MessagingCenter不订阅

Question

我对Xamarin很陌生(而且在编码方面也是新的)。

我在ListView中使用ListView来尝试获取LongPress菜单。

因为TouchEffect.LongPressCommand是一个命令，所以我只能出于某种原因将它绑定到模型页面。所以..。我正试图通过MessagingCenter将信息发送到代码隐藏。

我的问题是没有收到信息。

我读了很多，并试图弄清楚它，我想要能够接收到消息，订阅者需要先实例化/初始化。

我的主要问题是..。我不知道怎么做，哈哈。

还是我想做的一切都错了？

我会添加一些代码，但如果还需要什么，请告诉我。

注意:加载页面(当应用程序启动时)是一个GroupPage(运行良好)，问题在于ItemsPage。

非常感谢大家。

ItemsPage.xaml

<ContentPage.BindingContext>
    <localvm:ItemViewModel/>
</ContentPage.BindingContext>

<ListView ItemsSource="{Binding Items, Mode=TwoWay}" x:Name="lstView" 
                  AbsoluteLayout.LayoutBounds="0,0,1,1" 
                  AbsoluteLayout.LayoutFlags="All"
                  SelectedItem="{Binding SelectedItem}" HasUnevenRows="True" RowHeight="50">

    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell>

                <Grid Padding="0,0,8,0" Margin="4,0,4,0" xct:TouchEffect.LongPressCommand="{Binding LongPressItemCommand}">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto"/>
                        <ColumnDefinition Width="7*"/>
                    </Grid.ColumnDefinitions>

                    <Label Text="{Binding ItemName}" TextColor="Black" Grid.Column="1" FontSize="Medium"></Label>

                    <Label Text="{Binding ItemDescription}" Grid.Column="1" VerticalTextAlignment="End"/>

                </Grid>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

ItemsPage.cs

namespace MobileApp2
{
    public partial class ItemsPage : ContentPage
    {
        public ItemsPage()
        {
            InitializeComponent();
            MessagingCenter.Subscribe<Item, Guid>(this, "PopupMenuItemMsg",
                 (page, itemId) =>
                 {
                     Main_PopupMenu(itemId);
                 });
        }
        public async void Main_PopupMenu(Guid itemId)
        {
            DisplayActionSheet("Test", "Test", "OK");
        }
    }
}

Items.cs (模型)

namespace MobileApp2
{
    public class Item : INotifyPropertyChanged
    {
        public Command LongPressItemCommand { get; }

        public Guid ItemId { get; set; }

        public Guid GroupId { get; set; }

        private string itemName = string.Empty;
        public string ItemName
        {
            get { return itemName; }
            set
            {
                if (value != null) itemName = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("ItemName"));
            }
        }

        private string itemDescription = string.Empty;
        public string ItemDescription
        {
            get
            {

                return itemDescription.Trim();
            }
            set
            {
                if (value != null) itemDescription = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("ItemDescription"));
            }
        }


        public Item(string itemName, string itemDescription)
        {
            ItemName = itemName;
            ItemDescription = itemDescription;
        }

        public Item()
        {
            LongPressItemCommand = new Command(() =>
            {
                MessagingCenter.Send<Item, Guid>(this, "PopupMenuItemMsg", ItemId);
            });
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Answer 1

您可以使用命令的相对绑定来检查下面的代码。

型号：

public class Item
{
    public string ItemName { get; set; }
    public string ItemDescription { get; set; }
   

}

ViewModel：

public class ItemViewModel
{
    public ICommand LongPressItemCommand { get; set; }
    public Guid ItemId { get; set; }
    public ObservableCollection<Item> Items { get; set; }
    public ItemViewModel()
    {
         LongPressItemCommand = new Command(() =>
        {

            MessagingCenter.Send<ItemViewModel, Guid>(this, "PopupMenuItemMsg", ItemId);
        });
        
        CreateCollection();
    }



    public void LongPress()
    {

    }
    public void CreateCollection()
    {
        Items = new ObservableCollection<Item>()
        {
            new Item(){ ItemName="A", ItemDescription="AA"},
            new Item(){ ItemName="B", ItemDescription="BB"},
            new Item(){ ItemName="C", ItemDescription="CC"},
        };

    }
}

Xaml：

  <ContentPage.BindingContext>
    <localvm:ItemViewModel></localvm:ItemViewModel>
</ContentPage.BindingContext>
<StackLayout>
    <ListView ItemsSource="{Binding Items, Mode=TwoWay}" x:Name="lstView" 
              AbsoluteLayout.LayoutBounds="0,0,1,1" 
              AbsoluteLayout.LayoutFlags="All"
              SelectedItem="{Binding SelectedItem}" HasUnevenRows="True" RowHeight="50">

        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <Grid Padding="0,0,8,0" Margin="4,0,4,0" xct:TouchEffect.LongPressCommand="{Binding Path=BindingContext.LongPressItemCommand, Source={x:Reference Name=lstView}}">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="Auto"/>
                        </Grid.ColumnDefinitions>

                        <Label Text="{Binding ItemName}" TextColor="Black" Grid.Column="0" FontSize="Medium"></Label>

                        <Label Text="{Binding ItemDescription}" Grid.Column="1" VerticalTextAlignment="End"/>

                    </Grid>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
   
</StackLayout>

代码背后：

 MessagingCenter.Subscribe<ItemViewModel, Guid>(this, "PopupMenuItemMsg",
           (page, itemId) =>
           {
               Main_PopupMenu(itemId);
           });