主页 / coding / 问题 / 79370513
Accepted
Matthew Pans
Asked: 2025-01-20 15:06:31 +0800 CST

collectionview 的 SelectionChangedCommand 中 selecteditem 的问题

  • 772

我已经为 collectionview 添加了命令SelectionChangedCommand。SelectedItem

<CollectionView
        Margin="0,10,0,0"
        BackgroundColor="Transparent"
        IsVisible="{Binding Visibility}"
        ItemsSource="{Binding ImageItems}"
        ItemsLayout="VerticalGrid, 2"
        SelectionMode="Single"
        SelectionChangedCommand="{Binding ImageItemTappedCommand}"
        SelectedItem="{Binding LastImageTappedItem, Mode=TwoWay}"
        HeightRequest="{Binding ImageHeight}">
    <CollectionView.ItemTemplate>
        <DataTemplate>
            <StackLayout BackgroundColor="{Binding ImageBGColor}">

ImageItemTapped命令：

public NameMatchViewModel() {
    ImageItemTappedCommand = new Command((obj) = >{
        try {
            //reset the bg color 
            foreach(var item in ImageItems) {
                item.ImageBGColor = Colors.White;
            }
            NameMatchList imageList = obj as NameMatchList;
            if (imageList != null) {
                Debug.WriteLine("**not null**");
            }
            else {
                Debug.WriteLine("**null**");
            }
            int index = ImageItems.IndexOf(imageList);
            imageList.ImageBGColor = Color.FromArgb("#0091da");
            //Storing name and imageurl to local db
            if (Utility.IsInternet()) {
                Preferences.Default.Set("NameMatchImageList_Image", imageList.imageUrl);
            }
            else {
                Preferences.Default.Set("NameMatchImageList_Image", imageList.FullImageUrl);
            }
            Preferences.Default.Set("NameMatchImageList_Name", imageList.name);
            Preferences.Default.Set("ImageItem", imageList);
            isImageSelected = true;
            if (isImageSelected && isNameSelected) {
                //If both image and name selected by player startes checking the matching
                StartNameMatchCheck(imageList);
            }
        }
        catch(Exception imagetapEx) {
            Debug.WriteLine("imagetapEx:>>" + imagetapEx);
        }
    });
}

但是当我尝试获取 selecteditem 值时，我得到的是 null。对于上述代码，我得到以下异常：

以下行除外：

imageList.ImageBGColor = Color.FromArgb("#0091da");

**null**
12:30:36:070    [0:] imagetapEx:>>System.NullReferenceException: Object reference not set to an instance of an object.
12:30:36:070       at MyProjectName.Model.NameMatchViewModel.<.ctor>b__98_0(Object obj) in E:\My Projects\MAUI\MyProjectName-app-maui\MyProjectName\Model\NameMatchViewModel.cs:line 390

我如何获取选定项目的值？

c#

1 个回答

  1. Best Answer

    的参数SelectionChangedCommand将始终为空，因为您尚未定义SelectionChangedCommandParameter

    您可以设置SelectionChangedCommandParameter为SelectedItem，或者，由于您已经绑定SelectedItem到，LastImageTappedItem您可以直接使用它来设置ImageBGColor所选项目的颜色属性

    选项 1：设置SelectionChangedCommandParameter

    <CollectionView x:Name="myCollectionView"
    Margin="0,10,0,0"
    BackgroundColor="Transparent"
    IsVisible="{Binding Visibility}"
    ItemsSource="{Binding ImageItems}"
    ItemsLayout="VerticalGrid, 2"
    SelectionMode="Single"
    SelectionChangedCommand="{Binding ImageItemTappedCommand}"
    SelectedItem="{Binding LastImageTappedItem, Mode=TwoWay}"
    SelectionChangedCommandParameter="{Binding Source={x:Reference myCollectionView},Path=SelectedItem}"
...

    选项 2：使用LastImageTappedItem

    LastImageTappedItem.ImageBGColor = Color.FromArgb("#0091da");
    • 2

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