列出<T>在更改时触发事件

Question

我创建了一个继承List的Class EventList,每次添加,插入或删除时都触发一个Event:

public class EventList<T> : List<T>
{
    public event ListChangedEventDelegate ListChanged;
    public delegate void ListChangedEventDelegate();

    public new void Add(T item)
    {
        base.Add(item);
        if (ListChanged != null
            && ListChanged.GetInvocationList().Any())
        {
            ListChanged();
        }
    }
    ...
}

在片刻我将它用作这样的属性:

public EventList List
{
    get { return m_List; }
    set
    {
        m_List.ListChanged -= List_ListChanged;

        m_List = value;

        m_List.ListChanged += List_ListChanged;
        List_ListChanged();
    }
}

现在我的问题是,我可以以某种方式处理一个新的对象是否被引用或阻止它,所以我不必在setter中做事件接线？

当然,我可以将属性更改为"私有集",但我希望能够将该类用作变量.

Answer 1

您很少在类中创建集合类的新实例.实例化一次并清除它而不是创建新列表.(并使用ObservableCollection,因为它已经继承了INotifyCollectionChanged接口)

private readonly ObservableCollection<T> list;
public ctor() {
    list = new ObservableCollection<T>();
    list.CollectionChanged += listChanged;
}

public ObservableCollection<T> List { get { return list; } }

public void Clear() { list.Clear(); }

private void listChanged(object sender, NotifyCollectionChangedEventArgs args) {
   // list changed
}

这样,您只需连接事件一次,并且可以通过调用clear方法"重置它",而不是检查null或与属性的set访问器中的前一个列表相等.

通过C#6中的更改,您可以在没有支持字段的情况下从构造函数分配get属性(支持字段是隐式的)

所以上面的代码可以简化为

public ctor() {
    List = new ObservableCollection<T>();
    List.CollectionChanged += OnListChanged;
}

public ObservableCollection<T> List { get; }

public void Clear()
{
    List.Clear();
}

private void OnListChanged(object sender, NotifyCollectionChangedEventArgs args)
{
   // react to list changed
}

Answer 2

ObservableCollection是一个带有CollectionChanged事件的List

ObservableCollection.CollectionChanged事件

有关如何连接事件处理程序,请参阅Patrick的回答.+1

不知道你在寻找什么,但是我将它用于一个集合,其中包含一个在添加,删除和更改时触发的事件.

public class ObservableCollection<T>: INotifyPropertyChanged
{
    private BindingList<T> ts = new BindingList<T>();

    public event PropertyChangedEventHandler PropertyChanged;

    // This method is called by the Set accessor of each property. 
    // The CallerMemberName attribute that is applied to the optional propertyName 
    // parameter causes the property name of the caller to be substituted as an argument. 
    private void NotifyPropertyChanged( String propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public BindingList<T> Ts
    {
        get { return ts; }
        set
        {
            if (value != ts)
            {
                Ts = value;
                if (Ts != null)
                {
                    ts.ListChanged += delegate(object sender, ListChangedEventArgs args)
                    {
                        OnListChanged(this);
                    };
                }
                NotifyPropertyChanged("Ts");
            }
        }
    }

    private static void OnListChanged(ObservableCollection<T> vm)
    {
        // this will fire on add, remove, and change
        // if want to prevent an insert this in not the right spot for that 
        // the OPs use of word prevent is not clear 
        // -1 don't be a hater
        vm.NotifyPropertyChanged("Ts");
    }

    public ObservableCollection()
    {
        ts.ListChanged += delegate(object sender, ListChangedEventArgs args)
        {
            OnListChanged(this);
        };
    }
}

Answer 3

如果您不想或不能转换为Observable Collection,请尝试以下方法:

public class EventList<T> : IList<T> /* NOTE: Changed your List<T> to IList<T> */
{
  private List<T> list; // initialize this in your constructor.
  public event ListChangedEventDelegate ListChanged;
  public delegate void ListChangedEventDelegate();

  private void notify()
  {
      if (ListChanged != null
          && ListChanged.GetInvocationList().Any())
      {
        ListChanged();
      }
  }

  public new void Add(T item)
  {
      list.Add(item);
      notify();
  }

  public List<T> Items {
    get { return list; } 
    set {
      list = value; 
      notify();
    }
  }
  ...
}

现在,对于您的属性,您应该能够将代码减少到:

public EventList List
{
  get { return m_List.Items; }
  set
  {
      //m_List.ListChanged -= List_ListChanged;

      m_List.Items = value;

      //m_List.ListChanged += List_ListChanged;
      //List_ListChanged();
  }
}

为什么？在EventList.Items中设置任何内容都将调用您的私有notify()例程.