How can I display the navigation bar in Xamarin.Forms TabbedPage?

The following code is my Xaml:

<?xml version="1.0" encoding="utf-8" ?>
<TabbedPage
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:view="clr-namespace:FuturesMaster.View;assembly=FuturesMaster"
    x:Class="FuturesMaster.View.PortalMain_TabbedPage">
  <TabbedPage.Children>
    <ContentPage Title="Buy" />
  </TabbedPage.Children>
</TabbedPage>

And I've tried to call the following method which is matched above xaml in constructor:
NavigationPage.SetHasNavigationBar(this, true);

When I run my app under simulator but it seems not result in what I want to see. I expected that it would show a navigation bar on the top of my TabbedPage.

By the way, I've tried the same way in a ContentPage, it ran very well but TabbedPage.

Thanks.