I am coding a simple login UserControl with two TextBoxes (Username and Password) and a Login button. I want the Login button to be enabled only when the username and password fields are filled in. I am using Prism and MVVM. The LoginViewModel contains a property called LoginCommand that is bound to the Login button. I have a CanLoginExecute() method in my ViewModel but it fires only when the application comes up and then never again. So the Login button is never enabled. What am I missing?
Here's my xaml:
<TextBox x:Name="username"
Text="{Binding Path=Username, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
<TextBox x:Name="password"
Text="{Binding Path=Password, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
<Button Content="Login"
cmnd:Click.Command="{Binding LoginCommand}" />
Here's my ViewModel
class LoginViewModel : IDataErrorInfo, INotifyPropertyChanged
{
public LoginViewModel()
{
this.LoginCommand =
new DelegateCommand<object>(
this.LoginExecute, this.CanLoginExecute);
}
private Boolean CanLoginExecute(object dummyObject)
{
return (string.IsNullOrEmpty(Username) ||
string.IsNullOrEmpty(Password)) ? false : true;
}
private void LoginExecute(object dummyObject)
{
if (CheckCredentials(Username, Password))
{
....
}
}
#region IDataErrorInfo Members
public string Error
{
get { throw new NotImplementedException(); }
}
public string this[string columnName]
{
get
{
string result = null;
if (columnName == "Username")
{
if (string.IsNullOrEmpty(Username))
result = "Please enter a username";
}
else if (columnName == "Password")
{
if (string.IsNullOrEmpty(Password))
result = "Please enter a password";
}
return result;
}
}
#endregion // IDataErrorInfo Members
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#endregion // INotifyPropertyChanged Members
#region Properties
private String _username;
public String Username
{
get { return _username; }
set
{
if (value == _username)
return;
_username = value;
this.OnPropertyChanged("Username");
}
}
private String _password;
public String Password
{
get { return _password; }
set
{
if (value == _password)
return;
_password = value;
this.OnPropertyChanged("Password");
}
}
public ICommand LoginCommand { get; private set; }
#endregion // Properties
}
