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Collection objects must implement a GetEnumerator() method. This is required by the IEnumerable interface, which is the most basic interface for collection or list objects in the .NET Framework. In the case of SortedBindingList, both strongly typed and loosely typed GetEnumerator() methods must be implemented: public IEnumerator<T> GetEnumerator() { if (_sorted) return new SortedEnumerator(_list, _sortIndex, _sortOrder); else return _list.GetEnumerator(); } IEnumerator System.Collections.IEnumerable.GetEnumerator() { return GetEnumerator(); } These methods merely return an instance of an enumerator object for use by foreach statements that wish to iterate through the items in the collection. If the view is not currently sorted, then it can simply ask the original collection for its enumerator. The original collection s enumerator will already iterate through all the child objects in the collection in their original order: return _list.GetEnumerator(); On the other hand, if a sort has been applied, then an instance of the custom SortedEnumerator (implemented in the preceding code) is returned: return new SortedEnumerator(_list, _sortIndex, _sortOrder); Either way, the compiler generated code for the foreach statement has an enumerator object that iterates through the items in the collection.
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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The IBindingList interface allows for removal of the sort. The result should be that the items in the collection return to their original order. This is handled by an UndoSort() method: private void UndoSort() { _sortIndex.Clear(); _sortBy = null; _sortOrder = ListSortDirection.Ascending; _sorted = false; OnListChanged(new ListChangedEventArgs(ListChangedType.Reset, 0)); } Removing a sort is just a matter of setting _sorted to false and clearing the various sort-related fields. Most important is calling Clear() on _sortIndex, as that releases any possible object references to items in the original collection. Because removing the sort alters the order of items in the view, the ListChanged event is raised to tell the UI that it needs to refresh its display of the collection.
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Adding and Removing Items
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Now we get to the complex issues. Remember that SortedBindingList is an updatable view of the original collection. This means that when the user adds or removes an item from the original collection, that change is immediately reflected in the view; the view is even re-sorted, if appropriate. Conversely, if the user adds or removes an item from the view, that change is immediately reflected in the original collection. There s some work involved in keeping the view and collection in sync. Also remember that collections may raise ListChanged events as they are changed. Table 5-3 lists the add and remove operations and how they raise events. Table 5-3. Events Raised During Add and Remove Operations
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Operation
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AddNew() Insert() RemoveAt()
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Event Behavior
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Called by data binding to add an item to the end of the collection; an ItemAdded type ListChanged event is raised by the collection Inserts an item into the collection; an ItemAdded type ListChanged event is raised by the collection Removes an item from the collection; an ItemDeleted type ListChanged event is raised by the collection
A ListChanged event is raised when the user adds or removes an item from the original collection. This event must be handled and sometimes reraised by the view. This is illustrated in Figure 5-2. Figure 5-2 shows the simple case, in which both the original collection and the view are bound to separate controls on the UI, and an update to the original collection is made. However, when the user adds or removes an item through the view, the view raises a ListChanged event as well as updating the original collection. Of course, updating the original collection triggers its ListChanged event. If you re not careful, this could result in duplicate events being raised, as shown in Figure 5-3. In this case, the UI control bound to the sorted view gets the ListChanged event twice, which is wasteful. But when the change is applied to the original collection, its event could flow back to the view and then to the UI.
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