Question

如何更好地编写此代码：

void CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
    if (sender is ObservableCollection<PromotionPurchaseAmount>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionPurchaseAmount item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionPurchaseAmount item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
    else if (sender is ObservableCollection<PromotionItemPricing>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionItemPricing item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionItemPricing item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
    else if (sender is ObservableCollection<PromotionItem>)
    {
        if (e.Action == NotifyCollectionChangedAction.Remove)
        {
            foreach (PromotionItem item in e.NewItems)
            {
                //Removed items
                item.PropertyChanged -= EntityViewModelPropertyChanged;
            }
        }
        else if (e.Action == NotifyCollectionChangedAction.Add)
        {
            foreach (PromotionItem item in e.NewItems)
            {
                //Added items
                item.PropertyChanged += EntityViewModelPropertyChanged;
            }
        }
    }
}

Answer 1

当e.Action == NotifyCollectionChangedAction.Remove需要迭代e.OldItems而不是e.NewItems。
当e.Action == NotifyCollectionChangedAction.Replace需要迭代e.OldItems以从旧项中删除事件处理程序时，您需要迭代e.NewItems以将事件处理程序添加到新项目中。< / LI>

重构代码如下：

if (sender is ObservableCollection<PromotionPurchaseAmount> || 
    sender is ObservableCollection<PromotionItemPricing> || 
    sender is ObservableCollection<PromotionItem>)
{
    if (e.Action == NotifyCollectionChangedAction.Remove ||
        e.Action == NotifyCollectionChangedAction.Replace)
    {

        foreach (INotifyPropertyChanged item in e.OldItems)
        {
            //Removed items
            item.PropertyChanged -= EntityViewModelPropertyChanged;
        }
    }
    if (e.Action == NotifyCollectionChangedAction.Add ||
        e.Action == NotifyCollectionChangedAction.Replace)
    {
        foreach (INotifyPropertyChanged item in e.NewItems)
        {
            //Added items
            item.PropertyChanged += EntityViewModelPropertyChanged;
        }
    }
}

这很有效，因为所有的Promotion类都实现了INotifyPropertyChanged。

c＃中的observablecollection的CollectionChanged事件

1 个答案: