Question

请注意，我正在尝试使用NotifyCollectionChangedAction.Add操作而不是.Reset。后者确实有效，但对于大型集合来说效率不高。

所以我将子类化为ObservableCollection：

public class SuspendableObservableCollection<T> : ObservableCollection<T>

出于某种原因，这段代码：

private List<T> _cachedItems;
...

    public void FlushCache() {
        if (_cachedItems.Count > 0) {

        foreach (var item in _cachedItems)
            Items.Add(item);

        OnCollectionChanged(new NotifyCollectionChangedEventArgs(
            NotifyCollectionChangedAction.Add, (IList<T>)_cachedItems));
        }
    }

投掷 集合添加事件是指不属于集合的项目

这似乎是BCL中的一个错误？

我可以在调用OnCollectionChanged之前逐步查看新项目是否添加到this.Items

WOW

刚刚发现了一个惊人的发现。这些方法都不适合我（flush，addrange），因为只有当这个集合绑定到我的Listview时才会触发错误！

TestObservableCollection<Trade> testCollection = new TestObservableCollection<Trade>();
List<Trade> testTrades = new List<Trade>();

for (int i = 0; i < 200000; i++) 
    testTrades.Add(t);

testCollection.AddRange(testTrades); // no problems here.. 
_trades.AddRange(testTrades); // this one is bound to ListView .. BOOOM!!!

总之，ObservableCollection支持添加增量列表，但ListView不支持。 Andyp想出了一个解决方法，使其适用于下面的CollectionView，但是因为.Refresh（）被调用，这与调用OnCollectionChanged（.Reset）没什么不同。

Answer 1

你可以为ObservableCollection实现AddRange（），如图所示here：

public class RangeObservableCollection<T> : ObservableCollection<T>
{
    private bool _SuppressNotification;

    public override event NotifyCollectionChangedEventHandler CollectionChanged;

    protected virtual void OnCollectionChangedMultiItem(
        NotifyCollectionChangedEventArgs e)
    {
        NotifyCollectionChangedEventHandler handlers = this.CollectionChanged;
        if (handlers != null)
        {
            foreach (NotifyCollectionChangedEventHandler handler in 
                handlers.GetInvocationList())
            {
                if (handler.Target is CollectionView)
                    ((CollectionView)handler.Target).Refresh();
                else
                    handler(this, e);
            }
        }
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (!_SuppressNotification)
        {
            base.OnCollectionChanged(e);
            if (CollectionChanged != null)
                CollectionChanged.Invoke(this, e);
        }
    }

    public void AddRange(IEnumerable<T> list)
    {
        if (list == null)
            throw new ArgumentNullException("list");

        _SuppressNotification = true;

        foreach (T item in list)
        {
            Add(item);
        }
        _SuppressNotification = false;

        OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, list));
    }
}

更新：绑定到ListBox后，我也看到了InvalidOperationException（您看到的消息相同）。根据这个article，因为CollectionView不支持范围操作。幸运的是，这篇文章也提供了一个解决方案（尽管感觉有点像“hack-ish”）。

更新2：添加了一个修复程序，在重写的OnCollectionChanged（）实现中引发了重写的CollectionChanged事件。

Answer 2

感谢AndyP的灵感。我的实现有一些问题，例如在测试中使用CollectionView而不是ICollectionView，以及在元素上手动调用“Reset”。从CollectionView继承的元素可能实际上以比调用“this.Reset（）”更多的方式处理这些args，所以最好仍然触发它们的处理程序，只需要它们需要的Action = Reset args而不是改进的事件args包括已更改的项目列表。以下是我的（非常相似）实现。

public class BaseObservableCollection<T> : ObservableCollection<T>
{
    //Flag used to prevent OnCollectionChanged from firing during a bulk operation like Add(IEnumerable<T>) and Clear()
    private bool _SuppressCollectionChanged = false;

    /// Overridden so that we may manually call registered handlers and differentiate between those that do and don't require Action.Reset args.
    public override event NotifyCollectionChangedEventHandler CollectionChanged;

    public BaseObservableCollection() : base(){}
    public BaseObservableCollection(IEnumerable<T> data) : base(data){}

    #region Event Handlers
    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if( !_SuppressCollectionChanged )
        {
            base.OnCollectionChanged(e);
            if( CollectionChanged != null )
                CollectionChanged.Invoke(this, e);
        }
    }

    //CollectionViews raise an error when they are passed a NotifyCollectionChangedEventArgs that indicates more than
    //one element has been added or removed. They prefer to receive a "Action=Reset" notification, but this is not suitable
    //for applications in code, so we actually check the type we're notifying on and pass a customized event args.
    protected virtual void OnCollectionChangedMultiItem(NotifyCollectionChangedEventArgs e)
    {
        NotifyCollectionChangedEventHandler handlers = this.CollectionChanged;
        if( handlers != null )
            foreach( NotifyCollectionChangedEventHandler handler in handlers.GetInvocationList() )
                handler(this, !(handler.Target is ICollectionView) ? e : new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
    }
    #endregion

    #region Extended Collection Methods
    protected override void ClearItems()
    {
        if( this.Count == 0 ) return;

        List<T> removed = new List<T>(this);
        _SuppressCollectionChanged = true;
        base.ClearItems();
        _SuppressCollectionChanged = false;
        OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, removed));
    }

    public void Add(IEnumerable<T> toAdd)
    {
        if( this == toAdd )
            throw new Exception("Invalid operation. This would result in iterating over a collection as it is being modified.");

        _SuppressCollectionChanged = true;
        foreach( T item in toAdd )
            Add(item);
        _SuppressCollectionChanged = false;
        OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, new List<T>(toAdd)));
    }

    public void Remove(IEnumerable<T> toRemove)
    {
        if( this == toRemove )
            throw new Exception("Invalid operation. This would result in iterating over a collection as it is being modified.");

        _SuppressCollectionChanged = true;
        foreach( T item in toRemove )
            Remove(item);
        _SuppressCollectionChanged = false;
        OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, new List<T>(toRemove)));
    }
    #endregion
}

Answer 3

经过多次迭代后，我们最终得到了ObservableRangeCollection和ReadOnlyObservableRangeCollection这个版本，该版本基于接受答案的代码，我们在过去6个月内无需修改：

public class ObservableRangeCollection<T> : ObservableCollection<T>
{
    private bool suppressNotification;

    public ObservableRangeCollection() { }

    public ObservableRangeCollection(IEnumerable<T> items)
        : base(items)
    {
    }

    public override event NotifyCollectionChangedEventHandler CollectionChanged;

    protected virtual void OnCollectionChangedMultiItem(
        NotifyCollectionChangedEventArgs e)
    {
        var handlers = CollectionChanged;
        if (handlers == null) return;

        foreach (NotifyCollectionChangedEventHandler handler in handlers.GetInvocationList())
        {
            if (handler.Target is ReadOnlyObservableCollection<T>
                && !(handler.Target is ReadOnlyObservableRangeCollection<T>))
            {
                throw new NotSupportedException(
                    "ObservableRangeCollection is wrapped in ReadOnlyObservableCollection which might be bound to ItemsControl " +
                    "which is internally using ListCollectionView which does not support range actions.\n" +
                    "Instead of ReadOnlyObservableCollection, use ReadOnlyObservableRangeCollection");
            }
            var collectionView = handler.Target as ICollectionView;
            if (collectionView != null)
            {
                collectionView.Refresh();
            }
            else
            {
                handler(this, e);
            }
        }
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (suppressNotification) return;

        base.OnCollectionChanged(e);
        if (CollectionChanged != null)
        {
            CollectionChanged.Invoke(this, e);
        }
    }

    public void AddRange(IEnumerable<T> items)
    {
        if (items == null) return;

        suppressNotification = true;

        var itemList = items.ToList();

        foreach (var item in itemList)
        {
            Add(item);
        }
        suppressNotification = false;

        if (itemList.Any())
        {
            OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, itemList));
        }
    }

    public void AddRange(params T[] items)
    {
        AddRange((IEnumerable<T>)items);
    }

    public void ReplaceWithRange(IEnumerable<T> items)
    {
        Items.Clear();
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
        AddRange(items);
    }

    public void RemoveRange(IEnumerable<T> items)
    {
        suppressNotification = true;

        var removableItems = items.Where(x => Items.Contains(x)).ToList();

        foreach (var item in removableItems)
        {
            Remove(item);
        }

        suppressNotification = false;

        if (removableItems.Any())
        {
            OnCollectionChangedMultiItem(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, removableItems));
        }
    }
}

public class ReadOnlyObservableRangeCollection<T> : ReadOnlyObservableCollection<T>
{
    public ReadOnlyObservableRangeCollection(ObservableCollection<T> list)
        : base(list)
    {            
    }

    protected override event NotifyCollectionChangedEventHandler CollectionChanged;

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        var handlers = CollectionChanged;
        if (handlers == null) return;

        foreach (NotifyCollectionChangedEventHandler handler in handlers.GetInvocationList())
        {
            var collectionView = handler.Target as ICollectionView;
            if (collectionView != null)
            {
                collectionView.Refresh();
            }
            else
            {
                handler(this, e);
            }
        }
    }
}

我们基本上用ObservableCollection替换了我们应用中ObservableRangeCollection的所有用法，它就像一个魅力。

Answer 4

我相信您需要将其投放到IList：

base.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, (IList)_cachedItems));

ObservableCollection：使用多个新项调用OnCollectionChanged

4 个答案: