如何从 MVVM 中的异步 SQLite 数据源填充选择器选项

Question

我需要一个选择器（或其他控件）在 MVVM 实现（基于 Xamarin.Forms MVVM: How to Work with SQLite DB(C# — Xaml)）中提供用户选项，但看不到如何将异步选项加载到绑定中。

我有一个名为 viewings 的记录类型的详细信息编辑屏幕，其中直接输入字段绑定到 ViewingDetailViewingModel。 ...BindingContext = new ViewingDetailViewModel(viewModel ?? new ViewingViewModel(), viewingStore, pageService, viewingsRestStore); 在 XAML 的代码隐藏中。

除了绑定到 Viewing 模型属性的相对直接的输入单元格（如果我没有使用正确的命名法，请原谅我，这是新的），我想让其中一个字段使用选择器根据用户选择客户端保存客户端 ID。

所以我猜，由于 SQLite clientstore 的 get return 是异步的，我应该有一个从某处触发的命令，我理想地想象从代码隐藏的 onAppearing 方法更新客户端的 IList选择器绑定到 - 虽然我在这里遇到两个问题：

我不知道如何从那里触发命令，所以我现在尝试从视图模型的构造函数的末尾执行它。
这样做（虽然我不认为是因为那个），应用程序在尝试将第一个客户端添加到列表时崩溃，同时使用“...System.NullReferenceException 循环获取结果：你调用的对象是空的在 AgentApp.ViewingDetailViewModel.LoadClients（System.Collections.Generic.IList`1[T] 个客户端）..."

所以在我的 XAML 我有

<EntryCell Label="Agt_description" Text="{Binding Path=Viewing.Agt_description}" />
                <ViewCell >
                    <Picker Title="Select a client" ItemsSource="{Binding Clients}"  ItemDisplayBinding ="{Binding Agt_FirstName}" />
                </ViewCell>

在视图的代码隐藏中：

 public ViewingDetailPage(ViewingViewModel viewModel)
        {
            Console.WriteLine("ViewingDetailPage()");
            InitializeComponent();
            var viewingStore = new SQLiteViewingStore(DependencyService.Get<ISQLiteDb>());
            var viewingsRestStore = new RESTViewingStore();
            var pageService = new PageService();
            bool isNewViewing = (viewModel.Agt_name == " ");
            Title = (isNewViewing) ? "New Viewing" : "Edit Viewing";
            if (isNewViewing)
            {
                var app = Application.Current as App;
                viewModel.Agt_ac = app.AgencyIdInt;
                viewModel.Agt_b = app.BranchIDInt;
                viewModel.Agt_at = app.AccountIdInt;
            }
            BindingContext = new ViewingDetailViewModel(viewModel ?? new ViewingViewModel(), viewingStore, pageService, viewingsRestStore);
        }

然后是 ViewingDetailViewModel：

class ViewingDetailViewModel : BaseViewModel
    {
        private readonly IViewingStore _viewingStore;
        private readonly IViewingStore _viewingRestStore;
        private readonly IPageService _pageService;
        public Viewing Viewing { get; private set; }
        public ICommand SaveCommand { get; private set; }

        //trying
        public ICommand LoadClientsCommand { get; private set; }
        private IClientStore _clientStore;
        private IList<ClientViewModel> _clients;
        public IList<ClientViewModel> Clients
        {
            get { return _clients; }
            set { SetProperty(ref _clients, value); }   // from BaseViewModel, implements OnPropertyChanged()
        }
        //

        public ViewingDetailViewModel(ViewingViewModel viewModel, IViewingStore viewingStore, IPageService pageService, IViewingStore viewingRestStore)
        {
                        var app = Application.Current as App;
            if (viewModel == null)
                throw new ArgumentNullException(nameof(viewModel));

            _pageService = pageService;
            _viewingStore = viewingStore;
            _viewingRestStore = viewingRestStore;

            LoadClientsCommand = new Command(async () => await LoadClients());

            SaveCommand = new Command(async () =>
            {
                                var SaveDBTask = SaveDB("primary");
                var SaveRESTTask = SaveRest();
                                var saveTasks = new List<Task> { SaveDBTask, SaveRESTTask };
                while (saveTasks.Count > 0)
                {
                    Task finishedTask = await Task.WhenAny(saveTasks);
                    if (finishedTask == SaveDBTask)
                    {
                        Console.WriteLine("SaveDBTask finished:" + SaveDBTask);
                    }
                    else if (finishedTask == SaveRESTTask)
                    {
                        Console.WriteLine("SaveRESTTask finished, do db update:" + SaveRESTTask);
                        await SaveDB("secondary update");
                    }
                    saveTasks.Remove(finishedTask);
                }
                Console.WriteLine("Save tasks complete.");
            });

            Viewing = new Viewing
            {
                Id = viewModel.Id,
                Agt_description = viewModel.Agt_description,
                Agt_name = viewModel.Agt_name,
                RemoteId = viewModel.RemoteId,
                Agt_ac = viewModel.Agt_ac > 0 ? viewModel.Agt_ac : app.AgencyIdInt,
                Agt_b = viewModel.Agt_b > 0 ? viewModel.Agt_b : app.BranchIDInt,
                Agt_at = viewModel.Agt_at > 0 ? viewModel.Agt_at : app.AccountIdInt,
                Agt_pr = viewModel.Agt_pr,
                Agt_datetime_scheduled = viewModel.Agt_datetime_scheduled,
                Agt_datetime_start = viewModel.Agt_datetime_start,
                Agt_datetime_end = viewModel.Agt_datetime_end,
                StatusLocal = viewModel.StatusLocal,
                CreatedLocal = viewModel.CreatedLocal,
                ModifiedLocal = viewModel.ModifiedLocal,
                CreatedRemote = viewModel.CreatedRemote,
                ModifiedRemote = viewModel.ModifiedRemote,
                LastSync = viewModel.LastSync,
            };

            LoadClientsCommand.Execute(null);//doubt this is a good idea
        }

        async Task LoadClients()
        {
            _clientStore = new SQLiteClientStore(DependencyService.Get<ISQLiteDb>());
            var clients = await _clientStore.GetClientsAsync();
            foreach (var client in clients)
            {
                Console.WriteLine("for client:" + client.Agt_FirstName);
                Clients.Add(new ClientViewModel(client)); //crashes around here
            }
        }

        async Task SaveDB(string savetype)
        {
            if (String.IsNullOrWhiteSpace(Viewing.Agt_name))
            {
                await _pageService.DisplayAlert("Error", "Please enter the name.", "OK");
                return;
            }
            if (Viewing.Id == 0)
            {
                await _viewingStore.AddViewing(Viewing);
                MessagingCenter.Send(this, Events.ViewingAdded, Viewing);
            }
            else
            {
                await _viewingStore.UpdateViewing(Viewing);
                MessagingCenter.Send(this, Events.ViewingUpdated, Viewing);
            }
            if (savetype == "primary")
                await _pageService.PopAsync();
        }

        async Task SaveRest()
        {
            if (String.IsNullOrWhiteSpace(Viewing.Agt_name))
            {
                Console.WriteLine("REST save could not, invalid name.");
                return;
            }
            if (Viewing.RemoteId == 0)
            {
                await _viewingRestStore.AddViewing(Viewing);
            }
            else
            {
                await _viewingRestStore.UpdateViewing(Viewing);
                MessagingCenter.Send(this, Events.ViewingUpdated, Viewing);
            }
        }
    }

我是一个来自简单 php 背景的完全新手，所有这些面向对象的复杂性让我有些困惑；我可能犯了很多非常愚蠢的错误——也许我把事情搞得太复杂了——我希望有人能给我指出一个加载选择器异步数据的简单好方法。

提前致谢！

Answer 1

首先，您似乎没有在任何地方实例化 Clients 属性。这就是为什么你会得到 NullReferenceException

其次，如果您要将项目添加到 IList 中，则没有人会收到有关已将某些内容添加到该集合中的通知。相反，您应该考虑将类型更改为 ObservableCollection，这有助于您这样做。

因此将 Clients 的代码更改为：

public ObservableCollection<ClientViewModel> Clients { get; }
    = new ObservableCollection<ClientViewModel>();

这可能会解决您的大部分问题，即 UI 未更新且 Clients 在您尝试填充它时为 null。

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