c# - 如何仅从 WPF MVVM 中的调度程序线程以外的线程更新可观察集合中的属性？

Question

我有一个名为Employee 的类。我只需要从当前调度程序线程以外的线程更新属性 Status:

class Employee
{
    public string Name { get; set; }
    public string Status { get; set; }
}

Viewmodel(ViewModelBase 实现了INotifyPropertyChanged 接口(interface)):

    public abstract class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }


public class DisplayViewModel : ViewModelBase
{

    public static event EventHandler OnStatusChanged;

    ObservableCollection<Employee> _dailyEmployees;

    public ObservableCollection<Employee> DailyEmployees
    {
        get { return _dailyEmployees; }
        set
        {
            _dailyEmployees = value;
            OnPropertyChanged();
        }
    }

    public DisplayViewModel()
    {
        OnStatusChanged += DisplayViewModel_OnStatusChanged;
    }

    //invoked in other thread
    private void DisplayViewModel_OnStatusChanged(object sender, EventArgs e)
    {
        var d = sender as Employee;
        if (d == null)
            return;
        var em = DailyEmployees.FirstOrDefault(a => a.Name == d.Name);

        if(em == null)
        {
            DailyEmployees.Add(em);
        }
        else
        {
            em.Status = d.Status;
        }
    }
}

当我尝试向可观察集合添加值时，它抛出异常

“这种类型的 CollectionView 不支持从与 Dispatcher 线程不同的线程更改其 SourceCollection”

我该如何进行？谢谢。

Answer 1

要么使用调度器在 UI 线程上调用 Add 方法:

Application.Current.Dispatcher.BeginInvoke(() => DailyEmployees.Add(em)); 

或调用BindingOperations.EnableCollectionSynchronization UI 线程上的方法，使集合能够在多个线程上使用:

public class DisplayViewModel
{
    private readonly ObservableCollection<Employee> _dailyEmployees = new ObservableCollection<Employee>();
    private readonly object _lock = new object();

    public ObservableCollection<Employee> DailyEmployees
    {
        get { return _dailyEmployees; }
    }

    public DisplayViewModel()
    {
        System.Windows.Data.BindingOperations.EnableCollectionSynchronization(_dailyEmployees, _lock);
        OnStatusChanged += DisplayViewModel_OnStatusChanged;
    }

    //invoked in other thread
    private void DisplayViewModel_OnStatusChanged(object sender, EventArgs e)
    {
        var d = sender as Employee;
        if (d == null)
            return;
        var em = DailyEmployees.FirstOrDefault(a => a.Name == d.Name);

        if (em == null)
        {
            DailyEmployees.Add(em);
        }
        else
        {
            em.Status = d.Status;
        }
    }
}

另请注意，Employee 类应实现 INotifyPropertyChanged 接口(interface)并为 Status 引发 PropertyChanged 事件属性，如果你希望这个的值在你设置它时反射(reflect)在 View 中。