c# - 添加 MasterDetailPage 后 Xamarin.forms 应用程序中的 System.InvalidCastException

Question

我创建了一个 Xamarin 表单应用程序，它将用户输入的数据存储在 sqlite 数据库中，然后显示在列表中。到目前为止，一切都运行良好。数据被正确存储和检索。然后我按照教程添加一个 MasterDetailPage。应用程序构建成功，但出现 System.InvalidCastException:指定的转换在 mgmain JNI_OnLoad 中无效。 MainActivitiy.cs LoadApplication(new App()); 方法中的异常。仔细搜索后，我无法弄清楚问题出在哪里。

这是 App.cs，其中 MainMenuPage 是 MasterDetailPage

namespace MyListXamarinForms
{

  public class App : Application
    {

        public App()
        {

            MainPage = new MainMenuPage();
        }

        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
        }
    }
}

MainMenuPage.xaml(这是包含 MasterPage 和 Detail 的母版页 - 如下所示)`

<?xml version="1.0" encoding="utf-8" ?>
<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
                  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                  x:Class="MyListXamarinForms.MainMenuPage"
                  xmlns:local="clr-namespace:MyListXamarinForms">

  <MasterDetailPage.Master>
    <local:MasterPage x:Name="master"/>
  </MasterDetailPage.Master>

  <MasterDetailPage.Detail>
    <NavigationPage>
      <x:Arguments>
        <local:DetailPage/>
      </x:Arguments>
    </NavigationPage>
  </MasterDetailPage.Detail>
</MasterDetailPage>

MainMenuPage.xaml.cs`

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace MyListXamarinForms
{
    public partial class MainMenuPage : MasterDetailPage
    {
        public MainMenuPage()
        {
            InitializeComponent();

            master.ListViews.ItemSelected += OnItemSelected;
        }

        private void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
        {
            var item = e.SelectedItem as MasterPageItem;
            if(item!=null)
            {
                Detail = new NavigationPage((Page)Activator.CreateInstance(item.TargetType));
                master.ListViews.SelectedItem = null;
                IsPresented = false;
            }
        }
    }
}`

MasterPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MyListXamarinForms.MasterPage"
             Icon=""
             Title="Menu"
             Padding="0,50,0,0">
  <ContentPage.Content >
    <StackLayout VerticalOptions="FillAndExpand">
      <ListView x:Name="MenuList" VerticalOptions="FillAndExpand" SeparatorVisibility="None">
        <ListView.ItemTemplate>
          <DataTemplate>
            <Label x:Name="ItemNameLabel" Text="{Binding Title}"/>
          </DataTemplate>
        </ListView.ItemTemplate>            
      </ListView>          
    </StackLayout>
  </ContentPage.Content>
</ContentPage>

MasterPage.xaml.cs

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

using Xamarin.Forms;

namespace MyListXamarinForms
{
    public partial class MasterPage : ContentPage
    {
        public ListView ListViews { get { return MenuList; } }
        public MasterPage()
        {
            InitializeComponent();

            var masterPageItem = new List<MasterPageItem>();

            //AddItem and HomePage are two ContentPages with content
            masterPageItem.Add(new MasterPageItem {
                Title = "Add Item",
                TargetType = typeof(AddItem)
            });
            masterPageItem.Add(new MasterPageItem
            {
                Title = "View Item List",
                TargetType = typeof(HomePage)
            });

            MenuList.ItemsSource = masterPageItem;
        }
    }
}

MasterPageItem.cs

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

namespace MyListXamarinForms
{
    public class MasterPageItem
    {
        public string Title { get; set; }
        public Type TargetType { get; set; }
    }
}

MyListXamarinForms.Droid主事件.cs

using System;
using Android.App;
using Android.Content.PM;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;

namespace MyListXamarinForms.Droid
{
    [Activity(Label = "MyListXamarinForms", Icon = "@drawable/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.OnCreate(bundle);

            global::Xamarin.Forms.Forms.Init(this, bundle);
            LoadApplication(new App());  //Exception thrown here
        }
    }
}

`

Answer 1

在您的情况下，您不需要 ImageCell，因为您只是显示标签，但 ListView 的 DataTemplate 必须是 Cell，因此您可以这样做:

<DataTemplate> 
    <TextCell x:Name="ItemNameLabel" Text="{Binding Title}"/> 
</DataTemplate>

- TextCell 是一个带有标签的简单单元格。

或者这个:

 <DataTemplate> 
     <ViewCell>
        <Label x:Name="ItemNameLabel" Text="{Binding Title}"/>
    </ViewCell>
</DataTemplate>

- 注意这只是将标签包装在 ViewCell 中