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Xamarin.Forms MVVM TapGestureRecognizer 到 ListView 的 ViewCell 中的標簽（在部分文件中）

[英]Xamarin.Forms MVVM TapGestureRecognizer to a Label in a ViewCell of ListView (in partial files)

 原文  2020-10-01 08:44:34  8   2    c#/ listview/ xamarin/ xamarin.forms/ uitapgesturerecognizer

問題描述

我已經對這個問題進行了很多搜索，坦率地說，我對此很感興趣。 我有一個聊天應用程序。 在這個應用程序中有一個視圖，其中有來自我和其他聊天成員的消息。 從技術上講，它是一個帶有 ItemTemplate 的 ListView，它有一個 Binded (DataTemplateSelector) 類，它根據規則返回 ViewCells（要顯示的消息是 MINE 還是 OTHERS）

消息（入站或出站）位於單獨的文件中。

目前，TapGestureRecognizer 無法正常工作，並且命令 ChooseNameToMentionCommand 未觸發

有很多“類似”的問題，其中 TapGestureRecognizer 不能像這樣在 ListView 上工作：

TapGestureRecognizer 在 ListView 中不起作用

命令不起作用的答案（以及任何其他相關主題）是：

  • 在您的命令綁定上使用Source={x:Reference MessagesListView}

但是當我使用這個建議時，我的結尾是：

Xamarin.Forms.Xaml.XamlParseException: 'Position 30:21. Can not find the object referenced by MessagesListView'

我如何在我的情況下使用它（在單獨的文件中定義 ViewCell） 重要說明 - 我正在使用 MVVM 方法並且不想在 ViewCell 的代碼隱藏中做任何事情（然后我什至可以使用 Tapped 事件。我已經測試過了. 當然這種方法有效:) )

這是我的代碼：

MainViewModel.cs

public class MainViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public ObservableRangeCollection<MessageModel> Messages { get; set; }

    public Command ChooseNameToMentionCommand { get; set; }
    
    public string NewMessage {get; set;}

    public MainViewModel()
    {
        Messages = new ObservableRangeCollection<MessageModel>();
        ChooseNameToMentionCommand = new Command<string>(async (t) => await ChooseNameToMention(t));
    }

    private Task ChooseNameToMention(string name)
    {
        this.NewMessage += $"@{name}";
    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage
    x:Class="Chat.ClientLibrary.MainPage"
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:converters="clr-namespace:Chat.ClientLibrary.Converters"
    xmlns:local="clr-namespace:Chat.ClientLibrary.CustomCells"
    xmlns:partials="clr-namespace:Chat.ClientLibrary.Views.Partials"
    BackgroundColor="White"
    x:Name="MainChatPage">
    
<ContentPage.Resources>
    <ResourceDictionary>            
        <local:MyDataTemplateSelector x:Key="MessageTemplateSelector"/>
    </ResourceDictionary>
</ContentPage.Resources>

    /* REMOVED UNNECESSARY code */
    <Grid RowSpacing="0" ColumnSpacing="0">
    <Grid.RowDefinitions>
        <RowDefinition Height="50" />
        <RowDefinition Height="*" />
        <RowDefinition Height="1" />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
        <ListView   
            Grid.Row="1"
            FlowDirection="RightToLeft"
            Rotation="180"    
            x:Name="MessagesListView" 
            ItemTemplate="{StaticResource MessageTemplateSelector}" 
            ItemsSource="{Binding Messages}" 
            HasUnevenRows="True" 
            ItemSelected="MyListView_OnItemSelected" 
            ItemTapped="MyListView_OnItemTapped" 
            SeparatorVisibility="None" />
        /* REMOVED UNNECESSARY code */
    </Grid>
</ContentPage>

MyDataTemplateSelector.cs

class MyDataTemplateSelector : DataTemplateSelector
{
    public MyDataTemplateSelector()
    {
        // Retain instances!
        this.incomingDataTemplate = new DataTemplate(typeof(IncomingViewCell));
        this.outgoingDataTemplate = new DataTemplate(typeof(OutgoingViewCell));
    }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        var messageVm = item as MessageModel;
        if (messageVm == null)
            return null;
        return messageVm.IsOwnMessage ? this.incomingDataTemplate : this.outgoingDataTemplate;
    }

    private readonly DataTemplate incomingDataTemplate;
    private readonly DataTemplate outgoingDataTemplate;
}

IncomingViewCell.xaml （我不會發布 OutgoingViewCell - 它幾乎相同；）不同的顏色）

<?xml version="1.0" encoding="utf-8" ?>
<ViewCell xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="Chat.ClientLibrary.Views.CustomCells.IncomingViewCell"
             xmlns:forms9patch="clr-namespace:Forms9Patch;assembly=Forms9Patch">
    <Grid ColumnSpacing="2"
          Padding="5"
          FlowDirection="LeftToRight"
          Rotation="180"
          >
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="40"></ColumnDefinition>
            <ColumnDefinition Width="*"></ColumnDefinition>
            <ColumnDefinition Width="40"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"></RowDefinition>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="Auto"></RowDefinition>
        </Grid.RowDefinitions>
        <Label Grid.Row="0"  Grid.Column="1" HorizontalTextAlignment="Start"  Text="{Binding UserName}" TextColor="Blue">
            <Label.GestureRecognizers>
                <TapGestureRecognizer 
                    Command="{Binding Path= BindingContext.ChooseNameToMentionCommand, Source={x:Reference MessagesListView}}" CommandParameter="{Binding UserName}" />
            </Label.GestureRecognizers>
        </Label>
        /* REMOVED UNNECESSARY code */
    </Grid>
</ViewCell>

[編輯 12:12 01.10.2020] 我忘了把 MainPage.cs 放在這里，所以這里是：

MainPage.cs

public partial class MainPage : ContentPage
{
    MainViewModel vm;

    public MainPage()
    {
        this.BindingContext = vm = new MainViewModel();

        InitializeComponent();
    }

    void MyListView_OnItemSelected(object sender, SelectedItemChangedEventArgs e)
    {
        MessagesListView.SelectedItem = null;
    }

    void MyListView_OnItemTapped(object sender, ItemTappedEventArgs e)
    {
        MessagesListView.SelectedItem = null;

    }
}

添加了 ListOnItemTapped 事件（我忘了它 - 因為它是從一些聊天示例中獲取的。但我不認為它會破壞任何東西。當我直接為 Label 添加 OnTapped 時 - 它確實有效。

2 個解決方案

解決方案1
 1 已采納  2020-10-02 07:47:14

由於缺乏代碼的，我讓你參考使用類似的樣品TapGestureRecognizer ListView中ViewCell。

Xml：

<ContentPage
x:Class="Selector.HomePage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Selector"
x:Name="MainPage">
<ContentPage.Resources>
    <ResourceDictionary>
        <DataTemplate x:Key="validPersonTemplate">
            <ViewCell>
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="0.4*" />
                        <ColumnDefinition Width="0.3*" />
                        <ColumnDefinition Width="0.3*" />
                    </Grid.ColumnDefinitions>
                    <Label
                        FontAttributes="Bold"
                        Text="{Binding Name}"
                        TextColor="Green">
                        <Label.GestureRecognizers>
                            <TapGestureRecognizer Command="{Binding Path=BindingContext.TapCommand, Source={x:Reference MainPage}}" CommandParameter="false" />
                        </Label.GestureRecognizers>
                    </Label>
                    <Label
                        Grid.Column="1"
                        Text="{Binding DateOfBirth, StringFormat='{0:d}'}"
                        TextColor="Green" />
                    <Label
                        Grid.Column="2"
                        HorizontalTextAlignment="End"
                        Text="{Binding Location}"
                        TextColor="Green" />
                </Grid>
            </ViewCell>
        </DataTemplate>
        <DataTemplate x:Key="invalidPersonTemplate">
            <ViewCell>
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="0.4*" />
                        <ColumnDefinition Width="0.3*" />
                        <ColumnDefinition Width="0.3*" />
                    </Grid.ColumnDefinitions>
                    <Label
                        FontAttributes="Bold"
                        Text="{Binding Name}"
                        TextColor="Red">
                        <Label.GestureRecognizers>
                            <TapGestureRecognizer Command="{Binding Path=BindingContext.TapCommand, Source={x:Reference MainPage}}" CommandParameter="false" />
                        </Label.GestureRecognizers>
                    </Label>
                    <Label
                        Grid.Column="1"
                        Text="{Binding DateOfBirth, StringFormat='{0:d}'}"
                        TextColor="Red" />
                    <Label
                        Grid.Column="2"
                        HorizontalTextAlignment="End"
                        Text="{Binding Location}"
                        TextColor="Red" />
                </Grid>
            </ViewCell>
        </DataTemplate>
        <local:PersonDataTemplateSelector
            x:Key="personDataTemplateSelector"
            InvalidTemplate="{StaticResource invalidPersonTemplate}"
            ValidTemplate="{StaticResource validPersonTemplate}" />
    </ResourceDictionary>
</ContentPage.Resources>
<StackLayout Margin="20">
    <Label
        FontAttributes="Bold"
        HorizontalOptions="Center"
        Text="ListView with a DataTemplateSelector" />
    <ListView
        x:Name="listView"
        Margin="0,20,0,0"
        ItemTemplate="{StaticResource personDataTemplateSelector}" />
</StackLayout>
</ContentPage>

PersonDataTemplateSelector.cs：

public class PersonDataTemplateSelector : DataTemplateSelector
{
    public DataTemplate ValidTemplate { get; set; }

    public DataTemplate InvalidTemplate { get; set; }

    protected override DataTemplate OnSelectTemplate (object item, BindableObject container)
    {
        return ((Person)item).DateOfBirth.Year >= 1980 ? ValidTemplate : InvalidTemplate;
    }
}

人物.cs：

 public class Person
{
    public string Name { get; set; }
    public DateTime DateOfBirth { get; set; }
    public string Location { get; set; }
}

背后的代碼：

 public Command TapCommand
    {
        get
        {
            return new Command(val =>
            {
                DisplayAlert("Alert", val.ToString(), "OK");
            });
        }
        
    }
    public HomePage()
    {
        InitializeComponent();

        var people = new List<Person>
        {
            new Person { Name = "Kath", DateOfBirth = new DateTime(1985, 11, 20), Location = "France" },
            new Person { Name = "Steve", DateOfBirth = new DateTime(1975, 1, 15), Location = "USA" },
            new Person { Name = "Lucas", DateOfBirth = new DateTime(1988, 2, 5), Location = "Germany" },
            new Person { Name = "John", DateOfBirth = new DateTime(1976, 2, 20), Location = "USA" },
            new Person { Name = "Tariq", DateOfBirth = new DateTime(1987, 1, 10), Location = "UK" },
            new Person { Name = "Jane", DateOfBirth = new DateTime(1982, 8, 30), Location = "USA" },
            new Person { Name = "Tom", DateOfBirth = new DateTime(1977, 3, 10), Location = "UK" }
        };

        listView.ItemsSource = people;
        this.BindingContext = this;
    }

截屏：

在此處輸入圖片說明

更新：

使用資源字典創建單獨的文件。 我在這兩個文件中更改了命令的綁定路徑。

MyResourceDictionary.xaml：

<?xml version="1.0" encoding="UTF-8" ?>
<ResourceDictionary
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Selector">

<DataTemplate x:Key="validPersonTemplate">
    <ViewCell x:Name="MyCell">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="0.4*" />
                <ColumnDefinition Width="0.3*" />
                <ColumnDefinition Width="0.3*" />
            </Grid.ColumnDefinitions>
            <Label
                FontAttributes="Bold"
                Text="{Binding Name}"
                TextColor="Green">
                <Label.GestureRecognizers>
                    <TapGestureRecognizer Command="{Binding Path=Parent.BindingContext.TapCommand, Source={x:Reference MyCell}}" CommandParameter="false" />
                </Label.GestureRecognizers>
            </Label>
            <Label
                Grid.Column="1"
                Text="{Binding DateOfBirth, StringFormat='{0:d}'}"
                TextColor="Green" />
            <Label
                Grid.Column="2"
                HorizontalTextAlignment="End"
                Text="{Binding Location}"
                TextColor="Green" />
        </Grid>
    </ViewCell>
</DataTemplate>

</ResourceDictionary>

MyResourceDictionary2.xaml：

<?xml version="1.0" encoding="UTF-8" ?>
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">

<DataTemplate x:Key="invalidPersonTemplate">
    <ViewCell x:Name="MyCell2">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="0.4*" />
                <ColumnDefinition Width="0.3*" />
                <ColumnDefinition Width="0.3*" />
            </Grid.ColumnDefinitions>
            <Label
                FontAttributes="Bold"
                Text="{Binding Name}"
                TextColor="Red">
                <Label.GestureRecognizers>
                    <TapGestureRecognizer Command="{Binding Path=Parent.BindingContext.TapCommand, Source={x:Reference MyCell2}}" CommandParameter="false" />
                </Label.GestureRecognizers>
            </Label>
            <Label
                Grid.Column="1"
                Text="{Binding DateOfBirth, StringFormat='{0:d}'}"
                TextColor="Red" />
            <Label
                Grid.Column="2"
                HorizontalTextAlignment="End"
                Text="{Binding Location}"
                TextColor="Red" />
        </Grid>
    </ViewCell>
</DataTemplate>
</ResourceDictionary>

更改內容頁面：

<?xml version="1.0" encoding="UTF-8" ?>
<ContentPage
x:Class="Selector.HomePage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Selector">
<ContentPage.Resources>
    <ResourceDictionary> 
        <ResourceDictionary Source="MyResourceDictionary.xaml" />
        <ResourceDictionary Source="MyResourceDictionary2.xaml" />

        <local:PersonDataTemplateSelector
            x:Key="personDataTemplateSelector"
            InvalidTemplate="{StaticResource invalidPersonTemplate}"
            ValidTemplate="{StaticResource validPersonTemplate}" />

    </ResourceDictionary>
</ContentPage.Resources>
<StackLayout Margin="20">
    <Label
        FontAttributes="Bold"
        HorizontalOptions="Center"
        Text="ListView with a DataTemplateSelector" />
    <ListView
        x:Name="listView"
        Margin="0,20,0,0"
        ItemTemplate="{StaticResource personDataTemplateSelector}" />
</StackLayout>
</ContentPage>

選擇器和視圖模型沒有變化，請檢查。 如果您對此問題仍有疑問，請隨時告訴我。

解決方案2
 0  2020-10-04 04:42:31

添加一個類ViewModelLocator，我用的是MVVM Light

public class ViewModelLocator
{
   public ViewModelLocator()
   {
      ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);

      SimpleIoc.Default.Register<MainViewModel>();
   }
   public MainViewModel MainVM
   {
      get { return ServiceLocator.Current.GetInstance<MainViewModel>(); }
   }
}

然后你可以使用 BindingContext 而不使用頁面的引用

BindingContext="{Binding Path=MainVM, Source={StaticResource VMLocator}}"

App.Xaml 代碼

xmlns:vm="clr-namespace:xxx.xx.ViewModels"

<Application.Resources>
<vm:ViewModelLocator x:Key="VMLocator" />
</Application.Resources>

用法

選項1：

您想在列表視圖中綁定標簽，但列表視圖的綁定上下文指向集合，請使用 this。

<Label.GestureRecognizers>
       <TapGestureRecognizer Command="{Binding YourVM.YourCommand,Source={StaticResource VMLocator}}" CommandParameter="{Binding UserName}" />
</Label.GestureRecognizers>

選項2：

您想將其綁定到當前頁面的 Viewmodel（帶有頁面的引用）

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="xxxx.xx.xx.App"
             x:Name="MyViewName">

<Label Text="My Text" IsVisible="{Binding Path=BindingContext.IsLoading,Source={x:Reference MyViewName},Mode=TwoWay}"/>

選項2：

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