WPF MVVM command canexecute enable/disable button

Question

I want to enable RibbonButton when textbox property text isn't null. Disable RibbonButton when textbox property text is null. I want to use CanExecute method in ICommand for it. How can I do it?

View:

 <Custom:RibbonButton
                        LargeImageSource="..\Shared\img\save_diskete.png"
                        Label="Save"
                        Command="{Binding ButtonCommand}">
                    </Custom:RibbonButton>

ViewModel

class KomentarViewModel:BaseViewModel
    {
        #region Data
        private ICommand m_ButtonCommand;
        public ICommand ButtonCommand
        {
            get
            {
                return m_ButtonCommand;
            }
            set
            {
                m_ButtonCommand = value;
            }
        }
        private string textKomentar;
        public string TextKomentar
        {
            get
            {
                return this.textKomentar;
            }
            set
            {
                // Implement with property changed handling for INotifyPropertyChanged
                if (!string.Equals(this.textKomentar, value))
                {
                    textKomentar = value;
                    OnPropertyChanged("TextKomentar");
                }
            }
        }        
        private ObservableCollection<Komentar> allCommentsInc;
        public ObservableCollection<Komentar> AllCommentsInc
        {
            get
            {
                return allCommentsInc;
            }
            set
            {
                allCommentsInc = value;
                OnPropertyChanged("AllCommentsInc");
            }
        }

        public int idIncident { get; private set; }
        public Incident incident { get; private set; }
        #endregion

        #region Constructor
        public KomentarViewModel(int id)
        {
            CC_RK2Entities context = new CC_RK2Entities();
            this.idIncident = id;

            AllCommentsInc = new ObservableCollection<Komentar>(context.Komentar.Where(a => a.Incident_id == idIncident));
            incident = context.Incident.Where(a => a.id == idIncident).First();

            //ButtonCommand = new RelayCommand(new Action<object>(ShowMessage));
        }
        #endregion        

        #region Methods
        //ukaz napsany text
        public void ShowMessage(object obj)
        {
            //MessageBox.Show(obj.ToString());
            MessageBox.Show(this.TextKomentar);
        }
}

RelayCommand

namespace Admin.Shared.Commands
{
    class RelayCommand : ICommand
    {
        private Action<object> _action;

        public RelayCommand(Action<object> action)
        {
            _action = action;
        }

        #region ICommand Members

        public bool CanExecute(object parameter)
        {
            return true;            
        }

        public event EventHandler CanExecuteChanged;

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

        #endregion
    }
}

Answer 1

You need to modify your RelayCommand class like this

  class RelayCommand : ICommand
{
    private Action<object> _action;
    private Func<bool> _func;  

    public RelayCommand(Action<object> action,Func<bool> func)
    {
        _action = action;
        _func = func;
    }

    public void RaiseCanExecuteChanged()
    {
        if(CanExecuteChanged!=null)
            CanExecuteChanged(this,new EventArgs());
    }

    #region ICommand Members

    public bool CanExecute(object parameter)
    {
        if (_func != null)
           return _func();
        return true;
    }



    public event EventHandler CanExecuteChanged;

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

    #endregion
}

Initialize ButtonCommand as

ButtonCommand = new RelayCommand((s) => ShowMessage(s),()=>!string.IsNullOrEmpty(TextKomentar));

RaiseCanExcuteChanged from the setter of Text property

        public string TextKomentar
    {
        get
        {
            return this.textKomentar;
        }
        set
        {
            // Implement with property changed handling for INotifyPropertyChanged
            if (!string.Equals(this.textKomentar, value))
            {
                textKomentar = value;
                OnPropertyChanged("TextKomentar");
            }
            ButtonCommand.RaiseCanExecuteChanged();
        }
    }