WPF Simple Commanding Example

Question

I try not to post questions like this, but i\'ve really been struggling to find an answer or similar example. I have what I think is a really simple example I\'d like to setup.

Answer 1

Well, MVVM is just a pattern or philosophy, so I think your desire to avoid using a framework might be a little misguided. Even if you're not using one of those frameworks, you will essentially be writing your own framework in order to implement the MVVM pattern.

That being said, probably what you want to use is a DelegateCommand or one of the similar implementations. See: http://www.wpftutorial.net/DelegateCommand.html. The important part that I think you are looking for is that the command that the WPF button is binding to must raise the CanExecuteChanged event whenever there is a change made in the view model which affects whether the command can or cannot be executed.

So in your case, for example, you would want to add a call to the CanExecuteChanged of your AddPersonDelegateCommand to your OnPropertyChanged method (possibly filtered by the name of the property that was changed). This tells anything bound to the command to call CanExecute on the command, and then you would have your logic in that CanExecute that actually determines if a person with the entered name already exists.

So to add some sample code, it might look something like this:

class MainViewModel : INotifyPropertyChanged
{
    public MainViewModel()
    {
        People = new ObservableCollection();
        People.Add( new Person {Name = "jimmy"});
        AddPersonDelegateCommand = new DelegateCommand(AddPerson, CanAddPerson);
    }

  // Your existing code here

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
      if(propertyName == "NewNameTextBox") AddPersonDelegateCommand.RaiseCanExecuteChanged();

      PropertyChangedEventHandler handler = PropertyChanged;
      if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
  }

  public DelegateCommand AddPersonDelegateCommand { get; set; }

  public void AddPerson()
  { 
      // Code to add a person to the collection
  }

  public bool CanAddPerson()
  {
      return !People.Any(p=>p.Name == NewNameTextBox);
  }

  public string NewNameTextBox 
  { 
    get { return _newNameTextBox; }
    set 
    { 
       _newNameTextBox = value;
       OnPropertyChanged();
    }
  }
}

*Note: In this sample your would need to be bound to the NewNameTextBox property on the view model.