加载页面时打开 CollectionView 的 SelectionChanged 事件答案

【问题标题】：SelectionChanged event of the CollectionView opens when the page is loaded加载页面时打开 CollectionView 的 SelectionChanged 事件
【发布时间】：2021-01-24 09:48:45
【问题描述】：

我有一个带有 SelectionChanged 事件的 CollectionView，一旦选择或取消选择某些项目，就应该读取该事件。实际上，只要包含 CollectionView 的页面打开，应用程序就会进入该事件。如何解决？

xaml

<CollectionView               
            x:Name="CategoryView" 
            RelativeLayout.WidthConstraint="{ConstraintExpression
            Type=RelativeToParent,
            Property=Width,
            Factor=1}"
            RelativeLayout.YConstraint="{ConstraintExpression
            Type=Constant,
            Constant=60}"             
            Margin="10,0,10,0"
            HeightRequest="700"
            SelectionMode="Multiple"
            SelectionChanged="CategoryView_SelectionChanged">
            <CollectionView.Footer>
                <Button
                    HeightRequest="120"
                    BackgroundColor="Transparent"/>
            </CollectionView.Footer>
            <CollectionView.ItemsLayout>
                <GridItemsLayout Orientation="Vertical" Span="2" VerticalItemSpacing="1" HorizontalItemSpacing="4"/>
            </CollectionView.ItemsLayout>
            <CollectionView.ItemTemplate>
                <DataTemplate>
                    <Grid BackgroundColor="{Binding myBackGroundColor}">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="36"/>
                            <RowDefinition Height="33"/>
                        </Grid.RowDefinitions>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="30"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>

                        <Image Grid.Row="0" Grid.RowSpan="2" Grid.Column="0" Grid.ColumnSpan="2" Source="{Binding Image}"/>
                        <Label Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2"  Text="{Binding Titolo}" FontSize="19" FontAttributes="Bold" TextColor="White" Margin="13,0,0,0"/>
                        <Image Grid.Row="0" Grid.Column="0" Source="checked.png" IsVisible="{Binding Vis}" Margin="13,5,0,0"/>
                    </Grid>
                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>

 private async void CategoryView_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        var selected = e.CurrentSelection;         
        ClassCategory model = e.CurrentSelection.FirstOrDefault() as ClassCategory;

        WebClient clientw = new WebClient();
        clientw.Credentials = new NetworkCredential("account", "password");
        string Frasi1 = "ftp://epiz_27426656@ftpupload.net/htdocs/" + "Obiettivo" + ".json";
        string contents1 = await clientw.DownloadStringTaskAsync(Frasi1);
        ObservableCollection<FraseClass> FrasiJsonOnline1 = JsonConvert.DeserializeObject<ObservableCollection<FraseClass>>(contents1);
        Frasi.ItemsSource = FrasiJsonOnline1;
       
        ViewFrasi.IsVisible = true;
        
        model.myBackGroundColor = Color.Transparent;
        model.Vis = true;

        list.Clear();
        foreach (ClassCategory cat in selected)
        {
            list.Add(cat.Titolo);
        }
    }

【问题讨论】：

if (e.CurrentSelection == null) return;
我的应用程序的制作方式永远不会为空。我希望仅在选择其他项目而不是在页面启动时读取该事件
然后在页面加载后分配事件处理程序
默认情况下，CollectionView 选择被禁用。在您选择该项目之前，它不会触发 SelectionChanged 事件。我使用您提供的 xaml 进行测试。在我选择该项目之前，它不会触发该事件。你能提供更多关于这方面的信息吗？
我认为问题是因为我在进入该页面时对 CollectionView 的某些元素进行了预选，然后触发了 SelectionChanged。我该如何避免呢？

标签： c# xaml xamarin.forms collectionview

【解决方案1】：

我用零件制作一个代码示例供预选或参考。

Xaml：

  <ContentPage.Content>
    <CollectionView
        x:Name="CategoryView"
        Margin="10,0,10,0"
        HeightRequest="700"
        ItemsSource="{Binding classCategories}"
        SelectedItems="{Binding SelectedCategories}"
        SelectionChanged="CategoryView_SelectionChanged"
        SelectionMode="Multiple">
        <CollectionView.Footer>
            <Button BackgroundColor="Transparent" HeightRequest="120" />
        </CollectionView.Footer>
        <CollectionView.ItemsLayout>
            <GridItemsLayout
                HorizontalItemSpacing="4"
                Orientation="Vertical"
                Span="2"
                VerticalItemSpacing="1" />
        </CollectionView.ItemsLayout>
        <CollectionView.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <!--  BackgroundColor="{Binding myBackGroundColor}"  -->
                    <Grid.RowDefinitions>
                        <RowDefinition Height="36" />
                        <RowDefinition Height="33" />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="30" />
                        <ColumnDefinition Width="*" />
                    </Grid.ColumnDefinitions>

                    <Image
                        Grid.Row="0"
                        Grid.RowSpan="2"
                        Grid.Column="0"
                        Grid.ColumnSpan="2"
                        Source="{Binding Image}" />
                    <Label
                        Grid.Row="1"
                        Grid.Column="0"
                        Grid.ColumnSpan="2"
                        Margin="13,0,0,0"
                        FontAttributes="Bold"
                        FontSize="19"
                        Text="{Binding Titolo}"
                        TextColor="White" />
                    <Image
                        Grid.Row="0"
                        Grid.Column="0"
                        Margin="13,5,0,0"
                        IsVisible="{Binding Vis}"
                        Source="pink.jpg" />
                </Grid>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>

后面的代码：

 public partial class MainPage : ContentPage, INotifyPropertyChanged
{
    public ObservableCollection<ClassCategory> classCategories { get; set; }

    public ObservableCollection<object> SelectedCategories { get; set; }

  
    public MainPage()
    {
        InitializeComponent();
        classCategories = new ObservableCollection<ClassCategory>()
        {
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Red", Titolo="A", Vis=true},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Green", Titolo="B", Vis=false},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Red", Titolo="A", Vis=true},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Green", Titolo="B", Vis=false},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Red", Titolo="A", Vis=true},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Green", Titolo="B", Vis=false},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Red", Titolo="A", Vis=true},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Green", Titolo="B", Vis=false},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Red", Titolo="A", Vis=true},
            new ClassCategory{ Image="pink.jpg", myBackGroundColor="Green", Titolo="B", Vis=false},

        };

        SelectedCategories = new ObservableCollection<object>()
        {
            classCategories[0], classCategories[3]
        };

        this.BindingContext = this;
    }
    int i;
    private void CategoryView_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        i++;
        if (i <= 2)//this event would be trigglled twice before you load the page
        {
            return;
        }
        else
        {
            //    var selected = e.CurrentSelection;
            //    ClassCategory model = e.CurrentSelection.FirstOrDefault() as ClassCategory;

            //    WebClient clientw = new WebClient();
            //    clientw.Credentials = new NetworkCredential("account", "password");
            //    string Frasi1 = "ftp://epiz_27426656@ftpupload.net/htdocs/" + "Obiettivo" + ".json";
            //    string contents1 = await clientw.DownloadStringTaskAsync(Frasi1);
            //    ObservableCollection<FraseClass> FrasiJsonOnline1 = JsonConvert.DeserializeObject<ObservableCollection<FraseClass>>(contents1);
            //    Frasi.ItemsSource = FrasiJsonOnline1;

            //    ViewFrasi.IsVisible = true;

            //    model.myBackGroundColor = Color.Transparent;
            //    model.Vis = true;

            //    list.Clear();
            //    foreach (ClassCategory cat in selected)
            //    {
            //        list.Add(cat.Titolo);
            //    }
        }


    }      

}
public class ClassCategory
{
    public string myBackGroundColor { get; set; }
    public string Image { get; set; }
    public string Titolo { get; set; }
    public bool Vis { get; set; }
}

【讨论】：