Dynamically generating ViewModel for tooltips in WPF MVVM

Question

I have a list of items representing packages in an MVVM control.

When you hover over the tooltip it needs to go to the database for additional information, lets just call it 'PackageDetails' for simplicity. I know how to handle the database loading with a ViewModel class but I'm having trouble figuring out when to instantiate it.

Approach 1) Have a 'lazy-load' property in the 'Package' object so when the tooltip is triggered the viewmodel will be created and immediately access the database. This approach isn't ideal because each 'Package' object isn't a true viewmodel and came from WCF objects originally.

Approach 2) Use a converter as explained in this Josh Smith blog entry. His example seems to fit a converter well, but I don't think it really suits my situation well.

Approach 3) Somehow create a viewmodel in the XAML, but this seems like a bad idea.

What's a good approach to dynamically generate a viewmodel for a tooltip using MVVM ?

Answer 1

Binding models ( in your case the packages ) to the view only works for very simple situations where there is no more "processing" or business logic to implement.

I have experimented with a few options and in the ended up creating a VM wrapper for just about all my models. Going down this path makes having a tooltip property straight forward.

The other option that i have experimented with is to use partial classes to extend the wcf models. This works unless you are using dataannotations for validation ( wcf and dataannotations dont work together properly )

if you decide to wrap your models with a vm, then instantiating your list of VM wrappers is just one line of code using linq and lambdas

assuming you have a constructor on your VM that accepts your model as a parameter.

var listPackageVMs = new ObservableCollection<PackageVM> ( listPackageModels.Select(model=> new PackageVM(model)));

Answer 2

You could create a partial class to Package. I would avoid placing data access logic in an entity class, but this is the cheap and easy way.

namespace WCFServiceNamespace
{
    // Since WCF generated entities are partial classes, we can inject features
    public partial class Package
    {
        private readonly IDataAccessor _DataAccessor;

        public Package()
            : this(DataAccessor.Instance) // how you choose to inject a data accessor is up to you
        {
        }

        public Package(IDataAccessor dataAccessor)
        {
            _DataAccessor = dataAccessor;
            _ToolTip = new Lazy<string>(GetToolTip);
        }

        private readonly Lazy<string> _ToolTip;
        public string ToolTip
        {
            get
            {
                // executes GetToolTip when the Value property of Lazy<T> is accessed
                return _ToolTip.Value;
            }
        }

        private string GetToolTip()
        {
            // we're assuming we can retreive the tooltip by ID, and that PackageId is defined in the generated WCF entity
            return _DataAccessor.GetToolTipByPackageId(PackageId);
        }
    }
}