Exception on Xamarin: System.ObjectDisposedException: Cannot access a disposed object 'Xamarin.Forms.Platform.Android.FastRenderers.LabelRenderer'

Question

I am using Xamarin.Forms to build an app for Android and I'm currently getting the following error: System.ObjectDisposedException: Cannot access a disposed object 'Xamarin.Forms.Platform.Android.FastRenderers.LabelRenderer'. 我使用的Xamarin.Forms版本是4.8.0.1364

我的XAML代码如下：

<?xml version="1.0" encoding="UTF-8"?>
<ContentView
  x:Class="Peripass.YardAssetManagementApp.UI.Views.MainTaskListViewContent"
  xmlns="http://xamarin.com/schemas/2014/forms"
  xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
  xmlns:converters="clr-namespace:Peripass.Mobile.Framework.Converters;assembly=Peripass.Mobile.Framework"
  xmlns:d="http://xamarin.com/schemas/2014/forms/design"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:uiControls="clr-namespace:Peripass.Mobile.Framework.UIControls;assembly=Peripass.Mobile.Framework"
  mc:Ignorable="d"
  x:Name="tasksPage">
  <ContentView.Resources>
    <ResourceDictionary>
      <converters:ItemTappedEventArgsToTappedItemConverter x:Key="ItemTappedConverter" />
    </ResourceDictionary>
  </ContentView.Resources>
  <ContentView.Content>
    <!-- All Tasks -->
    <StackLayout
      BackgroundColor="White"
      VerticalOptions="FillAndExpand">
      <!-- ActivityIndicator -->
      <StackLayout
        Padding="0,20,0,0"
        BackgroundColor="White"
        IsVisible="{Binding Model.ShowLoadingIndicator}">
        <ActivityIndicator BackgroundColor="White" 
                               
                           Color="#28aa90" 
                           HeightRequest="40"
                           WidthRequest="40"
                           IsRunning="{Binding Model.ShowLoadingIndicator}"
                           IsVisible="{Binding Model.ShowLoadingIndicator}">

        </ActivityIndicator>
        <Label FontSize="14" Text="fetching tasks" TextColor="DimGray" HorizontalTextAlignment="Center" HorizontalOptions="FillAndExpand"></Label>
      </StackLayout>

      <RefreshView Command="{Binding RefreshCommand}" IsRefreshing="{Binding IsRefreshing, Mode=OneWay}">
        <CollectionView x:Name="tasks" ItemsSource="{Binding Model.AllTasks}" ItemSizingStrategy="MeasureAllItems" VerticalOptions="FillAndExpand">
          <CollectionView.ItemTemplate>
            <DataTemplate>
              <StackLayout BackgroundColor="{Binding RowBackGroundColor}">
                <StackLayout.GestureRecognizers>
                  <TapGestureRecognizer CommandParameter="{Binding TaskId}" Command="{Binding Source={x:Reference tasks}, Path=BindingContext.TappedGotoTaskDetailCommand}"/>
                </StackLayout.GestureRecognizers>
                <Grid>
                  <Grid.RowDefinitions>
                    <RowDefinition Height="73" />
                    <RowDefinition Height="*" />
                  </Grid.RowDefinitions>
                  <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="65" />
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="75" />
                  </Grid.ColumnDefinitions>
                  <uiControls:PpIcon
                            Grid.Row="0"
                            Grid.Column="0"
                            Margin="5"
                            IconChar="{Binding LeftIconChar}"
                            IconColor="{Binding LeftIconColor}"
                            IconSize="42" />

                  <Grid Grid.Row="0" Grid.Column="1">
                    <Grid.RowDefinitions>
                      <RowDefinition Height="50*" />
                      <RowDefinition Height="50*" />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                      <ColumnDefinition Width="*" />
                      <ColumnDefinition Width="Auto" />
                    </Grid.ColumnDefinitions>

                    <Label
                      FontAttributes="Bold"
                      FontSize="21"
                      HorizontalTextAlignment="Start"
                      LineBreakMode="TailTruncation"
                      Text="{Binding TaskLine1}"
                      TextColor="{StaticResource PeripassBlack}"
                      TranslationY="7"
                      VerticalTextAlignment="Center" />
                    

                    <StackLayout
                              Grid.Row="0"
                              Grid.RowSpan="2"
                              Grid.Column="1"
                              Margin="0"
                              Padding="0"
                              Orientation="Horizontal"
                              Spacing="0"
                              TranslationX="4">
                      <Label
                                FontAttributes="Bold"
                                FontSize="21"
                                HorizontalOptions="End"
                                HorizontalTextAlignment="Start"
                                Text="{Binding StepProgressLineStepsDone}"
                                TextColor="{Binding RightIconColor}"
                                VerticalTextAlignment="Center" />

                      <Label
                                FontSize="21"
                                HorizontalOptions="End"
                                HorizontalTextAlignment="Start"
                                Text="/"
                                TextColor="{Binding RightIconColor}"
                                VerticalTextAlignment="Center" />

                      <Label
                                FontSize="21"
                                HorizontalOptions="End"
                                HorizontalTextAlignment="Start"
                                Text="{Binding StepProgressLineStepsTotal}"
                                TextColor="{Binding RightIconColor}"
                                VerticalTextAlignment="Center" />
                    </StackLayout>

                    <Label
                              Grid.Row="1"
                              Grid.Column="0"
                              FontSize="16"
                              HorizontalTextAlignment="Start"
                              LineBreakMode="TailTruncation"
                              Text="{Binding TaskLine2}"
                              TextColor="{StaticResource PeripassBlack}"
                              VerticalOptions="StartAndExpand"
                              VerticalTextAlignment="Center" />


                  </Grid>

                  <uiControls:PpIcon
                            Grid.Row="0"
                            Grid.Column="2"
                            Margin="15"
                            IconChar="{Binding RightIconChar}"
                            IconColor="{Binding RightIconColor}"
                            IconSize="42" />

                  <Label
                    Grid.Row="1"
                    Grid.Column="1"
                    Grid.ColumnSpan="2"
                    FontSize="16"
                    HorizontalTextAlignment="Start"
                    LineBreakMode="TailTruncation"
                    Text="{Binding DisplayName}"
                    TextColor="{StaticResource PeripassBlack}"
                    VerticalOptions="StartAndExpand"
                    VerticalTextAlignment="Center"
                    IsVisible="{Binding Source={x:Reference tasksPage}, Path=BindingContext.Model.IsSearching}"
                    />
                </Grid>
              </StackLayout>
            </DataTemplate>
          </CollectionView.ItemTemplate>
          <CollectionView.EmptyView>
            <StackLayout>
              <StackLayout HeightRequest="100" BackgroundColor="#D6D6D6" Margin="10,10,10,10">
                <Label HorizontalOptions="Center" VerticalOptions="CenterAndExpand" FontSize="20" Text="No tasks available" TextColor="{StaticResource PeripassBlack}"/>
              </StackLayout>
            </StackLayout>
          </CollectionView.EmptyView>
        </CollectionView>
      </RefreshView>
    </StackLayout>
  </ContentView.Content>
</ContentView>

ViewModel的代码

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Globalization;
using System.Linq;
using System.Threading.Tasks;
using System.Windows.Input;
using Peripass.ApiModels;
using Peripass.Mobile.Framework.DependencyInterfaces;
using Peripass.YardAssetManagementApp.Data;
using Peripass.YardAssetManagementApp.Data.LocalDataServices;
using Peripass.YardAssetManagementApp.Data.Syncing.QueueItemSyncActions;
using Peripass.YardAssetManagementApp.DomainModel.V1;
using Peripass.YardAssetManagementApp.DomainModel.V1.Extensions;
using Peripass.YardAssetManagementApp.Events;
using Peripass.YardAssetManagementApp.Icons;
using Peripass.YardAssetManagementApp.Models;
using Peripass.YardAssetManagementApp.MVVM;
using Peripass.YardAssetManagementApp.UI.TopHeaders;
using Xamarin.Forms;

namespace Peripass.YardAssetManagementApp.ViewModels {
  public class MainTaskListViewModel : ViewModelBase<MainTaskListModel> {
    private readonly ILocalTaskDataService _localTaskDataService;
    private readonly ILocalLocationDataService _localLocationDataService;
    private readonly IToMobileTasksSyncer _toMobileTasksSyncer;
    private List<YardOperatorTask> _tasks;
    private readonly ILocalUserDataService _localUserDataService;

    public MainTaskListViewModel(ILocalTaskDataService localTaskDataService, ILocalLocationDataService localLocationDataService, IToMobileTasksSyncer toMobileTasksSyncer, ILocalUserDataService localUserDataService) {
      _localTaskDataService = localTaskDataService;
      _localLocationDataService = localLocationDataService;
      _toMobileTasksSyncer = toMobileTasksSyncer;
      _localUserDataService = localUserDataService;
      MessagingCenter.Unsubscribe<ToMobileTasksSyncer>(this, EventTypes.FetchingTasks.ToString());
      MessagingCenter.Unsubscribe<ToMobileTasksSyncer>(this, EventTypes.LocalTasksUpdated.ToString());
      MessagingCenter.Unsubscribe<IQueueItemSyncAction>(this, EventTypes.LocalTasksUpdated.ToString());
      MessagingCenter.Unsubscribe<ToMobileTasksSyncer>(this, EventTypes.FetchingTasksFailed.ToString());
      SubscribeToTaskFetches();
      _tasks = new List<YardOperatorTask>();
    }
    public ICommand TappedGotoTaskDetailCommand => new Command<int>(async (taskId) => await ExecuteGotoTaskDetail(taskId));
    public ICommand RefreshCommand => new Command(ExecuteRefreshCommand);

    private void SubscribeToTaskFetches() {
      MessagingCenter.Subscribe<ToMobileTasksSyncer>(this, EventTypes.FetchingTasks.ToString(), sender => {
        Model.ShowLoadingIndicator = true;
      });
      MessagingCenter.Subscribe<ToMobileTasksSyncer>(this, EventTypes.LocalTasksUpdated.ToString(), _ => OnTasksUpdated());
      MessagingCenter.Subscribe<IQueueItemSyncAction>(this, EventTypes.LocalTasksUpdated.ToString(), _ => OnTasksUpdated());
      MessagingCenter.Subscribe<ToMobileTasksSyncer>(this, EventTypes.FetchingTasksFailed.ToString(), sender => {
        Model.ShowLoadingIndicator = false;
      });
    }

    private void OnTasksUpdated() {
      Model.ShowLoadingIndicator = false;
      GetTasks();
      MapToModel();
    }

    public bool IsRefreshing { get; set; }

    private async void ExecuteRefreshCommand() {
      try {
        if (IsRefreshing) {
          return;
        }

        IsRefreshing = true;
        await _toMobileTasksSyncer.SyncAllData();
      }
      catch {
        await NavigationService.NavigateToViewModelAsync<GenericExceptionPageViewModel>();
      }
      finally {
        IsRefreshing = false;
      }
    }

    public override void OnInit(object param) {
      SetTopHeader();
    }

    private void SetTopHeader() {
      var topHeader = new TopHeaderForAllTasksContentView(SearchTextChangedAction, StopSearchAction);
      topHeader.SetLine("Task List");
      NavigationService.SetTopHeader(TopHeaderTappedBackCommand, topHeader);
      NavigationService.SetTopHeaderVisible(true);
    }

    public override void OnAppeared() {
      GetTasks();
      MapToModel();
    }

    private void GetTasks() {
      _tasks = _localTaskDataService
        .GetAllTasks()
        .Where(task => task.Status == TaskStatusEnum.Unassigned || task.Status == TaskStatusEnum.Started && task.AssignedOperatorId == _localUserDataService.GetCurrentUser().UserTechnicalId)
        .ToList();
    }

    private async Task ExecuteGotoTaskDetail(int taskId) {
      await NavigationService.NavigateToViewModelAsync<TaskDetailViewModel>(taskId);
    }

    private void StopSearchAction() {
      DependencyService.Get<IDeviceKeyboardHelper>().HideKeyboard();
      MapToModel();
    }

    private void SearchTextChangedAction(string searchText) {
      MapToModel(searchText);
    }

    private void TopHeaderTappedBackCommand() {
      NavigationService.ShowMenu();
    }

    private void MapToModel(string searchOn = "") {
      Model.IsSearching = searchOn != "";
      Model.AllTasks = new ObservableCollection<TaskItem>();
      Model.ShowMessageNoTasksAvailable = !Model.ShowLoadingIndicator && _tasks.All(task => task.Status == TaskStatusEnum.Finished);
      AddUncompletedTasks(searchOn);
    }

    private void AddUncompletedTasks(string searchOn = "") {
      if (_tasks == null) return;
      var tasks = _tasks.Where(c => c.Status != TaskStatusEnum.Finished).ToArray();
      if (searchOn != "") {
        tasks = _tasks.Where(task => task.GetSearchableTaskString(_localLocationDataService.GetLocations()).Contains(searchOn.ToUpper(CultureInfo.InvariantCulture))).ToArray();
      }

      for (var i = 0; i < tasks.Length; i++) {
        AddUncompletedTask(i, tasks[i]);
      }
    }

    private void AddUncompletedTask(int index, YardOperatorTask task) {
      var rowBackgroundColor = Color.FromHex("FFFFFF");
      if (index % 2 == 0) {
        rowBackgroundColor = Color.FromHex("F6F6F6");
      }

      var leftIconChar = CalcLeftIconChar(task);
      var iconColor = CalcIconColor(task);
      var rightIconChar = CalcRightIconChar(task);

      var stepProgressLineStepsDone = task.TaskSteps.Count(c => c.IsDone).ToString();
      var stepProgressLineStepsTotal = task.TaskSteps.Count().ToString();

      Model.AllTasks.Add(new TaskItem {
        TaskIsFinished = false,
        TaskId = task.Id,
        LeftIconChar = leftIconChar,
        LeftIconColor = iconColor,
        TaskLine1 = task.GetTaskDisplayName(_localLocationDataService.GetLocations()),
        TaskLine2 = task.Template.Name,
        RightIconChar = rightIconChar,
        RightIconColor = iconColor,
        RowBackGroundColor = rowBackgroundColor,
        StepProgressLineStepsDone = stepProgressLineStepsDone,
        StepProgressLineStepsTotal = stepProgressLineStepsTotal,
        DisplayName = task.Asset?.Name ?? task.Visitor.Name
      });
    }

    private static char CalcRightIconChar(YardOperatorTask task) {
      switch (task.Status) {
        case TaskStatusEnum.Unassigned:
          return IconChar.For(IconsEnum.TaskListTaskIsUnassiged);
        case TaskStatusEnum.Assigned:
        case TaskStatusEnum.Started:
          return IconChar.For(IconsEnum.TaskListTaskStarted);
        default:
          return ' ';
      }
    }

    private static Color CalcIconColor(YardOperatorTask task) {
      switch (task.Status) {
        case TaskStatusEnum.Unassigned:
          return IconColor.For(IconsEnum.TaskListTaskIsUnassiged);
        case TaskStatusEnum.Assigned:
        case TaskStatusEnum.Started:
          return IconColor.For(IconsEnum.TaskListTaskStarted);
        default:
          return Color.Black;
      }
    }

    private static char CalcLeftIconChar(YardOperatorTask task) {
      if (task.TaskSteps.Any(c => c.FieldDefinitionType == FieldDefinitionType.MoveToLocation)) {
        return IconChar.For(IconsEnum.TaskListTaskIsMove);
      }
      else {
        return IconChar.For(IconsEnum.TaskListTaskIsInspection);
      }
    }
  }
}

我相信XAML中的这段代码是罪魁祸首。 更具体地说，label 的IsVisible属性。 如果我删除它，似乎不会发生异常。

<Label
                    Grid.Row="1"
                    Grid.Column="1"
                    Grid.ColumnSpan="2"
                    FontSize="16"
                    HorizontalTextAlignment="Start"
                    LineBreakMode="TailTruncation"
                    Text="{Binding DisplayName}"
                    TextColor="{StaticResource PeripassBlack}"
                    VerticalOptions="StartAndExpand"
                    VerticalTextAlignment="Center"
                    IsVisible="{Binding Source={x:Reference tasksPage}, Path=BindingContext.Model.IsSearching}"
                    />

我已经搜索了一段时间并尝试了不同的东西，但似乎没有任何效果。 如果您需要更多信息，请询问。

Answer 1

此问题已在最新版本的Xamarin.forms 4.8.0.1687中修复。

任何遇到此问题的人请更新您的 Xamarin.forms 版本。

以下是 Github 中的一些相关线程。

无法访问已处置的 object

无法访问已处置的 object FastRenderers.LabelRenderer