Question

I know MVVM heavily uses the INotifyPropertyChanged, but I have never seen any usage of the INotifyPropertyChanging. Any reason why?

If I did want to use this, what would be a good way to integrate this into my MVVM Framework? I know you’re not supposed to use MessageBox on your ViewModel because then you can’t unit test it. So how would one go about throwing up an alert, then continuing on with the PropertyChange if applicable?

Answer 1

Something to keep in mind about INotifyPropertyChanging is you can’t stop the change from happening. This merely allows you to record that the change occurred.

I use it in a framework of mine for change tracking, but it isn’t an appropriate method for halting changes.

You could extend your ViewModelBase with a custom interface/event pair:

delegate void AcceptPendingChangeHandler(
    object sender,
    AcceptPendingChangeEventArgs e);

interface IAcceptPendingChange
{
    AcceptPendingChangeHandler PendingChange;
}

class AcceptPendingChangeEventArgs : EventArgs
{
    public string PropertyName { get; private set; }
    public object NewValue { get; private set; }
    public bool CancelPendingChange { get; set; }
    // flesh this puppy out
}

class ViewModelBase : IAcceptPendingChange, ...
{
    protected virtual bool RaiseAcceptPendingChange(
        string propertyName,
        object newValue)
    {
        var e = new AcceptPendingChangeEventArgs(propertyName, newValue)
        var handler = this.PendingChange;
        if (null != handler)
        {
            handler(this, e);
        }

        return !e.CancelPendingChange;
    }
}

At this point you’d need to add it by convention to your view models:

class SomeViewModel : ViewModelBase
{
     public string Foo
     {
         get { return this.foo; }
         set
         {
             if (this.RaiseAcceptPendingChange("Foo", value))
             {
                 this.RaiseNotifyPropertyChanging("Foo");
                 this.foo = value;
                 this.RaiseNotifyPropretyChanged("Foo");
             }
         }
     }
}