BooleanToVisibilityConverter.cs

public class BooleanToVisibilityConverter : IValueConverter
{
    public bool Inverted { get; set; }

    public object Convert(object value, Type targetType, object parameter, string language)
    {
        if (!(value is bool))
        {
            return Visibility.Collapsed;
        }
        return (bool)value ^ Inverted ? Visibility.Visible : Visibility.Collapsed;
    }

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

MainPage.xaml

<Page.Resources>
    <local:BooleanToVisibilityConverter x:Key="BoolToVisibilityConverter" />
    <local:BooleanToVisibilityConverter x:Key="InvertedBoolToVisibilityConverter" Inverted="True" />
</Page.Resources>

<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
    <ProgressRing IsActive="True"
                  Visibility="{Binding Initialized, Converter={StaticResource InvertedBoolToVisibilityConverter}}"/>
    <TextBlock Text="{Binding Text}" 
               TextWrapping="Wrap"
               Visibility="{Binding Initialized, Converter={StaticResource BoolToVisibilityConverter}}"
               Style="{StaticResource BasicTextStyle}" />
</Grid>

ViewModel.cs

public ViewModel()
{
    Init();
}

private async void Init()
{
    var file = await Package.Current.InstalledLocation.GetFileAsync(@"Assets\Text.txt");
    Text = await FileIO.ReadTextAsync(file);
    await Task.Delay(2000);
    Initialized = true;
}

public string Text
{
    get { return _text; }
    set
    {
        if (value == _text) return;
        _text = value;
        OnPropertyChanged();
    }
}

public bool Initialized
{
    get { return _initialized; }
    set
    {
        if (value.Equals(_initialized)) return;
        _initialized = value;
        OnPropertyChanged();
    }
}