demo-xf-app-1a/MobileApp1/Helpers/ContentPageHelper.cs

using System;
using Xamarin.Forms;

namespace MobileApp1.Views
{
    public class ContentPageHelper
    {

        public static App GetApp(ContentPage view) => GetParentElement<App>(view);

        public static Application GetApplication(ContentPage view) => GetParentElement<Application>(view);

        public static MainPage GetMainPage(ContentPage view) => GetParentElement<MainPage>(view);

        public static T GetParentElement<T>(ContentPage view)
        {
            if (view == null) return default(T);

            Element item = view.Parent;

            while (true)
            {
                if (item == null) break;
                if (item is T) break;

                item = item.Parent;
            }

            try
            {
                return (T)Convert.ChangeType(item, typeof(T));
            }
            catch (InvalidCastException)
            {
                return default(T);
            }
        }

        public static void SetDetailPage(ContentPage view, Type pageType, bool useNavigationPage = true) => SetDetailPage(view, (Page)Activator.CreateInstance(pageType), useNavigationPage);

        public static void SetDetailPage(ContentPage view, Page page, bool useNavigationPage = true)
        {
            if (view == null) return;
            if (view.Parent == null) return;
            if (page == null) return;

            MainPage rs = GetMainPage(view);
            if (rs == null)
            {
                return;
            }

            Device.BeginInvokeOnMainThread(() =>
            {
                if (useNavigationPage)
                {
                    rs.Detail = new NavigationPage(page);
                }
                else
                {
                    rs.Detail = page;
                }
            });
        }

        public static void SetMainPage(ContentPage view, Page page, bool useNavigationPage = false)
        {
            if (view == null) return;
            if (view.Parent == null) return;
            if (page == null) return;

            Application rs = GetApplication(view);
            if (rs == null)
            {
                return;
            }

            Device.BeginInvokeOnMainThread(() =>
            {
                if (useNavigationPage)
                {
                    rs.MainPage = new NavigationPage(page);
                }
                else
                {
                    rs.MainPage = page;
                }
            });
        }

        public static async void PushMainPageNavigation(ContentPage view, Page page)
        {
            if (view == null) return;
            if (view.Parent == null) return;
            if (page == null) return;

            Application rs = GetApplication(view);
            if (rs == null)
            {
                return;
            }

            await rs?.MainPage.Navigation.PushAsync(page);
        }

    }
}