Ich habe eine benutzerdefinierte Listenansicht für meine Xamarin.Forms-Apps, die die Schaltersteuerung für die Mehrfachauswahl hat. Jetzt habe ich dieses Problem, bei dem ich auf der ersten Seite meiner App einen Backbutton erhalte. Der Zurück-Button auf der ersten Seite ist nicht erforderlich, da er auf eine schwarze Seite umleitet. Ich weiß, dass es etwas mit Navigation.PushAsync zu tun hat. Aber ich konnte nicht herausfinden, wo die Änderung erforderlich ist. Wenn jemand mich auf die richtige Weise leiten kann, wird er hilfreich sein.

Hier ist die Seite SelectMultipleBasePage.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
//using System.Reflection.Emit;
using System.Text;
using Xamarin.Forms;

namespace _____
{
    public class SelectMultipleBasePage<T> : ContentPage
    {
        public class WrappedSelection<T> : INotifyPropertyChanged
        {
            public T Item { get; set; }
            bool isSelected = false;
            public bool IsSelected
            {
                get
                {
                    return isSelected;
                }
                set
                {
                    if (isSelected != value)
                    {
                        isSelected = value;
                        PropertyChanged(this, new PropertyChangedEventArgs("IsSelected"));
                        //                      PropertyChanged (this, new PropertyChangedEventArgs (nameof (IsSelected))); // C# 6
                    }
                }
            }
            public event PropertyChangedEventHandler PropertyChanged = delegate { };
        }
        public class WrappedItemSelectionTemplate : ViewCell
        {
            public WrappedItemSelectionTemplate()
                : base()
            {

                Grid objGrid = new Grid();
                objGrid.BackgroundColor = Color.Gray;
                objGrid.RowDefinitions.Add(new RowDefinition
                {
                    Height = new GridLength(1, GridUnitType.Star)
                });

                objGrid.ColumnDefinitions.Add(new ColumnDefinition
                {
                    Width = new GridLength(75, GridUnitType.Absolute)
                });
                objGrid.ColumnDefinitions.Add(new ColumnDefinition
                {
                    Width = new GridLength(1, GridUnitType.Star)
                });
                objGrid.ColumnDefinitions.Add(new ColumnDefinition
                {
                    Width = GridLength.Auto
                });

                //
                // Column 1:-
                Image objImage = new Image();
                objImage.BackgroundColor = Color.Green;
                objImage.SetBinding(Image.SourceProperty, new Binding("Item.Image"));
                objGrid.Children.Add(objImage, 0, 0);

                //
                // Column 2:-
                StackLayout objStackLayoutCol2 = new StackLayout();
                objGrid.Children.Add(objStackLayoutCol2, 1, 0);

                Label name = new Label()
                {
                    Text = "Name"
                };
                Label date = new Label()
                {
                    Text = "Date"
                };
                name.SetBinding(Label.TextProperty, new Binding("Item.Name"));
                date.SetBinding(Label.TextProperty, new Binding("Item.Date"));
                objStackLayoutCol2.Children.Add(name);
                objStackLayoutCol2.Children.Add(date);

                //
                // Column 3:-
                Switch mainSwitch = new Switch();
                mainSwitch.SetBinding(Switch.IsToggledProperty, new Binding("IsSelected"));
                objGrid.Children.Add(mainSwitch, 2, 0);

                View = objGrid;


            }
        }
        public List<WrappedSelection<T>> WrappedItems = new List<WrappedSelection<T>>();
        public SelectMultipleBasePage(List<T> items)
        {
            WrappedItems = items.Select(item => new WrappedSelection<T>() { Item = item, IsSelected = false }).ToList();
            ListView mainList = new ListView()
            {
                ItemsSource = WrappedItems,
                ItemTemplate = new DataTemplate(typeof(WrappedItemSelectionTemplate)),
            };

            mainList.ItemSelected += (sender, e) =>
            {
                if (e.SelectedItem == null) return;
                var o = (WrappedSelection<T>)e.SelectedItem;
                o.IsSelected = !o.IsSelected;
                ((ListView)sender).SelectedItem = null; //de-select
            };
            Content = mainList;

            if (Device.OS == TargetPlatform.WinPhone)
            {   // fix issue where rows are badly sized (as tall as the screen) on WinPhone8.1
                mainList.RowHeight = 40;
                // also need icons for Windows app bar (other platforms can just use text)
                ToolbarItems.Add(new ToolbarItem("All", "check.png", SelectAll, ToolbarItemOrder.Primary));
                ToolbarItems.Add(new ToolbarItem("None", "cancel.png", SelectNone, ToolbarItemOrder.Primary));
            }
            else
            {
                ToolbarItems.Add(new ToolbarItem("All", null, SelectAll, ToolbarItemOrder.Primary));
                ,ToolbarItems.Add(new ToolbarItem("None", null, SelectNone, ToolbarItemOrder.Primary));
            }
        }
        void SelectAll()
        {
            foreach (var wi in WrappedItems)
            {
                wi.IsSelected = true;
            }
        }
        void SelectNone()
        {
            foreach (var wi in WrappedItems)
            {
                wi.IsSelected = false;
            }
        }
        public List<T> GetSelection()
        {
            return WrappedItems.Where(item => item.IsSelected).Select(wrappedItem => wrappedItem.Item).ToList();
        }
    }
}

Listpage.cs:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
//using System.Reflection.Emit;
using System.Text;
using System.Xml.Serialization;
using Xamarin.Forms;

namespace ______
{
    public class ListPage : ContentPage
    { 
        SelectMultipleBasePage<CheckItem> multiPage= null;
        public ListPage ()
        {
            loadlist();
        }

        async void loadlist()
        {
            var items = new List<CheckItem>();
            items.Add(new CheckItem { Name = "Xamarin.com", Date = "01/01/2015", Image = "img.png" });
            items.Add(new CheckItem { Name = "Twitter", Date = "01/01/2015", Image = "img.png" });
            items.Add(new CheckItem { Name = "Facebook", Date = "01/01/2015", Image = "img.png" });

            if (multiPage == null)
                multiPage = new SelectMultipleBasePage<CheckItem>(items) { Title = "Check all that apply" };

            await Navigation.PushAsync(multiPage);
        }
    }
}

App.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Xamarin.Forms;

namespace ____
{
    public class App : Application
    {
        public App ()
        {
            MainPage = new NavigationPage(new ListPage());
        }

        protected override void OnStart ()
        {
            // Handle when your app starts
        }

        protected override void OnSleep ()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume ()
        {
            // Handle when your app resumes
        }
    }
}