Enable Button and DelegateCommand

Question

How can I disable a button until required data is entered in a TextBox?

I'm binding an Button to ICommand

        public ICommand LoginCommand
    {
        get
        {
            if (_loginCommand == null)
            {
                _loginCommand = new DelegateCommand<string>(this.Login, this.IsValid);
            }
            return _loginCommand;
        }
    }

in XAML like this:

<Button Style="{StaticResource LoginButton}" Content="Login" Command="{Binding LoginCommand}" CommandParameter="{Binding Text, ElementName=txtUserName}" />

Answer 1

Where is DelegateCommand defined? Check the constructor, it is probably expecting the second parameter to be a predicate, the result of which gets mapped to your ICommand's CanExecute.

I assume that this.IsValid is a delegate method that returns true if the string.Length > 0 for the Command parameter you are passing from the View... Try setting it to always return false just to check the UI is disabling the button.

EDIT: To get the param into the IsValid method, use the following syntax:

change the syntax of the IsValid method to accept a string parameter, then decalre your DelegateCommand like:

_loginCommand = new DelegateCommand<string>(this.Login, (param) => this.IsValid(param));

HTH :)

Answer 2

You just need to raise the requery event. This is easy with a DelegateCommand. Here's how your ViewModel would look.

Also, if there's not a compelling reason to make your property of type DelegateCommand, then you should do so.

public class MyViewModel : ViewModel
{

    private string _myTextField;
    public string MyTextField
    {
        get { return _myTextField; }
        set
        {

            _myTextField = value;
            OnPropertyChanged("MyTextField");

            //Here's the magic
            LoginCommand.RaiseCanExecuteChanged();

        }
    }

    public DelegateCommand<string> LoginCommand { get; set; }

    public MyViewModel()
    {
         LoginCommand = new DelegateCommand<string>(Login, CanLogin);
    }

    public bool CanLogin(string text)
    {
         return !string.IsNullOrEmpty(text);
    }

    public void Login(string text)
    {
         //login logic
    }

}

That's pretty much it.