WPF MVVM填充另一个组合框的组合框OnPropertyChanged

我想在combobox1选择更改事件之后填充我的combobox2。

这是我的XAML的一部分：

<ComboBox Name="cmbWorkcode"
        ItemsSource="{Binding Workcodes}" 
        DisplayMemberPath="WorkcodeName" 
        SelectedValuePath="WorkcodeID" 
        SelectedValue="{Binding Path=WorkcodeId, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
<ComboBox Name="cmbProcess" 
        ItemsSource="{Binding Processes}"
        DisplayMemberPath="ProcessName" SelectedValuePath="ProcessId"
        SelectedValue="{Binding Path=ProcessId, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>

我的ViewModel的一部分：

class MainWindowViewModel : ObservableObject 
{
    private ObservableCollection<Workcode> _workcodes = new ObservableCollection<Workcode>();
    public ObservableCollection<Workcode> Workcodes
    {
        get { return _workcodes; }
        set
        {
            _workcodes = value;
            OnPropertyChanged("Workcodes");
        }
    }

    private int _workcodeId;
    public int WorkcodeId
    {
        get { return _workcodeId; }
        set
        {
            _workcodeId = value;
            OnPropertyChanged("WorkcodeId");
        }
    }

    private ObservableCollection<Process> _processes = new ObservableCollection<Process>();
    public ObservableCollection<Process> Processes
    {
        get { return _processes; }
        set
        {
            _processes = value;
            OnPropertyChanged("Processes");
        }
    }

    private int _processId;
    public int ProcessId
    {
        get { return _processId; }
        set
        {
            _processId = value;
            OnPropertyChanged("ProcessId");
        }
    }

    public MainWindowViewModel()
    {
        PopulateWorkcode();
    }

    private void PopulateWorkcode()
    {
        using (var db = new DBAccess())
        {
            db.ConnectionString = ConfigurationManager.ConnectionStrings["connString"].ConnectionString;
            db.Query = @"SELECT workcodeId, workcode FROM workcode";
            DataTable data = db.GetData();
            if (data != null)
            {
                foreach (DataRow row in data.Rows)
                {
                    int workcodeId = Convert.ToInt32(row["workcodeId"].ToString());
                    string workcodeName = row["workcode"].ToString();
                    _workcodes.Add(new Workcode(workcodeId, workcodeName));
                }
            }
        }    
    }

    private void PopulateProcess()
    {
        using (var db = new DBAccess())
        {
            db.ConnectionString = ConfigurationManager.ConnectionStrings["connString"].ConnectionString;
            db.Query = @"SELECT ProcessId, ProcessName FROM `process` WHERE WorkcodeId = @workcodeId";
            DataTable data = db.GetData(new[] {new MySqlParameter("@workcodeId", _workcodeId.ToString())});
            if (data != null)
            {
                foreach (DataRow row in data.Rows)
                {
                    int id = Convert.ToInt32(row["ProcessId"].ToString());
                    string name = row["ProcessName"].ToString();
                    _processes.Add(new Process(id, name));
                }
            }
        }
    }
}

我的问题是我不知道我在哪里触发我的PopulateProcess（）方法，以便根据combobox1的选择来填充我的combobox2。感谢您一直以来的帮助！:)

- 编辑 -

public class ObservableObject : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class Workcode
{
    public int WorkcodeId { get; set; }
    public string WorkcodeName { get; set; }

    public Workcode(int id, string name)
    {
        WorkcodeId = id;
        WorkcodeName = name;
    }
}