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工具：我正在使用 Xamarin 表单和 MVVM

我正在关注视频：https ://www.youtube.com/watch?v=7mpe_1okwxk&list=PLwOF5UVsZWUiHY1CkRVjYJ6dm0iCvAlfw&index=12

NuGet：我已经安装了 NuGet > xamarin.community.toolkit

错误：

System.Reflection.TargetInvocationException: '调用的目标已抛出异常。'

您的应用已进入中断状态，但没有代码可显示，因为所有线程都在执行外部代码（通常是系统或框架代码）。

调试：

我正在尝试将 Collection ItemSelect 事件转换为命令。我知道问题是代码尝试将事件转换为命令的定义但我不确定如何调试此错误。由于应用程序已进入中断状态，我无法简单地添加断点。我在本主题的底部复制/粘贴了 outfile

请注意，如果我删除将事件转换为命令的代码，它工作正常。所以这意味着问题在于：CollectionView.Behaviors或xct:ItemSelectedEventArgsConverter

查看页面

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="TestApp02_MVVM_Product.Views.ProductPage"
         xmlns:xct="http://xamarin.com/schemas/2020/toolkit"
         xmlns:android="clr-namespace:Xamarin.Forms.PlatformConfiguration.AndroidSpecific;assembly=Xamarin.Forms.Core">

<ContentPage.Resources>
    <ResourceDictionary>
        <xct:ItemSelectedEventArgsConverter x:Key="ItemSelectedEventArgsConverter" />
    </ResourceDictionary>    
</ContentPage.Resources>

<ContentPage.Content>
        <CollectionView ItemsSource="{Binding ProductsList}"
                            SelectedItem="{Binding ItemSelectedEvent, Mode=TwoWay}"
                            ...>
                <!-- Turn Event Into Command-->
                <CollectionView.Behaviors>
                    <xct:EventToCommandBehavior 
                        EventName="ItemSelected"
                        Command="{Binding ItemSelectedCommand}"
                        EventArgsConverter="{StaticResource ItemSelectedEventArgsConverter}"/>
                </CollectionView.Behaviors>
                ...
        </CollectionView>
...

视图模型

using System.ComponentModel;
using System.Runtime.CompilerServices;
using TestApp02_MVVM_Product.Models;
using Xamarin.CommunityToolkit.ObjectModel;
using Xamarin.Forms;
using System.Threading.Tasks;

namespace TestApp02_MVVM_Product.ViewModels
{
    class ProductViewModel : INotifyPropertyChanged
    {
        public ObservableRangeCollection<ProductModel> ProductsList { get; set; }
        public AsyncCommand<ProductModel> ItemSelectedCommand { get; }
        
         public ProductViewModel()
        {
            ProductsList = new ObservableRangeCollection<ProductModel>();
            ProductsList.Add(new ProductModel { ProductId = 1, ProductName = "Name 1", ProductDescription = "Description 1" });
            ProductsList.Add(new ProductModel { ProductId = 1, ProductName = "Name 1", ProductDescription = "Description 1" });
            ProductsList.Add(new ProductModel { ProductId = 1, ProductName = "Name 1", ProductDescription = "Description 1" });
            ProductsList.Add(new ProductModel { ProductId = 1, ProductName = "Name 1", ProductDescription = "Description 1" });

            ItemSelectedCommand = new AsyncCommand<ProductModel>(OnItemSelectedCommand);
        }//end of con

        public ProductModel previousItemSelected;
        public ProductModel currentItemSelected;
        public ProductModel ItemSelectedEvent
        {
            get => currentItemSelected;
            set
            {
                currentItemSelected = value;
                OnPropertyChanged();
            }
        }
        
        async Task OnItemSelectedCommand(ProductModel productModel)
        {
            if (productModel == null)
                return;

             await Application.Current.MainPage.DisplayAlert("Selected works", "yayyy", "Cancel");
        }
        ...


#region INotifyPropertyChanged
        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var changed = PropertyChanged;
            if (changed == null)
                return;

            changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion
    }
}

** 输出 **

[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_new_object_array'.
[Mono] Probing 'java_interop_jnienv_new_object_array'.
[Mono] Found as 'java_interop_jnienv_new_object_array'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_set_object_array_element'.
[Mono] Probing 'java_interop_jnienv_set_object_array_element'.
[Mono] Found as 'java_interop_jnienv_set_object_array_element'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_is_assignable_from'.
[Mono] Probing 'java_interop_jnienv_is_assignable_from'.
[Mono] Found as 'java_interop_jnienv_is_assignable_from'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_new_float_array'.
[Mono] Probing 'java_interop_jnienv_new_float_array'.
[Mono] Found as 'java_interop_jnienv_new_float_array'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_set_float_array_region'.
[Mono] Probing 'java_interop_jnienv_set_float_array_region'.
[Mono] Found as 'java_interop_jnienv_set_float_array_region'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_get_float_array_region'.
[Mono] Probing 'java_interop_jnienv_get_float_array_region'.
[Mono] Found as 'java_interop_jnienv_get_float_array_region'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_call_float_method_a'.
[Mono] Probing 'java_interop_jnienv_call_float_method_a'.
[Mono] Found as 'java_interop_jnienv_call_float_method_a'.
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_new_local_ref'.
[Mono] Probing 'java_interop_jnienv_new_local_ref'.
[Mono] Found as 'java_interop_jnienv_new_local_ref'.
[Mono] Requesting loading reference 11 (of 12) of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.AndroidX.Fragment.dll
[Mono] Loading reference 11 of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.AndroidX.Fragment.dll asmctx DEFAULT, looking for Xamarin.AndroidX.ViewPager, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.AndroidX.Fragment[0x793c9b6900] -> Xamarin.AndroidX.ViewPager[0x793c9f8300]: 3
[Mono] Requesting loading reference 12 (of 15) of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.Google.Android.Material.dll
[Mono] Loading reference 12 of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.Google.Android.Material.dll asmctx DEFAULT, looking for Xamarin.AndroidX.ViewPager, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref Xamarin.Google.Android.Material[0x793c9f8b00] -> Xamarin.AndroidX.ViewPager[0x793c9f8300]: 4
[TabLayout] MODE_SCROLLABLE + GRAVITY_FILL is not supported, GRAVITY_START will be used instead
[Mono] Requesting loading reference 4 (of 11) of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.CommunityToolkit.dll
[Mono] Loading reference 4 of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.CommunityToolkit.dll asmctx DEFAULT, looking for System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref Xamarin.CommunityToolkit[0x793c9f8500] -> System[0x794c9eb480]: 9
[Mono] Requesting loading reference 9 (of 11) of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.CommunityToolkit.dll
[Mono] Loading reference 9 of /data/data/com.companyname.testapp02_mvvm_product/files/.__override__/Xamarin.CommunityToolkit.dll asmctx DEFAULT, looking for System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref Xamarin.CommunityToolkit[0x793c9f8500] -> System.Core[0x793c887980]: 5
[Mono] DllImport searching in: '__Internal' ('(null)').
[Mono] Searching for 'java_interop_jnienv_throw'.
[Mono] Probing 'java_interop_jnienv_throw'.
[Mono] Found as 'java_interop_jnienv_throw'.
**System.Reflection.TargetInvocationException:** 'Exception has been thrown by the target of an invocation.'
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1 回答 1

0

我把 ListView 和 CollectionView 混在一起了。CollectionView 已经有内置的 SelectionChangedCommand 所以不需要 <CollectionView.Behaviors> 东西

以下代码对我有用

看法

<CollectionView ItemsSource="{Binding ProductsList}"
             SelectedItem="{Binding ItemSelectedEvent, Mode=TwoWay}"
             SelectionChangedCommand="{Binding SelectionChangedCommand}">

视图模型

 public AsyncCommand<ProductModel> SelectionChangedCommand { get; }
 ...
 SelectionChangedCommand = new AsyncCommand<ProductModel>(OnSelectionChangedCommand);

 async Task OnSelectionChangedCommand(ProductModel productModel)
 {
        //item is not selected so do nothing
        if (productModel == null)
            return;

        //item is selcted so do logic here
           await Application.Current.MainPage.DisplayAlert("Selected works", "yayyy", "Cancel");
}
于 2021-05-07T16:58:58.050 回答