WPF Combobox的SelectedIndexChanged事件不触发（MVVM+Expression.Blend）

问题描述 #

刚接触WPF，已掌握MVVM与Expression.Blend，尝试通过InvokeCommandAction将ComboBox的SelectedIndexChanged事件绑定到ViewModel中的SetBuildingComponentConstruction命令，但命令代码始终未执行（myvalue值从未更新），求问题原因及解决办法。

相关代码 #

XAML代码 #

<UserControl x:Class="GDX.UI.Views.AnalyseView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:i1="http://schemas.microsoft.com/xaml/behaviors" 
             xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
             xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
             xmlns:local="clr-namespace:GDX.UI.Views"
             mc:Ignorable="d" 
             Height="720" Width="1210">
    <Control.Resources>
        <ResourceDictionary Source="pack://application:,,,/GDX.UI;component/ResourceDictionary/ResourceDictionary.xaml"/>
    </Control.Resources>
<materialDesign:Card Margin="15,15,15,15" Grid.Column ="0" Grid.Row="0" Grid.RowSpan="6" Grid.ColumnSpan="2">
            <ScrollViewer Grid.Row="0" Grid.RowSpan="5" Grid.Column="0" Grid.ColumnSpan="2" VerticalScrollBarVisibility="Hidden">
                <ItemsControl ItemsSource="{Binding buildingComponents}">
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Orientation="Vertical" Margin="10">
                                <TextBlock Margin="10" Text="{Binding Path=UIName}" FontWeight="Bold"/>
                                <ComboBox 
                                    ItemsSource="{Binding Path=ConstructionOptions}"
                                    SelectedItem="{Binding Path=Construction}"
                                    HorizontalAlignment="Center"
                                    Style="{StaticResource MaterialDesignOutlinedComboBox}"
                                    Width="256"
                                    Height="50"
                                    BorderBrush="Red"
                                    materialDesign:HintAssist.Hint="Baukonstruktion">
                                    <i:Interaction.Triggers>
                                        <i:EventTrigger EventName="SelectedIndexChanged">
                                            <i:InvokeCommandAction Command="{Binding SetBuildingComponentConstruction}"/>
                                        </i:EventTrigger>
                                    </i:Interaction.Triggers>
                                </ComboBox>
                            </StackPanel>
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                </ItemsControl>
            </ScrollViewer>
        </materialDesign:Card>
</UserControl>

ViewModel代码 #

using GDX.IO.Schemas;
using GDX.IO.Statics;
using GDX.MVVM;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;

namespace GDX.ViewModel.ViewModels
{
    public class AnalyseViewModel : BaseViewModel
    {
        public ObservableCollection<BuildingComponent> buildingComponents { get; set; }
        public double myvalue { get; set; }
        public ICommand SetBuildingComponentConstruction { get; set; }

        public AnalyseViewModel()
        {
            this.buildingComponents = new ObservableCollection<BuildingComponent>();
            if (Building.BuildingComponents.Count != 0) //if contains elements
            {
                foreach (BuildingComponent bcomp in Building.BuildingComponents)
                {
                    this.buildingComponents.Add(bcomp);
                }
            }
            

            this.SetBuildingComponentConstruction = new RelayCommand(this.setBuildingComponentConstruction);
            this.myvalue = 0;
        }

        public void setBuildingComponentConstruction()
        {
         myvalue += 1;
        }

    }
}

问题原因 #

1. 数据上下文不匹配：ComboBox位于ItemsControl的DataTemplate内部，此时它的DataContext是集合中的单个BuildingComponent对象，而SetBuildingComponentConstruction命令定义在AnalyseViewModel中，直接绑定会找不到命令，导致无法执行。
2. myvalue未实现属性通知：即使命令执行，myvalue属性没有实现INotifyPropertyChanged，UI也无法感知到值的变化。

解决办法 #

方法一：修正命令绑定的数据源 #

修改InvokeCommandAction的Command绑定，通过RelativeSource指定父级ItemsControl的DataContext（即AnalyseViewModel）：

<i:InvokeCommandAction Command="{Binding DataContext.SetBuildingComponentConstruction, RelativeSource={RelativeSource AncestorType=ItemsControl}}"/>

方法二：利用SelectedItem绑定（更符合MVVM规范） #

由于ComboBox已经绑定了SelectedItem="{Binding Construction}"，可以直接在BuildingComponent类的Construction属性的setter中处理逻辑，或者在ViewModel中监听属性变化：

方案1：修改BuildingComponent类（需继承INotifyPropertyChanged）

private Construction _construction;
public Construction Construction
{
    get => _construction;
    set
    {
        if (_construction != value)
        {
            _construction = value;
            OnPropertyChanged(nameof(Construction));
            // 在这里添加选中变化后的逻辑，或者通过事件通知ViewModel
        }
    }
}

方案2：在AnalyseViewModel中监听属性变化

public AnalyseViewModel()
{
    this.buildingComponents = new ObservableCollection<BuildingComponent>();
    if (Building.BuildingComponents.Count != 0)
    {
        foreach (BuildingComponent bcomp in Building.BuildingComponents)
        {
            this.buildingComponents.Add(bcomp);
            // 绑定PropertyChanged事件
            bcomp.PropertyChanged += BuildingComponent_PropertyChanged;
        }
    }

    this.SetBuildingComponentConstruction = new RelayCommand(this.setBuildingComponentConstruction);
    this.myvalue = 0;
}

private void BuildingComponent_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    if (e.PropertyName == nameof(BuildingComponent.Construction))
    {
        myvalue += 1;
        // 通知UI更新myvalue
        OnPropertyChanged(nameof(myvalue));
    }
}

补充：修复myvalue的属性通知 #

无论哪种方法，都需要给myvalue添加INotifyPropertyChanged实现，确保UI能更新：

private double _myvalue;
public double myvalue 
{ 
    get => _myvalue; 
    set 
    { 
        _myvalue = value; 
        OnPropertyChanged(nameof(myvalue)); 
    } 
}

内容的提问来源于stack exchange，提问作者aai