如何将ListView按钮的绑定上下文设置为Xamarin窗体中父级的绑定上下文

Question

基本上，我有一个带有DataTemplate选择器的ListView，它基于ListView项使用特定的DataTemplate。

现在，在DataTemplate中，我有一个带有命令的按钮，应该绑定到父视图（或ListView）本身的ViewModel上。

注意，我只想绑定按钮的Command属性，因为文本和其他属性需要绑定到按钮的当前绑定上下文。

DataTemplate的BindingContext是ListView项BindingContext（本例中为消息模型），但我希望能够将数据模板中的一个特定按钮绑定到父ListView的viewmodel。

我该怎么做呢？

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage 
        xmlns="http://xamarin.com/schemas/2014/forms"
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
        x:Class="MobileMobile.Views.MobileMessageListView"
        Title="Message List"
        NavigationPage.BarBackgroundColor="#FF9100"
        NavigationPage.BarTextColor="White"
        xmlns:ViewSwitchers="clr-namespace:MobileMobile.ViewSwitchers;assembly=MobileMobile"
        xmlns:ViewCells="clr-namespace:MobileMobile.ViewCells;assembly=MobileMobile"
        xmlns:Extensions="clr-namespace:MobileMobile.Extensions;assembly=MobileMobile"
        >

    <ContentPage.Resources>
        <ResourceDictionary>
            <DataTemplate x:Key="botMessageDataTemplate">
                <ViewCell>
                    <Button Text="Hello!" Command="{Binding TestCommand, Source=???}" CommandParameter="Hello"/>
                </ViewCell>
            </DataTemplate>

            <ViewSwitchers:MobileMessageTemplateSwitcher x:Key="MobileMessageTemplateSwitcher" BotMessageDataTemplate="{StaticResource botMessageDataTemplate}" />
        </ResourceDictionary>
    </ContentPage.Resources>

    <ContentPage.Content>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>

            <StackLayout Grid.Row="0" Orientation="Vertical" x:Name="MainViewStack">
                <ListView
                        CachingStrategy="RecycleElement"
                        BackgroundColor="Transparent"
                        ItemTemplate="{StaticResource MobileMessageTemplateSwitcher}"
                        IsPullToRefreshEnabled="true"
                        RefreshCommand="{Binding RefreshCommand}"
                        ItemsSource="{Binding Messages}"
                        HasUnevenRows="true"
                        IsRefreshing="{Binding IsLoading, Mode=OneWay}"
                        SeparatorVisibility="None">
                    <ListView.Footer/>
                </ListView>
            </StackLayout>
            <StackLayout Grid.Row="1" Orientation="Horizontal" HeightRequest="50">
                <Entry Text="{Binding CurrentMessageText}" Placeholder="{Binding MessageTextPlaceHolder}" HorizontalOptions="FillAndExpand"/>
                <Button Text="SEND" Command="{Binding SendMessageCommand}"/>
            </StackLayout>
        </Grid>
    </ContentPage.Content>
</ContentPage>

Answer 1

不要在内容字典中将datatemplate声明为静态资源，而是将datatemplate放在列表本身的itemtemplate中。

我不是100%确定引擎盖下发生了什么，但当声明为静态资源时，datatemplate似乎与页面的绑定上下文没有关联。这意味着，当您离开并返回页面或更改listview的内容时，您可能会在呈现项目更新时遇到问题。

这将使您能够将按钮命令绑定到父级，但也可以在您离开并重新访问页面时停止任何问题

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage 
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    x:Class="MobileMobile.Views.MobileMessageListView"
    Title="Message List"
    NavigationPage.BarBackgroundColor="#FF9100"
    NavigationPage.BarTextColor="White"
    xmlns:ViewSwitchers="clr-namespace:MobileMobile.ViewSwitchers;assembly=MobileMobile"
    xmlns:ViewCells="clr-namespace:MobileMobile.ViewCells;assembly=MobileMobile"
    xmlns:Extensions="clr-namespace:MobileMobile.Extensions;assembly=MobileMobile"
    x:Name="DeclareYourCodeBehindHereOrDeclareAViewModel"
    >

<ContentPage.Resources>
    <ResourceDictionary>
        <ViewSwitchers:MobileMessageTemplateSwitcher x:Key="MobileMessageTemplateSwitcher" BotMessageDataTemplate="{StaticResource botMessageDataTemplate}" />
    </ResourceDictionary>
</ContentPage.Resources>

<ContentPage.Content>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <StackLayout Grid.Row="0" Orientation="Vertical" x:Name="MainViewStack">
            <ListView
                    CachingStrategy="RecycleElement"
                    BackgroundColor="Transparent"
                    IsPullToRefreshEnabled="true"
                    RefreshCommand="{Binding RefreshCommand}"
                    ItemsSource="{Binding Messages}"
                    HasUnevenRows="true"
                    IsRefreshing="{Binding IsLoading, Mode=OneWay}"
                    SeparatorVisibility="None">
                <ListView.ItemTemplate>
<DataTemplate x:Key="botMessageDataTemplate">
            <ViewCell>
                <Button Text="Hello!" Command="{Binding Source={x:Reference DeclareYourCodeBehindHereOrDeclareAViewModel}, Path=BindingContext.CommandName}" CommandParameter="Hello"/>
            </ViewCell>
        </DataTemplate>
                </ListView.ItemTemplate>
                <ListView.Footer/>
            </ListView>
        </StackLayout>
        <StackLayout Grid.Row="1" Orientation="Horizontal" HeightRequest="50">
            <Entry Text="{Binding CurrentMessageText}" Placeholder="{Binding MessageTextPlaceHolder}" HorizontalOptions="FillAndExpand"/>
            <Button Text="SEND" Command="{Binding SendMessageCommand}"/>
        </StackLayout>
    </Grid>
</ContentPage.Content>

Answer 2

试试这个：

<Button
    Text="Hello!"
    Command="{Binding Path=BindingContext. TestCommand, Source={x:Reference Name=MessageListPage}}"
    CommandParameter="Hello" />

您必须给页面一个x： Name属性，其值为MessageListPage，所以它会稍微扰乱您的纯MVVM，但是由于Xamarin XAML不支持相对绑定（据我所知...）这是要走的路。