xamarin.forms - 弹出另一个页面后，Xamarin.Forms 页面不会立即出现

Question

我的应用程序目前有三个页面。我将它们称为EventsPage、NewEventPage 和ListPage。EventsPage 是应用程序的第一页，您可以从那里打开一个 NewEventPage。在这个 NewEventPage 上有一个按钮，它从堆栈中弹出 NewEventPage 并且应该在之后立即创建一个 ListPage，但是 ListPage 没有出现，尽管我发现它的构造函数正在运行。

这是 NewEventPage 的代码：

using Partylist.Models;

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Partylist.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class NewEventPage : ContentPage
    {
        // Constructor.
        public NewEventPage()
        {
            InitializeComponent();
        }

        // Event handlr for when the "Cancel" button is clicked.
        async void OnCancelClicked(object sender, EventArgs e)
        {
            // Goes back to the previous page.
            await Navigation.PopAsync();
        }

        // Event handler for when the "Create" button gets clicked.
        async void OnCreateClicked(object sender, EventArgs e)
        {
            // Make sure there is something in the text entry.
            if (string.IsNullOrWhiteSpace(EventNameEntry.Text))
            {
                // If there is nothing there, print an error message.
                ErrorLabel.Text = "Your event needs a name!";
            }
            // If there is something in the text entry, try to create 
            // a new event with the text as its name.
            else
            {
                // Variable to store the created event.
                Event newEvent = new Event();
                // Variable to store its folder.
                DirectoryInfo newEventFolder;
                // Flag to see if the event was created sccessfully.
                bool eventCreated;
                try
                {
                    // If there's already an event with that name, let the
                    // user know.
                    if 
(Directory.Exists(Path.Combine(Environment.GetFolderPath
                        (Environment.SpecialFolder.LocalApplicationData),
                        EventNameEntry.Text)))
                    {
                        ErrorLabel.Text = "You already have an event with that name.";
                        eventCreated = false;
                    }
                    // Otherwise, try to creaate the folder.
                    else
                    {
                        newEventFolder = Directory.CreateDirectory(
                            Path.Combine(Environment.GetFolderPath
                         (Environment.SpecialFolder.LocalApplicationData),
                            EventNameEntry.Text));
                        // Then, create the event based on that folder.
                        newEvent = new Event
                        {
                            FolderName = EventNameEntry.Text,
                            DateCreated = newEventFolder.CreationTime,
                            DateEdited = newEventFolder.LastWriteTime
                        };
                        // Get rid of any error messages that might be on screen.
                        ErrorLabel.Text = "";
                        eventCreated = true;
                    }
                }
                // Should throw an ArgumentException in most cases where there is 
                // an invalid character.
                catch (ArgumentException)
                {
                    // Get all the invalid characters and turn them into a string.
                    char[] invalidChars = Path.GetInvalidPathChars();
                    string invalid = "";
                    foreach(char currentChar in invalidChars)
                    {
                        invalid += currentChar;
                    }
                    // Change the text of the error label.
                    ErrorLabel.Text = "Your event name can't have these characters: \""
                        + invalid + "\".";
                    eventCreated = false;
                }
                // If the event was created successfully, select it, pop the "New Event" 
                // page, and open a "List" page for the event.
                if (eventCreated)
                {
                    App.selectedEvent = newEvent;
                    await Navigation.PopAsync();
                    await Navigation.PushAsync(new ListsPage());
                }
            }
        }
    }
}

这是 ListPage 的代码：

using Partylist.Models;

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Partylist.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class ListsPage : ContentPage
    {
        // List of lists, used to populate 
        // the page's ListView (see the XAML).
        public ObservableCollection<PartylistList> ListList { get; set; }

        // Constructor.
        public ListsPage()
        {
            // Does all the stuff to make the page
            // exist that doesn't involve anything 
            // specific to this particular page in
            // this particular app.
            InitializeComponent();
        }

        // Override for OnAppearing().
        protected override void OnAppearing()
        {
            // Regular OnAppearing() method.
            base.OnAppearing();

            // Set the title to be the name of the selected event.
            Title = App.selectedEvent.FolderName;

            // Set the BindingContext of the page to itself.
            this.BindingContext = this;

            // Set the ItemsSource of the ListView in the 
            // XAML to the ObservableCollection.
            ListList = new ObservableCollection<PartylistList>();
            ListListView.ItemsSource = ListList;

            // Loop to populate the ObservableCollection.
            for (int i = 0; i < Directory.GetFiles(
                Path.Combine(Environment.GetFolderPath
                (Environment.SpecialFolder.LocalApplicationData),
                App.selectedEvent.FolderName))
                .Length; i++)
            {
                // Add a new list.
                ListList.Add(new ContactList());
                // Set the filename to the name of the file 
                // that the list corresponds to.
                ListList.ElementAt(i).Filename =
                    Directory.GetFiles(
                    Path.Combine(Environment.GetFolderPath
                    (Environment.SpecialFolder.LocalApplicationData),
                    App.selectedEvent.FolderName))[i];
                // Sets the date/time created to the file's
                // creation date.
                ListList.ElementAt(i).DateCreated = Directory
                    .GetCreationTime(Directory.GetFiles(
                    Path.Combine(Environment.GetFolderPath
                    (Environment.SpecialFolder.LocalApplicationData),
                    App.selectedEvent.FolderName))[i]);
                // Sets the date/time last edited to the 
                // folder's write date.
                ListList.ElementAt(i).DateEdited = Directory
                    .GetLastWriteTime(Directory.GetFiles(
                    Path.Combine(Environment.GetFolderPath
                    (Environment.SpecialFolder.LocalApplicationData),
                    App.selectedEvent.FolderName))[i]);
            }
        }
    }
}

标签： xamarin.formsnavigation