在WPF的MVVM设计中有仅有一个控件ComboBox，其IsEditable属性设置为true，该控件绑定后台ViewModel中的数据类型是public ObservableCollection<string> Source，Source的来源是sqlite数据库中的MSJ.db数据库的表Source的数据，该表只有一列数据，列名为Source，表里有两行数据分别为“上级配发”和“本级自购”，要求实现第一点：ComboBoxItem选择发生改变时，弹窗提示用户选择前后的数据；第二点：ComboBox的输入框在接受自定义输入后，判断用户输入不属于ComboBoxItem后，自动将其加入ComboBoxItem中，同时更新后台sqlite数据库的MSJ.db数据库的表Source的数据。请给出实现上述功能的完整实例代码

ViewModel代码如下：

public class MainViewModel : INotifyPropertyChanged
{
    private ObservableCollection<string> _source;
    private string _selectedSource;

    public ObservableCollection<string> Source
    {
        get { return _source; }
        set
        {
            if (_source != value)
            {
                _source = value;
                OnPropertyChanged("Source");
            }
        }
    }

    public string SelectedSource
    {
        get { return _selectedSource; }
        set
        {
            if (_selectedSource != value)
            {
                if (!string.IsNullOrEmpty(_selectedSource))
                {
                    MessageBox.Show($"您选择了{_selectedSource}，下一步选择{_selectedSource}");
                }
                _selectedSource = value;
                OnPropertyChanged("SelectedSource");
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

XAML代码如下：

<Window x:Class="ComboBoxDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <ComboBox x:Name="comboBox" IsEditable="True" Text="{Binding SelectedSource, Mode=TwoWay}">
            <ComboBox.ItemsSource>
                <Binding Path="Source"/>
            </ComboBox.ItemsSource>
            <ComboBox.Style>
                <Style TargetType="ComboBox">
                    <Setter Property="IsDropDownOpen" Value="True" />
                    <Style.Triggers>
                        <Trigger Property="Text" Value="{x:Null}">
                            <Setter Property="IsDropDownOpen" Value="False" />
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </ComboBox.Style>
            <ComboBox.InputBindings>
                <KeyBinding Key="Enter" Command="{Binding AddSourceCommand}"/>
            </ComboBox.InputBindings>
        </ComboBox>
    </Grid>
</Window>

MainWindow.xaml.cs代码如下：

public partial class MainWindow : Window
{
    private MainViewModel _viewModel;

    public MainWindow()
    {
        InitializeComponent();

        _viewModel = new MainViewModel();
        DataContext = _viewModel;

        LoadData();
    }

    private void LoadData()
    {
        string connectionString = "Data Source=MSJ.db;Version=3;";
        using (SQLiteConnection connection = new SQLiteConnection(connectionString))
        {
            connection.Open();
            string sql = "SELECT * FROM Source;";
            SQLiteCommand command = new SQLiteCommand(sql, connection);
            SQLiteDataReader reader = command.ExecuteReader();
            ObservableCollection<string> source = new ObservableCollection<string>();
            while (reader.Read())
            {
                source.Add(reader[0].ToString());
            }
            _viewModel.Source = source;
            reader.Close();
            connection.Close();
        }
    }
}

在MainWindow.cs中用SQLiteConnection从数据库中读取数据，并将数据绑定到ComboBox上，同时添加了一个按Enter键的InputBinding，用于触发添加自定义输入的Command。AddSourceCommand在ViewModel中实现，代码如下：

public class MainViewModel : INotifyPropertyChanged
{
    // ...
    public ICommand AddSourceCommand { get; set; }

    public MainViewModel()
    {
        AddSourceCommand = new RelayCommand(AddSource);
    }

    private void AddSource()
    {
        if (!string.IsNullOrEmpty(SelectedSource) && !Source.Contains(SelectedSource))
        {
            Source.Add(SelectedSource);
            SaveData();
        }
    }

    private void SaveData()
    {
        string connectionString = "Data Source=MSJ.db;Version=3;";
        using (SQLiteConnection connection = new SQLiteConnection(connectionString))
        {
            connection.Open();
            string sql = "DELETE FROM Source;";
            SQLiteCommand command = new SQLiteCommand(sql, connection);
            command.ExecuteNonQuery();
            foreach (string source in Source)
            {
                sql = $"INSERT INTO Source(Source) VALUES('{source}');";
                command = new SQLiteCommand(sql, connection);
                command.ExecuteNonQuery();
            }
            connection.Close();
        }
    }
}

AddSourceCommand的实现简单地判断用户输入是否为空，是否已经存在于ComboBox中，如果不是则添加到Source中，并保存到数据库中。SaveData方法用SQLiteCommand执行SQL语句，删除原有数据并插入新数据。

其中，RelayCommand是一个常用的ICommand实现，请自行搜索相关资料获取代码。