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Hopefully this question isn't too general, but I have just started using WPF and I'm banging my head against this.

I am developing an application that uses dynamically created controls. However currently I can not figure out how to make the commands create and add more controls to the current window because the commands only work when created in the ViewModel which can not see the View. However I can't keep everything in the XAML because all controls except for a few initially empty stack panels are dynamic. I feel like I'm missing something easy here though.

So here I have the binding 

<MenuItem Header="LabelMenuItem" Command="{Binding Path=SpawnLabel}"/>

And here I have the command 

 public ICommand SpawnLabel { get { return new DelegateCommand(OnSpawnLabel); } }

Delegate command works like a relay command as defined here.

public class DelegateCommand : ICommand
    {
        private readonly Action _command;
        private readonly Func<bool> _canExecute;
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }


       remove { CommandManager.RequerySuggested -= value; }
    }

    public DelegateCommand(Action command, Func<bool> canExecute = null)
    {
        if (command == null)
            throw new ArgumentNullException();
        _canExecute = canExecute;
        _command = command;
    }

    public void Execute(object parameter)
    {
        _command();
    }

    public bool CanExecute(object parameter)
    {
        return _canExecute == null || _canExecute();
    }

}

This works in the view Model but I can't figure out how on earth to make it work in the View (or talk to the view without breaking MVVM principles) so that I can actually change the UI using the current controls created in c#.

Currently when I do it I get a BindingExpression path error which makes sense but I can not figure out how to bind it to look for the command in the view.

Smeasum
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1 Answers1

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You indirectly communicate with the UI via events, if you change a property you fire PropertyChanged, the UI will update a binding, if you change a collection you fire CollectionChanged and a new control is added or an old one removed.

The important interfaces are INotifyPropertyChanged and INotifyCollectionChanged, just bind an ItemsControl (via ItemsSource) to an ObservableCollection<T> in your view-model and add to that collection in your command (you may need to use the ItemsControl.ItemTemplate to specify the view for your view-models, depending on what MVVM framework you use).

H.B.
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  • So I would execute the command code in the ViewModel and onExecute would then fire the CollectionChanged which gets triggered in the View where I could then add the new UI elements? – Smeasum May 08 '13 at 21:54
  • @Smeasum: No, you just add an item to the `ObservableCollection`, which then internally will fire the event, upon which the `ItemsControl` will create what is specified as `ItemTemplate` or what is [implicitly specified for the item type](http://stackoverflow.com/questions/5644392/conditional-list-itemtemplate-or-datatemplate-in-wpf/5644414#5644414). There are a lot of examples of this if you search SO or the web, see also [MSDN](http://msdn.microsoft.com/en-us/library/ms752347.aspx#binding_to_collections). – H.B. May 08 '13 at 22:08