Custom navigation bar in Xamarin.Forms PCL

BasePage.cs

using System;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace {YOURNAMESPACE}
{
    public class BasePage : ContentPage
    {
        ContentView _header;
        public ContentView Header
        {
            get
            {
                return _header;
            }
            set
            {
                if (_mainLayout != null)
                {
                    if (_header != null && _mainLayout.Children.Contains(_header))
                        _mainLayout.Children.Remove(_header);

                    _header = value;
                    if (Device.OS == TargetPlatform.iOS)
                        _mainLayout.Children.Add(_header, 0, 1);
                    else
                        _mainLayout.Children.Add(_header, 0, 0);
                }
            }
        }

        ContentView _footer;
        public ContentView Footer
        {
            get
            {
                return _footer;
            }
            set
            {
                if (_mainLayout != null)
                {
                    if (_footer != null && _mainLayout.Children.Contains(_footer))
                        _mainLayout.Children.Remove(_footer);

                    _footer = value;
                    if (Device.OS == TargetPlatform.iOS)
                        _mainLayout.Children.Add(_footer, 0, 3);
                    else
                        _mainLayout.Children.Add(_footer, 0, 2);
                }
            }
        }

        ContentView _content;
        public ContentView ContentPresenter
        {
            get
            {
                return _content;
            }
            set
            {
                if (_mainLayout != null)
                {
                    if (_content != null && _mainLayout.Children.Contains(_content))
                        _mainLayout.Children.Remove(_content);

                    _content = value;
                    if (Device.OS == TargetPlatform.iOS)
                        _mainLayout.Children.Add(_content, 0, 2);
                    else
                        _mainLayout.Children.Add(_content, 0, 1);
                }
            }
        }

        ContentView _popUpView;
        BoxView _maskView;
        public ContentView PopUpView
        {
            get
            {
                return _popUpView;
            }
            set
            {
                if (_mainLayout != null)
                {
                    if (_maskView != null && _mainLayout.Children.Contains(_maskView))
                        _mainLayout.Children.Remove(_maskView);

                    if (_popUpView != null && _mainLayout.Children.Contains(_popUpView))
                        _mainLayout.Children.Remove(_popUpView);

                    _popUpView = value;
                    if (_popUpView == null)
                        return;
                    
                    if (_maskView == null)
                        _maskView = new BoxView { Color = Color.Black.MultiplyAlpha(0.5), IsVisible = false };
                    if (Device.OS == TargetPlatform.iOS)
                        _mainLayout.Children.Add(_maskView, 0, 1, 0, 4);
                    else
                        _mainLayout.Children.Add(_maskView, 0, 1, 0, 3);
                    _popUpView.IsVisible = false;
                    if (Device.OS == TargetPlatform.iOS)
                        _mainLayout.Children.Add(_popUpView, 0, 1, 0, 4);
                    else
                        _mainLayout.Children.Add(_popUpView, 0, 1, 0, 3);
                }
            }
        }

        public static readonly BindableProperty ShowPopupProperty = BindableProperty.Create(nameof(ShowPopup), typeof(bool), typeof(BasePage), default(bool));
        public bool ShowPopup
        {
            get { return (bool)GetValue(ShowPopupProperty); }
            set { SetValue(ShowPopupProperty, value); }
        }

        Grid _mainLayout;

        public BasePage()
        {
            NavigationPage.SetHasNavigationBar(this, false);
            _mainLayout = new Grid
            {
                Padding = new Thickness(0),
                RowSpacing = 0,
            };
            if (Device.OS == TargetPlatform.iOS)
                _mainLayout.RowDefinitions.Add(new RowDefinition { Height = new GridLength(20, GridUnitType.Absolute) });
            _mainLayout.RowDefinitions.Add(new RowDefinition { Height = new GridLength(44, GridUnitType.Absolute) });
            _mainLayout.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
            _mainLayout.RowDefinitions.Add(new RowDefinition { Height = new GridLength(44, GridUnitType.Absolute) });

            Content = new ContentView { Content = _mainLayout };
        }

        protected override void OnPropertyChanged(string propertyName = null)
        {
            base.OnPropertyChanged(propertyName);
            if (propertyName == ShowPopupProperty.PropertyName)
            {
                if (_maskView != null && PopUpView != null)
                {
                    _maskView.IsVisible = ShowPopup;
                    _popUpView.IsVisible = ShowPopup;
                }
            }
        }

        public Task<bool> ShowAlert(FormattedString message, string rightButtonText = null, string leftButtonText = null, string middleButtonText = null)
        {
            var mainLaout = new Grid
            {
                RowSpacing = 0,
                RowDefinitions = new RowDefinitionCollection {
                    new RowDefinition{Height = new GridLength(1, GridUnitType.Star)},
                    new RowDefinition{Height = new GridLength(1, GridUnitType.Absolute)},
                    new RowDefinition{Height = new GridLength(40, GridUnitType.Absolute)},
                },
                ColumnDefinitions = new ColumnDefinitionCollection
                {
                    new ColumnDefinition{Width = new GridLength(1, GridUnitType.Star)},
                    new ColumnDefinition{Width = new GridLength(1, GridUnitType.Star)},
                    new ColumnDefinition{Width = new GridLength(1, GridUnitType.Star)},
                }
            };

            var messageLabel = new Label
            {
                HorizontalTextAlignment = TextAlignment.Center,
                VerticalTextAlignment = TextAlignment.Center,
                FormattedText = message,
                Margin = new Thickness(10),
            };
            mainLaout.Children.Add(messageLabel, 0, 3, 0, 1);

            var line = new BoxView { Color = Color.Black, HeightRequest = 1 };
            mainLaout.Children.Add(line, 0, 3, 1, 2);

            var background = new BoxView { Color = Color.FromHex("#F7F7F7") };
            mainLaout.Children.Add(background, 0, 3, 2, 3);

            TaskCompletionSource<bool> completionSource = null;

            if (!string.IsNullOrEmpty(leftButtonText))
            {
                var leftButton = new Button
                {
                    BackgroundColor = Color.FromHex("#F7F7F7"),
                    Text = leftButtonText,
                    TextColor = Color.FromHex("#06A2D5"),
                };
                completionSource = new TaskCompletionSource<bool>();
                leftButton.Clicked += (sender, e) =>
                {
                    completionSource.SetResult(false);
                    ShowPopup = false;
                };
                mainLaout.Children.Add(leftButton, 0, 1, 2, 3);
            }

            if (!string.IsNullOrEmpty(middleButtonText))
            {
                var middleButton = new Button
                {
                    BackgroundColor = Color.FromHex("#F7F7F7"),
                    Text = middleButtonText,
                    TextColor = Color.FromHex("#06A2D5"),
                };
                completionSource = new TaskCompletionSource<bool>();
                middleButton.Clicked += (sender, e) =>
                {
                    completionSource.SetResult(true);
                    ShowPopup = false;
                };
                mainLaout.Children.Add(middleButton, 1, 2, 2, 3);
            }

            if (!string.IsNullOrEmpty(rightButtonText))
            {
                var rightButton = new Button
                {
                    BackgroundColor = Color.FromHex("#F7F7F7"),
                    Text = rightButtonText,
                    TextColor = Color.FromHex("#06A2D5"),
                };
                completionSource = new TaskCompletionSource<bool>();
                rightButton.Clicked += (sender, e) =>
                {
                    completionSource.SetResult(true);
                    ShowPopup = false;
                };
                mainLaout.Children.Add(rightButton, 2, 3, 2, 3);
            }

            // No buttons set here
            if (completionSource == null)
            {
                completionSource = new TaskCompletionSource<bool>();
                Task.Run(async () =>
                {
                    await Task.Delay(3000);
                    completionSource.SetResult(true);
                    ShowPopup = false;
                });
            }
            PopUpView = new ContentView
            {
                Padding = new Thickness(0),
                VerticalOptions = LayoutOptions.CenterAndExpand,
                Margin = new Thickness(5, 0),
                Content = new Frame
                {
                    Content = mainLaout,
                    IsClippedToBounds = true,
                    Padding = new Thickness(0),
                    BackgroundColor = Color.White
                },
            };
            ShowPopup = true;
            return completionSource.Task;
        }
        
        public void ShowLoading(string title = null)
        {
            var mainLayout = new StackLayout();
            var indicator = new ActivityIndicator { IsRunning = true };
            mainLayout.Children.Add(indicator);
            if (!string.IsNullOrEmpty(title))
            {
                var label = new Label
                {
                    Text = title,
                    HorizontalTextAlignment = TextAlignment.Center,
                    TextColor = Color.White
                };
                mainLayout.Children.Add(label);
            }
            PopUpView = new ContentView
            {
                Content = mainLayout,
                VerticalOptions = LayoutOptions.CenterAndExpand,
                BackgroundColor = Color.Transparent
            };
            ShowPopup = true; ;
        }

        public void HideAllPopup()
        {
            PopUpView = null;
            ShowPopup = false;
        }

    }
}

Demo.xaml

<?xml version="1.0" encoding="UTF-8"?>
<local:BasePage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
	xmlns:local="clr-namespace:{YOURNAMESPACE};assembly={YOURNAMESPACE}"
	xmlns:fe="clr-namespace:FreshEssentials;assembly=FreshEssentials"
	x:Class="Proofpoint.MessageListPage">
	<local:BasePage.Header>
		<ContentView BackgroundColor="{StaticResource GlobalBlue}">
			<Grid>
				<Grid.ColumnDefinitions>
					<ColumnDefinition Width="40" />
					<ColumnDefinition Width="*" />
				</Grid.ColumnDefinitions>
				<Image Source="Back.png" 
					fe:TappedGestureAttached.Command="{Binding GoBackCommand}"
					HeightRequest="25" WidthRequest="25" 
					HorizontalOptions="Center" VerticalOptions="Center"/>
				<Label Text="{Binding RightToolbarTitle}" Grid.ColumnSpan="2" HorizontalTextAlignment="End" Margin="14,0" Style="{StaticResource TitleWhite}" VerticalTextAlignment="Center"/>
			</Grid>
		</ContentView>
	</local:BasePage.Header>

	<local:BasePage.Footer>
		<ContentView BackgroundColor="{StaticResource GlobalBlue}">
			<Grid>
				<Grid.ColumnDefinitions>
					<ColumnDefinition Width="44" />
					<ColumnDefinition Width="*" />
					<ColumnDefinition Width="44" />
				</Grid.ColumnDefinitions>
				<Image Source="SortLeft.png" Margin="10,0" WidthRequest="30"
						IsVisible="{Binding ShowLeftButton}"
						HorizontalOptions="Start" fe:TappedGestureAttached.Command="{Binding LeftButtonCommand}" />
				<Label Text="{Binding BottomTitle}" VerticalTextAlignment="Center" HorizontalTextAlignment="Center" Grid.Column="1" Style="{StaticResource TitleWhite}" />

				<Image Source="SortRight.png" Margin="10,0" WidthRequest="30"
						IsVisible="{Binding ShowRightButton}" Grid.Column="2"
						HorizontalOptions="End" fe:TappedGestureAttached.Command="{Binding RightButtonCommand}" />				
			</Grid>
		</ContentView>
	</local:BasePage.Footer>
	
	<local:BasePage.ContentPresenter>
		<ContentView >
			<ListView ItemsSource="{Binding ItemSource}" SelectedItem="{Binding SelectedItem}" >
				<ListView.RowHeight>
					<OnPlatform x:TypeArguments="x:Int32" iOS="76" Android="90" />
				</ListView.RowHeight>
				<ListView.ItemTemplate>
					<DataTemplate>
						<ViewCell>
							<Grid Padding="10" RowSpacing="10" ColumnSpacing="5">
								<Grid.ColumnDefinitions>
									<ColumnDefinition Width="*" />
									<ColumnDefinition Width="15" />
									<ColumnDefinition Width="60" />
								</Grid.ColumnDefinitions>
								<Grid.RowDefinitions>
									<RowDefinition Height="*" />
									<RowDefinition Height="*" />
									<RowDefinition Height="*" />
								</Grid.RowDefinitions>
								<Label Text="{Binding Summary.Subject}" FontSize="16" TextColor="{Binding TitleTextColor}" LineBreakMode="TailTruncation" >
									<Label.FontFamily>
										<OnPlatform x:TypeArguments="x:String" iOS="SourceSansPro-Semibold" Android="SourceSansPro-Semibold.ttf#SourceSansPro-Semibold" />
									</Label.FontFamily>
								</Label>
								<Image Source="Attachment.png" Grid.Column="1" IsVisible="{Binding Summary.HasAttachments}" />
								<Label Text="{Binding SendDate}" FontSize="16" TextColor="#06A2D5" Grid.Column="2"/>
								<Label Text="{Binding Sender}" TextColor="#7A7A7B" Grid.Row="1" Grid.ColumnSpan="3" />
								<Label Text="{Binding To}" TextColor="#7A7A7B" Grid.Row="2" Grid.ColumnSpan="3" />
							</Grid>
						</ViewCell>
					</DataTemplate>
				</ListView.ItemTemplate>
			</ListView>
		</ContentView>
	</local:BasePage.ContentPresenter>	
</local:BasePage>

Hi,
I am getting white color above the Navigation bar in iOs XS Max. How to solve.