VisualStateManager 不适用于 Xamarin 表单上 CollectionView 中的 SelectionChanged答案

【问题标题】：VisualStateManager not work with SelectionChanged in a CollectionView on Xamarin FormsVisualStateManager 不适用于 Xamarin 表单上 CollectionView 中的 SelectionChanged
【发布时间】：2021-01-15 12:00:36
【问题描述】：

当我在集合视图中添加SelectionChanged 函数时，VisualStateManager 不起作用。

<ContentPage.Resources>
    <Style TargetType="StackLayout">
        <Setter Property="VisualStateManager.VisualStateGroups">
            <VisualStateGroupList>
                <VisualStateGroup x:Name="CommonStates">
                    <VisualState x:Name="Normal" />
                    <VisualState x:Name="Selected">
                        <VisualState.Setters>
                            <Setter Property="BackgroundColor" Value="LightSkyBlue"/>
                        </VisualState.Setters>
                    </VisualState>
                </VisualStateGroup>
            </VisualStateGroupList>
        </Setter>
    </Style>
</ContentPage.Resources>

当我在CollectionView:SelectionChanged="OnCollectionViewSelectionChanged" 中添加此代码时，VisualStateManager 停止工作。有谁知道为什么？

【问题讨论】：

标签： xaml xamarin.forms collectionview visualstatemanager

【解决方案1】：

问题导致SelectionChanged 无法获取选定的元素类型，如StackLayout 或Label。你可以改用TapGestureRecognizer。

Xaml：

   <ContentPage.Resources>
    <Style TargetType="StackLayout">
        <Setter Property="VisualStateManager.VisualStateGroups">
            <VisualStateGroupList>
                <VisualStateGroup>
                    <VisualState x:Name="Selected">
                        <VisualState.Setters>
                            <Setter Property="BackgroundColor" Value="Accent" />
                        </VisualState.Setters>
                    </VisualState>
                    <VisualState x:Name="UnSelected">
                        <VisualState.Setters>
                            <Setter Property="BackgroundColor" Value="Blue" />
                        </VisualState.Setters>
                    </VisualState>
                </VisualStateGroup>
            </VisualStateGroupList>
        </Setter>
    </Style>
</ContentPage.Resources>
<ContentPage.Content>
    <StackLayout
        Padding="10"
        HorizontalOptions="FillAndExpand"
        VerticalOptions="FillAndExpand">
        <CollectionView
            x:Name="MenuCollection"
            ItemsSource="{Binding Infos}"
            SelectionChanged="MenuCollection_OnSelectionChanged"
            SelectionMode="Single"
            VerticalScrollBarVisibility="Always">

            <CollectionView.ItemsLayout>
                <GridItemsLayout
                    HorizontalItemSpacing="15"
                    Orientation="Vertical"
                    Span="2"
                    VerticalItemSpacing="15" />
            </CollectionView.ItemsLayout>

            <CollectionView.ItemTemplate>
                <DataTemplate>

                    <StackLayout BackgroundColor="Blue">
                        <Label Text="{Binding Title}" />
                        <StackLayout.GestureRecognizers>
                            <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped" />
                        </StackLayout.GestureRecognizers>
                    </StackLayout>

                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>
    </StackLayout>
</ContentPage.Content>

背后的代码：

 public partial class Page4 : ContentPage
{
    public ObservableCollection<Info> Infos { get; set; }
    public Page4()
    {
        InitializeComponent();
        Infos = new ObservableCollection<Info>
    {
        new Info(){ Title="A"},
        new Info(){ Title="B"},
        new Info(){ Title="B"},
        new Info(){ Title="C"},
        new Info(){ Title="D"}
    };

        this.BindingContext = this;
    }
    StackLayout lastElementSelected;
    private void MenuCollection_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
    {      

     

    }

    private void TapGestureRecognizer_Tapped(object sender, EventArgs e)
    {

        if (lastElementSelected != null)
            VisualStateManager.GoToState(lastElementSelected, "UnSelected");

        VisualStateManager.GoToState((StackLayout)sender, "Selected");

        lastElementSelected = (StackLayout)sender;
    }
}
public class Info
{
    public string Title { get; set; }
}

截图：

【讨论】：