Xamarin MVVM 从另一个页面删除列表视图项

Question

你好 Xamarin 表单的新手。我有一个列表视图，我可以在选择项目时提取信息，类似于 Visual Studio 2017 xamarin 项目中提供的模板。

希望模板更完整，因为添加和更新效果很好，但我在创建删除方法时遇到了问题。数据已从 SQLite 中删除，但 ObservableCollection 未刷新。

我尝试将类似的逻辑从添加和更新复制到删除。我尝试将另一个消息中心方法添加到我的构造函数中，但是在像添加一样调试时它没有被命中。不知道我错过了什么。还在我头上。

这是列表视图，效果很好

        <ContentPage.ToolbarItems>
        <ToolbarItem Text="-" Command="{Binding ExecuteDeleteAllCommand}"></ToolbarItem>
        <ToolbarItem Text="+" Clicked="InsertAuditClicked"></ToolbarItem>
    </ContentPage.ToolbarItems>
    <ContentPage.Padding>
        <OnIdiom>10, 10, 10, 10</OnIdiom>
    </ContentPage.Padding>
    <ContentPage.Content>
        <StackLayout>
            <ListView x:Name="AuditsListView"
                      ItemsSource="{Binding AuditsCollection}"
                      VerticalOptions="FillAndExpand"
                      HasUnevenRows="true"
                      RefreshCommand="{Binding LoadAuditsCommand}"
                      IsPullToRefreshEnabled="true"
                      IsRefreshing="{Binding IsBusy, Mode=OneWay}"
                      CachingStrategy="RecycleElement"
                      ItemSelected="OnAuditSelected">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <Grid>
                                <StackLayout Grid.Column="0">
                                    <!-- <Label Text="{Binding Id}"
                                       LineBreakMode="NoWrap"
                                       Style="{DynamicResource ListAuditTextStyle}"
                                       FontSize="16"></Label>-->
                                    <Label Text="{Binding AuditId}"
                                       Style="{DynamicResource ListAuditTextStyle}"
                                       FontSize="16"></Label>
                                    <Label Text="{Binding AuditCommand}"
                                       LineBreakMode="NoWrap"
                                       Style="{DynamicResource ListAuditDetailTextStyle}"
                                       FontSize="13"></Label>
                                </StackLayout>
                                <StackLayout Grid.Column="1" VerticalOptions="CenterAndExpand" HorizontalOptions="EndAndExpand">
                                    <Label Text=">" FontSize="18"></Label>
                                </StackLayout>
                            </Grid>

                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>
    </ContentPage.Content>

这里是后面的代码

    {
        AuditsViewModel ViewModel;

        public AuditsPage()
        {
            InitializeComponent();

            BindingContext = ViewModel = new AuditsViewModel();
        }

        async void OnAuditSelected(object s, SelectedItemChangedEventArgs e)
        {
            var m = e.SelectedItem as AuditModel;

            if (m == null)
                return;

            await Navigation.PushAsync(new AuditDetailPage(new AuditDetailViewModel(m))); // (new AuditDetailPage(new AuditDetailViewModel(m)));

            AuditsListView.SelectedItem = null;
        }

        async void InsertAuditClicked(object sender, EventArgs e)
        {

            await Navigation.PushModalAsync(new NavigationPage(new AuditNewPage()));

        }

        protected override void OnAppearing()
        {
            base.OnAppearing();

            if (ViewModel.AuditsCollection.Count == 0)
                ViewModel.LoadAuditsCommand.Execute(null);
        }


    }

这是视图模型

namespace AuditTest.ViewModels
{
    public class AuditsViewModel : BaseViewModel
    {
        public ObservableCollection<AuditModel> AuditsCollection { get; set; }

        public Command LoadAuditsCommand { get; set; }

        public AuditsViewModel()
        {
            Title = "Browse";
            AuditsCollection = new ObservableCollection<AuditModel>();
            LoadAuditsCommand = new Command(async () => await ExecuteLoadAuditsCommand());

            MessagingCenter.Subscribe<AuditNewPage, AuditModel>(this, "AddAudit", async (o, a) =>
            {
                var n = a as AuditModel;
                AuditsCollection.Add(n);
                await App.MobileDataBase.InsertAuditAsync(n);
            });

            //MessagingCenter.Subscribe<AuditDetailPage, AuditModel>(this, "DeleteAudit", async (o, a) =>
            //{
            //    var n = a as AuditModel;
            //    AuditsCollection.Remove(n);
            //    await App.MobileDataBase.DeleteAuditAsync(n);


            //});

        }

        async Task ExecuteLoadAuditsCommand()
        {
            if (IsBusy)
                return;

            IsBusy = true;

            try
            {
                AuditsCollection.Clear();
                var a = await App.MobileDataBase.GetAuditsAsync(true); // var a = await DataSource.GetAuditsAsync(true);

                foreach (var i in a)
                {
                    AuditsCollection.Add(i);
                }
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
            }
            finally
            {
                IsBusy = false;
            }
        }

    }
}

获得了带有点击事件的工具栏。这是背后的代码

    using AuditTest.Models;
using Xamarin.Forms;
using AuditTest.Views;
using System.Threading.Tasks;

namespace AuditTest.ViewModels
{
    public class AuditDetailViewModel : BaseViewModel
    {
        public AuditModel AuditDetail { get; set; }

        public AuditDetailViewModel(AuditModel m = null)
        {

            Title = m.AuditId.ToString(); // m?.AuditId;
            AuditDetail = m;

        }

        public Command ExecuteEditAuditCommand
        {
            get
            {
                return new Command(async () =>
                {
                    await DataSource.UpdateAuditAsync(AuditDetail);
                });
            }
        }

        public Command ExecuteDeleteAuditCommand
        {
            get
            {
                return new Command(async () =>
                {
                    await App.MobileDataBase.DeleteAuditAsync(AuditDetail); // DataSource.DeleteAuditAsync(AuditDetail.AuditId));
                });
            }
        }

        public Task TestUpdateCommand()
        {
            Task<bool> tt = App.MobileDataBase.UpdateAuditAsync(AuditDetail);

            return tt;
        }

        public Task TestDeleteCommand()
        {
            Task<bool> tt = App.MobileDataBase.DeleteAuditAsync(AuditDetail);
            MessagingCenter.Send(this, "DeleteAudit", this.AuditDetail); // await ViewModel.ExecuteDeleteAuditCommand(); // App.MobileDataBase.DeleteAuditAsync();
            return tt;

        }
    }
}

尝试使用不同的属性和方法，但这里是内容页面的视图模型

    using AuditTest.Models;
using Xamarin.Forms;
using AuditTest.Views;
using System.Threading.Tasks;

namespace AuditTest.ViewModels
{
    public class AuditDetailViewModel : BaseViewModel
    {
        public AuditModel AuditDetail { get; set; }

        public AuditDetailViewModel(AuditModel m = null)
        {

            Title = m.AuditId.ToString(); // m?.AuditId;
            AuditDetail = m;

        }

        public Command ExecuteEditAuditCommand
        {
            get
            {
                return new Command(async () =>
                {
                    await DataSource.UpdateAuditAsync(AuditDetail);
                });
            }
        }

        public Command ExecuteDeleteAuditCommand
        {
            get
            {
                return new Command(async () =>
                {
                    await App.MobileDataBase.DeleteAuditAsync(AuditDetail); // DataSource.DeleteAuditAsync(AuditDetail.AuditId));
                });
            }
        }

        public Task TestUpdateCommand()
        {
            Task<bool> tt = App.MobileDataBase.UpdateAuditAsync(AuditDetail);

            return tt;
        }

        public Task TestDeleteCommand()
        {
            Task<bool> tt = App.MobileDataBase.DeleteAuditAsync(AuditDetail);
            MessagingCenter.Send(this, "DeleteAudit", this.AuditDetail); // await ViewModel.ExecuteDeleteAuditCommand(); // App.MobileDataBase.DeleteAuditAsync();
            return tt;

        }
    }
}

Answer 1

已修复。谢谢杰森，你太棒了！现在可以使用了。

取消评论并将 ViewModel 构造函数上的消息中心订阅更改为指向其他 ViewModel 发送

MessagingCenter.Subscribe<AuditDetailViewModel, AuditModel>(this, "DeleteAudit", async (o, a) =>
            {
                var n = a as AuditModel;
                AuditsCollection.Remove(n);
                await App.MobileDataBase.DeleteAuditAsync(n);


            });

Public 其他 ViewModel 上的方法仍然如此

        public void MethodDeleteCommand()
        {
            MessagingCenter.Send(this, "DeleteAudit", this.AuditDetail);
        }

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