c# pdf417 open source s COMPLETING THE FRAMEWORK in Visual C#

Generate PDF417 in Visual C# s COMPLETING THE FRAMEWORK

CHAPTER 5 s COMPLETING THE FRAMEWORK
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Figure 5-2. Flow of events when the user changes the original collection
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Figure 5-3. Duplicate events raised when the user changes the view
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Figure 5-4 shows what should happen when the user changes the view. Making this happen means keeping track of whether the user added or removed an object directly in the original collection or through the view. The view needs to know whether the change was initiated locally, on the view, or not. This is tracked by the _initiatedLocally field, which is set to true before SortedBindingList performs any add or remove operations on the original collection, and is set to false when it is done.
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Figure 5-4. Flow of events with no duplicate events being raised
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Adding and removing items to the view is done through the AddNew(), Insert(), and RemoveAt() methods. AddNew() and RemoveAt() are handled in a similar manner: public object AddNew() { object result; if (_supportsBinding) { _initiatedLocally = true; result = _bindingList.AddNew(); _initiatedLocally = false; OnListChanged( new ListChangedEventArgs( ListChangedType.ItemAdded, _bindingList.Count - 1)); } else result = null; return result; } public void RemoveAt(int index) { if (_sorted) { _initiatedLocally = true; int baseIndex = OriginalIndex(index); // remove the item from the source list _list.RemoveAt(baseIndex);
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// delete the corresponding value in the sort index _sortIndex.RemoveAt(index); // now fix up all index pointers in the sort index foreach (ListItem item in _sortIndex) if (item.BaseIndex > baseIndex) item.BaseIndex -= 1; OnListChanged(new ListChangedEventArgs( ListChangedType.ItemDeleted, index)); _initiatedLocally = false; } else _list.RemoveAt(index); } Remember that _bindingList is a reference to the original collection object s implementation of the IBindingList interface. So this code merely sets _initiatedLocally to true and then delegates the AddNew() call to the original collection. Similarly, the RemoveAt() call is delegated to the original collection through its IList<T> interface. I ve also optimized the RemoveAt() implementation, so when an item is removed from the middle of the list, the entire sorted index isn t rebuilt. This offers substantial performance improvements when dealing with larger-sized lists.
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The important thing here is that SortedBindingList doesn t maintain a local copy of the collection s data. Instead, it delegates all calls directly to the original collection itself.
The original collection performs the requested operation and adds or removes the child object. Of course, that triggers a ListChanged event from the original collection. Recall that in the constructor of SortedBindingList, the original collection s ListChanged event was handled by the SourceChanged() method. I ll cover the SourceChanged() method in a moment, and you ll see how the ListChanged event is suppressed when the add or remove operation is initiated by the view itself. The Insert() method is simpler: public void Insert(int index, T item) { _list.Insert(index, item); } void System.Collections.IList.Insert(int index, object value) { Insert(index, (T)value); } When a new item is inserted into the view, it is really inserted into the original collection. This results in the original collection raising its ListChanged event, and in turn the view then raises its ListChanged event (in the SourceChanged() method in the following code). The end result is that the view raises the ListChanged event exactly once, which is the desired goal.
Responding to Changed Data
The source collection s ListChanged event is handed by the SourceChanged() method. This allows SortedBindingList to re-sort the data if it is changed in the original collection, thus keeping the view current. It also means that the event can be reraised by the view so any UI components bound to the sorted view are also aware that the underlying data has changed.
CHAPTER 5 s COMPLETING THE FRAMEWORK
If no sort has been applied, then the only thing the SourceChanged() method needs to do is reraise the event: private void SourceChanged( object sender, ListChangedEventArgs e) { if (_sorted) { switch (e.ListChangedType) { // update sorted view based on type of change } } else OnListChanged(e); } The OnListChanged() method raises the ListChanged event from the SortedBindingList object. Notice that the exact same event is raised, so in this case, the UI is blissfully unaware that SortedBindingList is a view over the original collection. However, if the view is sorted, then things are far more complex. In this case, the view must be updated appropriately based on the underlying change in the original collection. The ListChanged event can indicate different types of changes, each of which must be handled in a different manner. The code that goes in the preceding switch block takes care of these details. Let s go through each case of that block.
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