topbar.cs

namespace mynamespace
{
    using Xamarin.Forms;

    public class TopBar : View
    {
        private string Title, LeftImage, RightImage;
        Page currentPage;

        public TopBar(string text = "", Page current = null, string leftImage = "", string rightImage = "")
        {
            Title = text;
            LeftImage = leftImage;
            RightImage = rightImage;
            currentPage = current;
        }

        public Grid CreateTopBar()
        {
            var leftCell = new Image();

            if (!string.IsNullOrEmpty(LeftImage))
            {
                leftCell.Source = LeftImage;
                leftCell.HeightRequest = leftCell.WidthRequest = 32;
                leftCell.HorizontalOptions = LayoutOptions.StartAndExpand;

                var gestLeft = new TapGestureRecognizer
                {
                    NumberOfTapsRequired = 1,
                };
                gestLeft.Tapped += async delegate
                {
                    await currentPage.Navigation.PopAsync();   
                };
                leftCell.GestureRecognizers.Add(gestLeft);
            }

            var rightCell = new Image();

            if (!string.IsNullOrEmpty(RightImage))
            {
                rightCell.Source = RightImage;
                rightCell.HeightRequest = rightCell.WidthRequest = 32;
                rightCell.HorizontalOptions = LayoutOptions.EndAndExpand;
            }

            dynamic title;
            if (!string.IsNullOrEmpty(Title))
            {
                title = new Label
                {
                    Font = Font.SystemFontOfSize(22),
                    TextColor = Color.White,
                    XAlign = TextAlignment.Center,
                    YAlign = TextAlignment.Center,
                    MinimumWidthRequest = App.ScreenSize.Width - 64,
                    Text = Title
                };
            }
            else
            {
                title = new Image
                {
                    Source = "logo.png",
                };
            }

            var grid = new Grid
            {
                VerticalOptions = LayoutOptions.CenterAndExpand,
                BackgroundColor = App.Self.Green,
                HeightRequest = 52,
                MinimumHeightRequest = 52,
                WidthRequest = App.ScreenSize.Width,
            };
            grid.Children.Add(leftCell, 0, 0);
            grid.Children.Add(title, 1, 0);
            grid.Children.Add(rightCell, 2, 0);

            return grid;
        }
    }
}

Usage

var topbar = new TopBar("", this, "icoback.png").CreateTopBar();  
or
var topbar = new TopBar().CreateTopBar();