adding/editing records in the repository. The service locator passes the instances of the repositories automaticaly so I have no idea how it passes the instance of the repository to the ViewModels. Does it pass the same instance?

Matt Whetton Matt Whetton · Accepted Answer · 2013-02-12T10:05:44

The view models wont automatically refresh when you add something to a repository.

All the observable collection does is inform the UI when a new item is added / removed to the observable collection (roughly speaking). All the INotifyPropertyChange does is inform the UI that a specific property has changed.

You have a few options for getting what you want to work (if I understand you correctly):

You could refresh the entire observable collection when an item is added
You could get the repository to inform your datagrid view model with an event when a new item is added - then your datagrid view model can update its observable collection
You could get the Add Form view model talk to the Data Grid view model and tell it when a new item is added - in this situation you'll need some mechanism for marshaling the information from one view model to another.

Does this help?

Edit 12/02/2013 17:30 GMT:

Here's a very quick and dirty example of what option 2 could look like. It requires that the same instance of the repository is shared between the 2 view models - in this case I have injected it on the constructors.

public interface IParentsRepository{
    event EventHandler<MyItemAddedEventArgs> ItemAdded;     

    //your normal interface implementation here
}

public class ParentsRepository : IParentsRepository
{
    public event EventHandler<MyItemAddedEventArgs> ItemAdded;

    public List<MyItem> GetAllItems()
    {
        //logic that returns all your items here
        return new List<MyItem>();
    }

    public void AddItem(MyItem item)
    {
        //logic that adds you item here

        //fire the item added event
        OnItemAdded(item);
    }

    private void OnItemAdded(MyItem item)
    {
        if(ItemAdded != null)
            ItemAdded(this, new MyItemAddedEventArgs(item));
    }
}

public class MyItemAddedEventArgs : EventArgs
{
    public MyItemAddedEventArgs(MyItem itemAdded)
    {

    }

    public MyItem ItemAdded { get; set; }
}

public class MyItem
{
    public string SomeProperty { get; set; }    
}

public class MyDataGridViewModel
{
    private readonly IParentsRepository _parentsRepository;

    public MyDataGridViewModel(IParentsRepository parentsRepository)
    {
        _parentsRepository = parentsRepository;
        _parentsRepository.ItemAdded += _parentsRepository_ItemAdded;

        var myItems = _parentsRepository.GetAllItems();
        MyItems = new ObservableCollection<MyItem>(myItems);
    }

    void _parentsRepository_ItemAdded(object sender, MyItemAddedEventArgs e)
    {
        if(!MyItems.Contains(e.ItemAdded))
            MyItems.Add(e.ItemAdded);
    }

    public ObservableCollection<MyItem> MyItems { get; set; }
}

public class MyAddItemViewModel
{
    private readonly IParentsRepository _parentsRepository;

    public MyAddItemViewModel(IParentsRepository parentsRepository)
    {
        _parentsRepository = parentsRepository;
    }

    //your logic to add an item here
}

This method could easily be adjusted to method 1, so that when the _parentsRepository_ItemAdded event fires, rather than just adding the new item, you fetch the whole dataset again.

WPF MVVM Light DataGrid and separate Add Item View

1 Answers