WPF Frame 的 DataContext 不能被 Page 继承

var frame = page.Parent as Frame;
// frame.DataContext == foo

<Frame Name="frame"
       LoadCompleted="Frame_LoadCompleted"
       DataContextChanged="Frame_DataContextChanged"/>

private void Frame_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    UpdateFrameDataContext(sender, e);
}
private void Frame_LoadCompleted(object sender, NavigationEventArgs e)
{
    UpdateFrameDataContext(sender, e);
}
private void UpdateFrameDataContext(object sender, NavigationEventArgs e)
{
    var content = frame.Content as FrameworkElement;
    if (content == null)
    {
        return;
    }
    
    content.DataContext = frame.DataContext;
}