Communicate Between Two View in WPF MVVM Only

Question

I Have two views in MVVM(WPF). First View contains two Text boxes: User Name, Password, second view is having two Buttons: Submit and Clear. Both Views now set on On Form. When I press 'Clear' button both textboxes are cleared and in Submit a message of UserName and Password is displayed. I am using only MVVM+WPF, not prism.

ModelView Of First View:

class LoginView:ViewModelBase
    {
        string _userName;
        public string  UserName 
        {
            get {return _userName ; }
            set {
                if (_userName != value)
                {
                    _userName = value;
                }
                base.OnPropertyChanged(UserName);
            }
        }

        string _Pwd;
        public string PWD
        {
            get { return _Pwd; }
            set
            {

                _Pwd = value;
                base.OnPropertyChanged(_Pwd);
            }
        }
    }

and For Button

 class ButtonHandler
    {
        private DelegateCommand _ClearData;
        public ICommand ClearCommand
        {
            get
            {
                if (_ClearData == null)
                {
                    _ClearData = new DelegateCommand(ClearText);
                }
                return _ClearData;
            }
        }
        LoginView lg = new LoginView();
        private void ClearText()
        {
            lg.UserName = "";
            lg.PWD = "";
        }
    }

and View Code of First Control

   <Label Content="Login" Grid.Row="0" Grid.Column="1" VerticalAlignment="Top" HorizontalAlignment="Left"
           FontFamily="Georgia" FontSize="24" FontWeight="UltraBold" ></Label>
    <Label Content="User Name" Grid.Row="1" Grid.Column="1" VerticalAlignment="Center" HorizontalAlignment="Left"></Label>
    <Label Content="Password" Grid.Row="2" Grid.Column="1" VerticalAlignment="Center" HorizontalAlignment="Left"></Label>
    <TextBox  Name="username" Grid.Row="1" Grid.Column="1" Margin="100,0,0,0" VerticalAlignment="Center" HorizontalAlignment="Left" Text="{Binding Path=UserName,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" ></TextBox>
    <TextBox  Name="pwd" Grid.Row="2" Grid.Column="1" Margin="100,0,0,0" VerticalAlignment="Center" HorizontalAlignment="Left" Text="{Binding Path=PWD,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"></TextBox>
    <Separator Grid.Row="0" Grid.Column="1" Height="5" Margin="0,40,0,0" Background="Green"></Separator>

and Button View

     <Button x:Name="Submit" Content="Submit" Grid.Column="1"></Button>
     <Button x:Name="Clear" Content="Clear" Grid.Column="2" 
             Command="{Binding Path=ClearCommand, Mode=OneWay, 
             UpdateSourceTrigger=PropertyChanged}" >
     </Button>

Why it is not working?

Answer 1

You are not using the MVVM pattern correctly, with this pattern the ViewModel should not have a reference to the View. A command is part of your ViewModel, therefore your reference to LoginView violates the pattern.

So you have two input fields and a button? for this I would have a single ViewModel and a single View. The ViewModel would expose two string properties (username & password) and a command that binds to the clear button. When the command executes it would clear the username and password texts on the ViewModel. The View will then update accordingly.

Answer 2

The basic principle of MVVM is to have a class that the view can bind to that has all the application logic inside of it. One of the main reasons is to have a separation of concerns. So if you want a username you expose a property that the view binds to and then when you want to log in you create a function that uses those bound values to submit to you business logic layer of your application.

This would seem to be one way to utilize MVVM in your example:

public class LoginViewModel
{
  public string UserName {get;set;}//Implement INotifyPropertyChanged
  public string PWD {get;set;}

    private DelegateCommand _ClearData;
    public ICommand ClearCommand
    {
        get
        {
            if (_ClearData == null)
            {
                _ClearData = new DelegateCommand(ClearText);
            }
            return _ClearData;
        }
    }

    private void ClearText()
    {
        UserName = "";
        PWD = "";
    }
}

and then in your xaml:

<TextBox Text={Binding UserName} />
<TextBox Text={Binding PWD} />
<Button Command={Binding ClearCommand}/>