How to data-bind my View's button to the Model, via its interface in the Presenter

Question

Apologies for the lengthy code, this is as condensed as I could get it. A contrived example below, followed by an explanation:

namespace Contrived.Example
{
    internal class Model : INotifyPropertyChanged
    {
        private bool _isAccountEmpty;

        public bool IsAccountEmpty
        {
            get { return _isAccountEmpty; }
            set
            {
                if (_isAccountEmpty == value) return;
                _isAccountEmpty = value;
                OnPropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    internal interface IView
    {
        event EventHandler WithdrawalButtonClick;
    }

    internal class View : IView
    {
        private Button WithdrawalButton;
        private void InitializeComponent()
        {
            WithdrawalButton = new Button();
            WithdrawalButton.Location = new System.Drawing.Point(0, 0);
            WithdrawalButton.Size = new System.Drawing.Size(75, 25);
            WithdrawalButton.Click += WithdrawalButton_Click;
        }

        private void WithdrawalButton_Click(object sender, EventArgs e)
        {
            WithdrawalButtonClick?.Invoke(sender, e);
        }

        public event EventHandler WithdrawalButtonClick;
    }

    internal class Presenter
    {
        private readonly IView _view;
        private readonly Model _model;
        //private readonly BindingSource bindSource;


        public Presenter(IView view)
        {
            _view = view;
            _model = new Model();
            view.WithdrawalButtonClick += OnWithDrawalButtonClick;

            /* The behavior that follows is what I am after: 
             * how do I bind my button control to its model counterpart here? 
             * You can't expose the control directly on the IView interface, can you?
             * 
             * bindSource = new BindingSource {DataSource = typeof(Model)};
             * bindSource.Add(_model);
             * 
             * This next line would throw an exception because 
             * _view isn't actually exposing the button:
             * _view.WithdrawalButton
             *     .DataBindings
             *     .Add("Enabled", bindSource, "IsAccountEmpty");
             */
        }

        private void OnWithDrawalButtonClick(object sender, EventArgs e)
        {
            // Pretend we withdrew too much money
            _model.IsAccountEmpty = true;
        }
    }
}

So this is basically the structure for a program of mine. I want to bind my view's button to a property on its model, via the presenter. The commented code works fine for me without an interface, and in vb.Net (controls have Friend modifier instead of Private, so they are visible to the presenter).

Now in C# the button is private, and in addition I am trying to practice loose coupling by throwing an interface into the mix. Now I have run into the problem that I can no longer bind my button to the model the same way I used to. Can anyone show me what the correct way to fix this is? Thank you so much.

P.S. If this setup is entirely wrong to begin with I would like to know that also, this is the best I have been able to come up with on my own without any real concentrated training on the subject in school.

Answer 1

At first let me tell you that your construct isn't entirely loosely coupled. If you take a look at your Presenter you can find this line:

_model = new Model();

Think of the new Keyword as evil, if you want to create loosely coupled programs. Because here you are bound to the Model. Not to any model but exactly to that one Model. If you want to UnitTest your Presenter you won't be able to Mock a model. So you should better create a further Interface IModel and pass this in the constructor.

public Presenter(IView view, IModel model)
{
   this.model = model;
   this.view = view;
}

To learn more about why you should do this and how you can handle this as well read sth. about DependencyInjection. With this step done you are able to pass any Model which fits your Interface.

After this i should come to your question ;-). At first you could create a new Interface which handles the DataBinding ability. Then you create a new Button which inerhits from Standardbutton and implements the new Interface. Your IView then contains a Property which holds an IBindingObject or whatever your new Interface is called. This could look like this:

    public interface IBindingObject
    {
       ControlBindingsCollection DataBinding {get;}   
    }

    public class MyButton : Button, IBindingObject
    {
       //Nothing to do here because the Button contains a DataBinding Property by default
    }

    public interface IView
    {
       IBindingObject WithdrawalButton {get;}
    }

    public class View : Form, IView
    {
      public IBindingObject WithdrawalButton {get {return new MyButton()}}
    }    

    public class Presenter
    {
        public Presenter(IView view, IModel model)
        {
           view.WithdrawalButton.DataBindings.Add(//do your binding);
        }
    }

Note this approach is coupled to ControlBindingCollection but this compromise is ok I think. Otherwise you always have to give your View access to your Model or vice versa I think. This would violate the Pattern. DataBinding and Winforms isn't that comfortable as in WPF. So if you have the opportunity to use WPF I would recommend that.