“TouchBehavior.CommandParameter”和“TouchBehavior.LongPressCommandParameter”始终作为“null”传递给“Command”和“LongPressCommand”

Question

TouchBehavior.CommandParameter

和

TouchBehavior.LongPressCommandParameter

始终作为

null

传递到

Command

和

LongPressCommand

。

在以下示例中，当点击

CollectionView

中的项目时，生成的警报将始终显示

Selected value:

，因为 CommandParameter 始终为

null

。例如，当用户点击

Selected vale: Item 1

时，它应该显示

Item 1

。

<VerticalStackLayout Padding="30,0"
                     Spacing="25">
    <Label Text="{Binding Title}"
           Style="{StaticResource Headline}"
           SemanticProperties.HeadingLevel="Level1" />

    <Label Text="Tapping or long pressing an item does not correctly pass that item as CommandParameter or LongTouchCommandParameter."
           Style="{StaticResource SubHeadline}"
           SemanticProperties.HeadingLevel="Level1"  />

    <CollectionView x:Name="collectionView"
                    ItemsSource="{Binding Items}">
        <CollectionView.ItemTemplate>
            <DataTemplate x:DataType="{x:Type x:String}">
                <Label Padding="10"
                       Text="{Binding .}"
                       x:Name="Label">
                    <Label.Behaviors>
                        <toolkit:TouchBehavior Command="{Binding Source={x:Reference collectionView}, Path=BindingContext.ShowItemCommand}"
                                               CommandParameter="{Binding .}"
                                               LongPressCommand="{Binding Source={x:Reference collectionView}, Path=BindingContext.ShowItemCommand}"
                                               LongPressCommandParameter="{Binding .}"
                        <!--
                        CommandParameter and LongPressCommandParameter are always passed as null in the CommunityToolkit.Maui TouchBehavior.
                        this was not the case with https://github.com/Axemasta/Maui.TouchEffect -->
                    </Label.Behaviors>
                </Label>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>

</VerticalStackLayout>

public partial class MainViewModel : ObservableObject
{
    public string Title => "Hello, Maui!";

    public ObservableCollection<string> Items { get; set; } = ["Item 1", "Item 2", "Item 3"];

    /// <summary>
    /// item is always null. This was not the case in https://github.com/Axemasta/Maui.TouchEffect
    /// </summary>
    /// <param name="item"></param>
    /// <returns></returns>
    [RelayCommand]
    private Task ShowItem(string item) => Application.Current.MainPage.DisplayAlert($"Selected value:", item, "OK");
}

样本复制

以下是此错误的重现示例的链接：https://github.com/hansmbakker/bugrepro-communitytoolkit-maui-touchbehavior-parameter

Answer 1

说明

在 .NET MAUI 中，您需要为每个

BindingContext

手动设置

Behavior

。换句话说，行为不会继承它们所附加的

BindingContext

的

VisualElement

。

来源： https://learn.microsoft.com/dotnet/maui/fundamentals/behaviors?view=net-maui-8.0#create-a-net-maui-behavior

更新代码

这里是更新的 XAML，可以让您的代码正常运行。它使用视图到视图绑定将

TouchBehavior.BindingContext

绑定到

Label.BindingContext

:

<VerticalStackLayout Padding="30,0"
                     Spacing="25">
    <Label Text="{Binding Title}"
           Style="{StaticResource Headline}"
           SemanticProperties.HeadingLevel="Level1" />

    <Label Text="Tapping or long pressing an item does not correctly pass that item as CommandParameter or LongTouchCommandParameter."
           Style="{StaticResource SubHeadline}"
           SemanticProperties.HeadingLevel="Level1"  />

    <CollectionView x:Name="collectionView"
                    ItemsSource="{Binding Items}">
        <CollectionView.ItemTemplate>
            <DataTemplate x:DataType="{x:Type x:String}">
                <Label Padding="10"
                       Text="{Binding .}"
                       x:Name="label">
                    <Label.Behaviors>
                        <toolkit:TouchBehavior Command="{Binding Source={x:Reference collectionView}, Path=BindingContext.ShowItemCommand}"
                                               CommandParameter="{Binding .}"
                                               LongPressCommand="{Binding Source={x:Reference collectionView}, Path=BindingContext.ShowItemCommand}"
                                               LongPressCommandParameter="{Binding .}"
                                               BindingContext="{Binding Source={x:Reference label}, Path=BindingContext}"/>
                    </Label.Behaviors>
                </Label>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>

</VerticalStackLayout>

与Behavior BindingContext问题有点无关，这是我对ViewModel进行小幅性能改进的建议（

Title

和

Items

都可以是只读属性）：

public partial class MainViewModel : ObservableObject
{
    public string Title { get; } = "Hello, Maui!";

    public ObservableCollection<string> Items { get; } = ["Item 1", "Item 2", "Item 3"];

    [RelayCommand]
    private Task ShowItem(string item) => Application.Current.MainPage.DisplayAlert($"Selected value:", item, "OK");
}

更新后的应用程序截图

这是更新代码后应用程序按预期工作的屏幕截图：

.NET MAUI 社区工具包单元测试

这是我为

TouchBehavior

中的

CommunityToolkit.Maui

编写的单元测试，我们验证

LongPressCommand

确实通过了

LongPressCommandParameter

；

https://github.com/CommunityToolkit/Maui/blob/51dc63a7642370849e413d87072b668fa022e303/src/CommunityToolkit.Maui.UnitTests/Behaviors/TouchBehaviorTests.cs#L612-L674

这是我为

TouchBehavior

中的

CommunityToolkit.Maui

编写的单元测试，我们验证

Command

确实通过了

CommandParameter

；

https://github.com/CommunityToolkit/Maui/blob/51dc63a7642370849e413d87072b668fa022e303/src/CommunityToolkit.Maui.UnitTests/Behaviors/TouchBehaviorTests.cs#L550-L610