.NET(C#): 為Collection加入成員添加刪除事件

通過繼承Collection<T>類型，並為其加入兩個事件ItemAdded和ItemRemoved，使用這兩個事件可以覺察到容器內項目的添加和刪除的操作。事件提供元素值和索引值的資料，使用起來如下：

static void Main(string[] args)

{

    var list = new MyCollection<string>();

    list.ItemAdded += list_ItemAdded;

    list.ItemRemoved += list_ItemRemoved;

 

    Console.WriteLine("list.Add(\"Mgen\");");

    list.Add("Mgen");

 

    Console.WriteLine("list.Add(\"Zhang\");");

    list.Add("Zhang");

 

    Console.WriteLine("list.Insert(0, \"Tony\");");

    list.Insert(0, "Tony");

 

    Console.WriteLine("list[2] = \"Chen\";");

    list[2] = "Chen";

 

    Console.WriteLine("list.Remove(\"Tony\");");

    list.Remove("Tony");

 

    Console.WriteLine("list.Clear();");

    list.Clear();

}

 

static void list_ItemRemoved(object sender, CollectionItemEventArgs<string> e)

{

    Console.WriteLine("移除 - 索引值 {0} 值 {1}", e.Index, e.Item);

}

 

static void list_ItemAdded(object sender, CollectionItemEventArgs<string> e)

{

    Console.WriteLine("添加 - 索引值 {0} 值 {1}", e.Index, e.Item);

}

 

輸出：

list.Add("Mgen");

添加 - 索引值 0 值 Mgen

list.Add("Zhang");

添加 - 索引值 1 值 Zhang

list.Insert(0, "Tony");

添加 - 索引值 0 值 Tony

list[2] = "Chen";

移除 - 索引值 2 值 Zhang

添加 - 索引值 2 值 Chen

list.Remove("Tony");

移除 - 索引值 0 值 Tony

list.Clear();

移除 - 索引值 1 值 Chen

移除 - 索引值 0 值 Mgen

 

實現起來很簡單，通過改寫Collection<T>類型的：SetItem，InsertItem，RemoveItem和ClearItems方法，然後把相應添加或刪除操作調用相應事件就可以了。

 

完整代碼：

class CollectionItemEventArgs<T> : EventArgs

{

    public int Index { get; private set; }

    public T Item { get; private set; }

 

    public CollectionItemEventArgs(int idx, T item)

    {

        Index = idx;

        Item = item;

    }

}

 

class MyCollection<T> : Collection<T>

{

    public event EventHandler<CollectionItemEventArgs<T>> ItemAdded;

 

    protected virtual void OnItemAdded(CollectionItemEventArgs<T> e)

    {

        var handler = this.ItemAdded;

        if (handler != null)

            handler(this, e);

    }

 

    public event EventHandler<CollectionItemEventArgs<T>> ItemRemoved;

 

    protected virtual void OnItemRemoved(CollectionItemEventArgs<T> e)

    {

        var handler = this.ItemRemoved;

        if (handler != null)

            handler(this, e);

    }

 

 

    protected override void ClearItems()

    {

        for (int i = Count - 1; i >= 0; i--)

        {

            OnItemRemoved(new CollectionItemEventArgs<T>(i, this[i]));

        }

        base.ClearItems();

    }

 

    protected override void RemoveItem(int index)

    {

        OnItemRemoved(new CollectionItemEventArgs<T>(index, this[index]));

        base.RemoveItem(index);

    }

 

    protected override void InsertItem(int index, T item)

    {

        OnItemAdded(new CollectionItemEventArgs<T>(index, item));

        base.InsertItem(index, item);

    }

 

    protected override void SetItem(int index, T item)

    {

        OnItemRemoved(new CollectionItemEventArgs<T>(index, this[index]));

        OnItemAdded(new CollectionItemEventArgs<T>(index, item));

        base.SetItem(index, item);

    }

}