rur2641 rur2641 - 3 months ago 39
C# Question

Trouble binding XAML uwp

Hi I am following this tutorial,http://blogs.u2u.be/diederik/post/2011/11/14/null.aspx, to bind the visibility of an element to a Boolean property. The program is not working. Here is the code:

<Page.Resources>
<local:BooleanToVisibilityConverter x:Key="TrueToVisibleConverter"/>
</Page.Resources>

<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<StackPanel>
<TextBlock Text=" Hello World"
Visibility="{Binding Path=Show_element, Converter={StaticResource TrueToVisibleConverter}}"/>
<Button Click="Button_Click">press button</Button>
</StackPanel>
</Grid>

public sealed partial class MainPage : Page , INotifyPropertyChanged
{
private bool show_element ;
public bool Show_element
{
get { return show_element; }
set
{
show_element = value;
this.OnPropertyChanged();
Debug.WriteLine("Show_element value changed");
}
}
public MainPage()
{
this.InitializeComponent();
}

private void Button_Click(object sender, RoutedEventArgs e)
{
Show_element = !Show_element;
}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
public void OnPropertyChanged(string propertyName = null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public class BooleanToVisibilityConverter : IValueConverter
{
public bool IsReversed { get; set; }

public object Convert(object value, Type typeName, object parameter, string language)
{
var val = System.Convert.ToBoolean(value);
if (this.IsReversed)
{
val = !val;
}

if (val)
{
return Visibility.Visible;
}

return Visibility.Collapsed;
}

public object ConvertBack(object value, Type targetType, object parameter, string language)
{
throw new NotImplementedException();
}
}


The visibility does not change with the property. I was having an error due to intellisense (Error Xaml namespace) which was resolved. Not sure what is wrong with this code.

Thank you.

Answer

change

this.OnPropertyChanged();

to

this.OnPropertyChanged("Show_element");

edit: besides that, you don't have a ViewModel (sorry, missed that when I was checking your code), so you need to create one and set it as DataContext:

ViewModel.cs:

public class ViewModel : INotifyPropertyChanged
{
    private bool show_element;
    public bool Show_element
    {
        get { return show_element; }
        set
        {
            show_element = value;
            this.OnPropertyChanged("Show_element");
            Debug.WriteLine("Show_element value changed");
        }
    }
    public ViewModel()
    {
    }

    public void ButtonClicked()
    {
        Show_element = !Show_element;
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };
    public void OnPropertyChanged(string propertyName = null)
    {
        this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

and your MainPage.xaml.cs should look somehow like that:

public sealed partial class MainPage : Page
{        
    private ViewModel _viewModel;

    public MainPage()
    {
        this.InitializeComponent();
        _viewModel = new ViewModel();
        DataContext = _viewModel;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        _viewModel.ButtonClicked();
    }        
}