Changing property value added to DefaultViewModel not affect the view

Question

I am trying to understand the MVVM model in Windows Store Application.

The problem is:

1. I've created an dependency property:

   public string text
   {
       get { return (string)GetValue(textProperty); }
       set { SetValue(textProperty, value); }
   }
   public static readonly DependencyProperty textProperty =
       DependencyProperty.Register("text", typeof(string), typeof(MainPage),  new PropertyMetadata("INIT"));
   

2. Added it to my DefaultViewModel which is an ObservableDictionary automatically generated via Visual Studio 2013:

   public MainPage()
   {
       this.InitializeComponent();
       this.DefaultViewModel.Add("text", text);
       this.navigationHelper = new NavigationHelper(this);
       this.navigationHelper.LoadState += navigationHelper_LoadState;
       this.navigationHelper.SaveState += navigationHelper_SaveState;
   }
   

3. Next binding to a textBox in XAML file:

   <TextBox Name="textBox" Height="50" Text="{Binding text}"></TextBox>
   

Where DefaultViewModel is binded as datacontext in xaml file:

    DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"

Untill this. Everything is perfect. But now i am adding a button:

    <Button Width="500" Height="500" Click="btn_Click">zmien</Button>

    private void btn_Click(object sender, RoutedEventArgs e)
    {
        text = "change it!";
    }

And this is not affecting my text of TextBox when application is running. Can You explain a bit this mechanism for me ? Please.

Answer 1

You should read the following Tutorial for Windows Store Apps with C# and XAML:

Data binding overview (Windows Store apps using C#/VB/C++ and XAML)

For changes to the source object to propagate to the target, the source must implement the INotifyPropertyChanged interface.

Code sample from the link:

// Create a class that implements INotifyPropertyChanged.
public class DefaultViewModel: INotifyPropertyChanged
{
    private String _text;

    // Declare the PropertyChanged event.
    public event PropertyChangedEventHandler PropertyChanged;

    // Create the property that will be the source of the binding.
    public String Text
    {
        get { return _text; }
        set
        {
            _text = value;
            // Call NotifyPropertyChanged when the source property 
            // is updated.
            NotifyPropertyChanged("Text");
        }
    }

    // NotifyPropertyChanged will fire the PropertyChanged event, 
    // passing the source property that is being updated.
    public void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, 
                new PropertyChangedEventArgs(propertyName));
        }
    }
}

If you now change the value of the Text property of the class DefaultViewModel the Textbox will update.

Answer 2

Try to enable TwoWay binding:

<TextBox Name="textBox" Height="50" Text="{Binding text, Mode=TwoWay}}" />

If this will not work you can System.Diagnostics.PresentationTraceSources.TraceLevel for binding.