MvvmCross; How to RaisePropertyChange from another ViewModel

Question

I have a ShoppingCart listView with items that is bound to ShopingCartViewModel. When I click to an item it takes me to the ItemInfoFragment which is bound to ItemInfoViewModel.

In ItemInfoFragment I have a button which deletes the item and removes it from the ShoppingCart listview.

My problem is; After i delete the item and press backbutton to return to my previously activity, the ShoppingCart listView still shows the Item that I deleted.

My Question is; How to RaisePropertyChange in ShoppingCartViewModel when i exit the ItemInfoFragment?

Answer 1

I believe you have a few options:

Shared Persistent Storage

If you use a storage/caching solution like SQLite or Realm etc. Which can be used to read and modify the same shopping cart data between pages. You can then use view life cycle events (OnResume[Android] or ViewWillAppear[iOS]) to retrieve the latest from the cache.

Alternatively if the shopping cart data size is small you could read/write it to MvvmCross Settings Plugin. You will just have to serialize and deserialize your objects as you can only save basic types like strings, bools, int etc.

Dependency Injection Shared Instance

You can create an in memory cache via using a shared class instance the can be shared between multiple ViewModels. This classes properties can bind directly to your various views. Any changes to the list will update all views that bind to it. One thing to note is that you will have to manually handle clean up if you require the memory space occupied by the this instance class.

Example:

Example model

public class ItemInfo
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Price { get; set; }
}

Shared class instance and interface

public interface ISharedShoppingCart
{
    MvxObservableCollection<ItemInfo> ShoppingCartItems { get; set; }
}

public class SharedShoppingCart : MvxNotifyPropertyChanged, ISharedShoppingCart
{
    MvxObservableCollection<ItemInfo> _shoppingCartItems;
    public MvxObservableCollection<ItemInfo> ShoppingCartItems
    {
        get { return _shoppingCartItems; }
        set { SetProperty(ref _shoppingCartItems, value); }
    }
}

Make sure to register the class and interface

public class App : MvxApplication
{
    public override void Initialize()
    {
        /* Other registerations*/

        Mvx.LazyConstructAndRegisterSingleton<ISharedShoppingCart, SharedShoppingCart>();
    }
}

Example usage in shared ViewModels

public class ShopingCartViewModel : MvxViewModel
{
    readonly ISharedShoppingCart _sharedShoppingChart;

    public ShopingCartViewModel(ISharedShoppingCart sharedShoppingChart)
    {
        _sharedShoppingChart = sharedShoppingChart;
    }

    public MvxObservableCollection<ItemInfo> ShoppingCartItems
    {
        get { return _sharedShoppingChart.ShoppingCartItems; }
        set { _sharedShoppingChart.ShoppingCartItems = value; }
    }
}

public class ItemInfoViewModel : MvxViewModel
{
    readonly ISharedShoppingCart _sharedShoppingCart;

    public ItemInfoViewModel(ISharedShoppingCart sharedShoppingCart)
    {
        _sharedShoppingCart = sharedShoppingCart;
    }

    void RemoveItemFromCart(int id)
    {
        _sharedShoppingCart.ShoppingCartItems
            .Remove(_sharedShoppingCart.ShoppingCartItems.Single(x => x.Id == id));
    }
}

Pub/Sub

You could send messages back to the shopping cart ViewModel using the MvvmCross Messenger Plugin.