同步命令与相同的异步方法相反

Question

我构建了一个非常简单的列表制作应用程序，它使用xamarin.forms和sqlite-net-pcl nuget软件包。我有一个单一的型号ListItem，带有布尔属性Active。有两个列表：activeItems，其中Active = true和inactiveItems，其中Active = false。当用户点击activeItems列表中的项目时，将其设置为Active = false，并且两个列表将重新加载。当用户点击inactiveItems列表中的项目时，将其设置为Active = true，并重新加载这两个列表。所有这些都很好。

但是，我还想提供永久删除项目的选项，而不仅仅是标记为完整。当用户长时间持有物品的MenuItem（在列表上）并选择"删除"时，ListItem会按预期从数据库中删除，但并未从activeItems列表中删除，inactiveItems列表完全消失（当i时重新加载页面，inactiveItems列表返回，activeItems列表缺少已删除的项目，如预期）。这真的很奇怪，因为我将" setActive/intactive"方法从字面上复制并将其复制到我的" deletelistitem"方法中。唯一的区别是" deletelistItem"是同步的，但是我不明白为什么会做它正在做的事情。

sidenote：对于每种方法，我重新加载了这两个列表的HeightRequest，因此它们完全适合其内容。

有人看到了这个问题吗？或者，如果有人知道如何使我的" deletelistitem"方法异步，那也很棒，这是一个问题。

update

我尝试使我的" deletelistItem"异步制作，但行为没有改变。我还尝试使用App.ListItemRepo.GetActiveListItemsAsync());而不是App.ListItemRepo.GetActiveListItems());，因为这是在工作方法中使用的方法。但是后来我得到了这个错误：

Error   CS1503
Argument 1: cannot convert from 'System.Threading.Tasks.Task<System.Collections.Generic.List<Myapp.Models.ListItem>>' to 'System.Collections.Generic.List<Myapp.Models.ListItem>'

---

XAML：

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="..."
             xmlns:x="..."
             x:Class="Myapp.ListPage"
             x:Name="ListPage">
    <ScrollView Orientation="Vertical">
    <StackLayout>
            <!-- CURRENT ITEMS LIST -->
        <StackLayout VerticalOptions="Fill" Orientation="Vertical" HorizontalOptions="StartAndExpand">
            <ListView x:Name="activeList" VerticalOptions="Start" ItemTapped="SetInactive">
                <ListView.ItemTemplate>
                    <DataTemplate>
                            <TextCell Text="{Binding Name}">
                                <TextCell.ContextActions>
                                    <MenuItem Command="{Binding Source={x:Reference ListPage}, Path=DeleteListItem}"
                                    CommandParameter="{Binding .}" Text="delete" />
                                </TextCell.ContextActions>
                            </TextCell>
                        </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>
        <!-- PAST ITEMS LIST -->
        <ListView x:Name="inactiveList" VerticalOptions="Start" ItemTapped="SetActive" >
            <ListView.ItemTemplate>
                <DataTemplate>
                <TextCell Text="{Binding Name}">
                    <TextCell.ContextActions>
                        <MenuItem Command="{Binding Source={x:Reference ListPage}, Path=DeleteListItem}"
                            CommandParameter="{Binding .}" Text="delete" />
                    </TextCell.ContextActions>
                </TextCell>
              </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout>
    </ScrollView>
</ContentPage>

code-behind ：（使用listitemrepository.cs中定义的方法，定义为" app.listitemrepo"）

using Myapp.Models;
using System;
using System.Collections.ObjectModel;
using System.Linq;
using SQLite;
using System.Threading.Tasks;
using System.IO;
using Xamarin.Forms;
using System.Diagnostics;
namespace Myapp
{
    public partial class ListPage
    {
        public Command DeleteListItem { get; set; }
        public ListPage()
        {
            InitializeComponent();
            ObservableCollection<ListItem> activeItems =
                new ObservableCollection<ListItem>(
                    App.ListItemRepo.GetActiveListItems());
            activeList.ItemsSource = activeItems;
            activeList.HeightRequest = 50 * activeItems.Count;
            ObservableCollection<ListItem> inactiveItems =
                new ObservableCollection<ListItem>(
                    App.ListItemRepo.GetInactiveListItems());
            inactiveList.ItemsSource = inactiveItems;
            inactiveList.HeightRequest = 50 * inactiveItems.Count;
            });
            // DELETE METHOD
            DeleteListItem = new Command((parameter) => {
                ListItem item = (ListItem)parameter as ListItem;
                App.ListItemRepo.DeleteListItemAsync(item);
                ObservableCollection<ListItem> commandActiveItems =
                    new ObservableCollection<ListItem>(
                        App.ListItemRepo.GetActiveListItems());
                activeList.ItemsSource = commandActiveItems;
                activeList.HeightRequest = 50 * commandActiveItems.Count;
                ObservableCollection<ListItem> commandInactiveItems =
                    new ObservableCollection<ListItem>(
                        App.ListItemRepo.GetInactiveListItems());
                inactiveList.ItemsSource = commandInactiveItems;
                inactiveList.HeightRequest = 50 * commandInactiveItems.Count;
            });
        }
        // SET AS ACTIVE METHOD
        public async void SetActive(object sender, ItemTappedEventArgs e)
        {
            var selectedListItem = e.Item as ListItem;
            await App.ListItemRepo.SetListItemActive(selectedListItem);
            ObservableCollection<ListItem> activeItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetActiveListItemsAsync());
            activeList.ItemsSource = activeItems;
            activeList.HeightRequest = 50 * activeItems.Count;
            ObservableCollection<ListItem> inactiveItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetInactiveListItemsAsync());
            inactiveList.ItemsSource = inactiveItems;
            inactiveList.HeightRequest = 50 * inactiveItems.Count;
        }
        // SET AS INACTIVE METHOD
        public async void SetInactive(object sender, ItemTappedEventArgs e)
        {
            var selectedListItem = e.Item as ListItem;
            await App.ListItemRepo.SetListItemInactive(selectedListItem);
            ObservableCollection<ListItem> activeItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetActiveListItemsAsync());
            activeList.ItemsSource = activeItems;
            activeList.HeightRequest = 50 * activeItems.Count;
            ObservableCollection<ListItem> inactiveItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetInactiveListItemsAsync());
            inactiveList.ItemsSource = inactiveItems;
            inactiveList.HeightRequest = 50 * inactiveItems.Count;
        }
    }
}

资产/listItemrepository.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Myapp.Models;
using SQLite;
using System.Threading.Tasks;
namespace Myapp
{
    public class ListItemRepository
    {
        private readonly SQLiteAsyncConnection conn;
        private readonly SQLiteConnection syncconn;
        public ListItemRepository(string dbPath)
        {
            conn = new SQLiteAsyncConnection(dbPath);
            conn.CreateTableAsync<ListItem>().Wait();
            syncconn = new SQLiteConnection(dbPath);
            syncconn.CreateTable<ListItem>();
        }

        public async Task SetListItemActive(ListItem listItem)
        {
            ListItem item = (ListItem)listItem as ListItem;
            var result = await conn.UpdateAsync(new ListItem { Id = item.Id, Active = true, Name = item.Name }).ConfigureAwait(continueOnCapturedContext: false);
        }
        public async Task SetListItemInactive(ListItem listItem)
        {
            ListItem item = (ListItem)listItem as ListItem;
            var result = await conn.UpdateAsync(new ListItem { Id = item.Id, Active = false, Name = item.Name }).ConfigureAwait(continueOnCapturedContext: false);
        }
        public async Task DeleteListItemAsync(ListItem listItem)
        {
            ListItem item = (ListItem)listItem as ListItem;
            var result = await conn.DeleteAsync(new ListItem { Id = item.Id }).ConfigureAwait(continueOnCapturedContext: false);
        }
        public Task<List<ListItem>> GetActiveListItemsAsync()
        {
            return conn.QueryAsync<ListItem>("select * from list_items where Active = 1");
        }
        public Task<List<ListItem>> GetInactiveListItemsAsync()
        {
            return conn.QueryAsync<ListItem>("select * from list_items where Active = 0");
        }
        public List<ListItem> GetActiveListItems()
        {
            return syncconn.Query<ListItem>("select * from list_items where Active = 1");
        }
        public List<ListItem> GetInactiveListItems()
        {
            return syncconn.Query<ListItem>("select * from list_items where Active = 0");
        }
    }
}

Answer 1

您可以异步调用命令。只需在DELETELISTITEM命令中（参数）添加async关键字。然后，您还应该不同步调用DeletelistIteMasync方法。最后，调用getActiveListItemSasync而不是getActivelistItemsasync。

        DeleteListItem = new Command(async (parameter) => {
            ListItem item = (ListItem)parameter as ListItem;
            await App.ListItemRepo.DeleteListItemAsync(item);
            ObservableCollection<ListItem> commandActiveItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetActiveListItemsAsync());
            activeList.ItemsSource = commandActiveItems;
            activeList.HeightRequest = 50 * commandActiveItems.Count;
            ObservableCollection<ListItem> commandInactiveItems =
                new ObservableCollection<ListItem>(
                    await App.ListItemRepo.GetInactiveListItemsAsync());
            inactiveList.ItemsSource = commandInactiveItems;
            inactiveList.HeightRequest = 50 * commandInactiveItems.Count;
        });

我希望它有帮助！如果没有，请让我知道：）