我需要使用带有一些工作表的excel工作簿。要将可用的工作表加载到组合框中，我需要打开工作簿并遍历所有工作表，然后将它们添加到组合框中。这花费了大量时间，用户需要等待。如果用户不想更改预加载的属性，则此时间很短。因此，我只想在需要时填写列表项，但显示预选属性，该属性是在应用启动时从设置文件加载的。
那么有没有办法在下拉菜单中加载项目？或者更好的是，如何绑定下拉菜单中触发的命令来加载项目？
<ComboBox SelectedItem="{Binding Worksheet}" ItemsSource="{Binding WorkSheets}" x:Name="cbWorkSheet" Padding="4">
...哦，我知道如何将命令绑定到按钮，所以我需要组合框的特殊提示，而不是一般的命令：）
UPDATE我解决了触发“加载”的问题，并且列表已填充，但是我需要显示最后一个选定的值（在启动时作为绑定属性工作表加载）
<ComboBox
x:Name="cbWorkSheet"
SelectedItem="{Binding Worksheet}"
ItemsSource="{Binding WorkSheets}"
IsEditable="True"
Padding="4">
<i:Interaction.Triggers>
<i:EventTrigger EventName="DropDownOpened" SourceObject="{Binding ElementName=cbWorkSheet}">
<i:InvokeCommandAction Command="{Binding LoadBACnetWsCombobox}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</ComboBox>
有了该绑定，组合框保持为空，直到我将其下拉：（
原始答案：不正确，发布日期2020/06/02：
如果您需要在打开下拉菜单之前填充SelectedItem，则可能应该将EventTrigger的EventName从DropDownOpened更改为Loaded。
那样，在向用户显示组合框之前，将运行LoadBACnetWsComboBox，希望可以设置您的Worksheet。然后，组合框将显示给用户。
修改后的答案，发布日期2020/06/03：
根据您对我的原始答案的评论，我已经使用我的这份工作副本进行了修改。我选择删除触发器和命令操作（我没有Interaction命名空间），而是采用了ComboBox的IsDropDownOpen属性之外的方式使用MVVM模式。
MainWindow.xaml：
<Window x:Class="ComboBoxDropdownTest.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:ComboBoxDropdownTest"
mc:Ignorable="d"
Title="{Binding Title}" Height="450" Width="800">
<Window.Resources>
<DataTemplate DataType="{x:Type local:WorksheetViewModel}">
<Grid>
<TextBlock Text="{Binding Name,FallbackValue=FixYourBinding}" />
</Grid>
</DataTemplate>
</Window.Resources>
<Grid>
<ComboBox x:Name="cbWorkSheet"
SelectedItem="{Binding Worksheet, Mode=OneWay}"
ItemsSource="{Binding Worksheets}"
IsEditable="True"
Padding="4"
IsDropDownOpen="{Binding IsDropDownOpen}"
VerticalAlignment="Center"
Margin="5"
Width="400"
Text="{Binding SelectedAtStartup}"
>
<ComboBox.ItemContainerStyle>
<Style TargetType="ComboBoxItem">
<Setter Property="IsSelected" Value="{Binding Selected}"/>
</Style>
</ComboBox.ItemContainerStyle>
</ComboBox>
</Grid>
</Window>
MainWindow.xaml.cs：
using System.Windows;
namespace ComboBoxDropdownTest
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindowViewModel ViewModel
{
get { return this.DataContext as MainWindowViewModel; }
}
public MainWindow()
{
InitializeComponent();
var vm = new MainWindowViewModel();
this.DataContext = vm;
this.Loaded += OnLoaded;
}
private void OnLoaded(object sender, RoutedEventArgs e)
{
this.ViewModel.Initialize();
}
}
}
MainWindowViewModel.cs：
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Threading.Tasks;
namespace ComboBoxDropdownTest
{
public class MainWindowViewModel : ViewModelBase
{
private string selectedAtStartup;
private bool isDropDownOpen;
private ObservableCollection<WorksheetViewModel> worksheets;
private bool loadedOnce;
public MainWindowViewModel()
{
this.PropertyChanged += OnViewModelPropertyChanged;
}
public string Title { get => "This is a combobox dropdown test"; }
public string SelectedAtStartup { get => this.selectedAtStartup; set { this.selectedAtStartup = value; OnPropertyChanged(); } }
public bool IsDropDownOpen { get => this.isDropDownOpen; set { this.isDropDownOpen = value; OnPropertyChanged(); } }
public WorksheetViewModel Worksheet
{
get { return this.Worksheets.FirstOrDefault(o => o.Selected); }
}
public ObservableCollection<WorksheetViewModel> Worksheets
{
get
{
if (this.worksheets == null)
{
this.worksheets = new ObservableCollection<WorksheetViewModel>();
}
return this.worksheets;
}
}
public void Initialize()
{
// Simulates the loading of your selected property at startup
this.SelectedAtStartup = "Worksheet3";
}
private async Task LoadBACnetWsCombobox()
{
for (int i = 0; i < 10; i++)
{
var worksheet = new Worksheet() { Name = $"Worksheet{i}" };
this.Worksheets.Add(new WorksheetViewModel(worksheet));
}
// simulate a long 2-second delay load time
await Task.Delay(2000);
}
private async void OnViewModelPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == nameof(this.IsDropDownOpen) && this.IsDropDownOpen)
{
// Do your Command action here
if (!loadedOnce)
{
this.loadedOnce = true;
await this.LoadBACnetWsCombobox();
}
}
}
}
}
WorksheetViewModel.cs：
namespace ComboBoxDropdownTest
{
public class WorksheetViewModel : ViewModelBase<Worksheet>
{
public WorksheetViewModel(Worksheet model) : base(model)
{
}
public string Name { get => this.Model.Name; set { this.Model.Name = value; OnPropertyChanged(); } }
public override string ToString()
{
return this.Name;
}
}
}
ViewModelBase.cs：
using System.ComponentModel;
using System.Runtime.CompilerServices;
namespace ComboBoxDropdownTest
{
public class ViewModelBase : INotifyPropertyChanged
{
private bool selected;
public event PropertyChangedEventHandler PropertyChanged;
public bool Selected { get => this.selected; set { this.selected = value; OnPropertyChanged(); } }
public void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class ViewModelBase<T> : ViewModelBase
{
public T Model { get; private set; }
public ViewModelBase(T model) : base()
{
this.Model = model;
}
}
}
Worksheet.cs：
namespace ComboBoxDropdownTest
{
public class Worksheet
{
public string Name { get; set; }
}
}