When working with UserControls on Xamarin Forms / WPF, values of the control should be bindable by the consumer. Here’s a quick example of this in action and how to specify a default value if the consumer does not bind to the property.

public LoadingView : UserControl
{

    public static readonly DependencyProperty LoadingTextProperty = DependencyProperty.Register("LoadingText",
        typeof (string), typeof (LoadingView), new PropertyMetadata("Loading", null));


    public string Title
    {
        get { return GetValue(LoadingText).ToString(); }
        set { SetValue(LoadingText, value); }
    }

}

In the above sample you will see that a default value is passed into the constructor of PropertyMetadata. If a client does not bind/specify a value then Loading will be default value. For more information see MSDN - PropertyMetadata Constructor.