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CHAPTER 9 PARE NT-C HILD RELA TION SHIPS
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The end result is that the child object is created or retrieved on demand, using lazy loading. At this point you should understand how an editable object can act as a parent or a child and how a parent object manages the references to its child objects. In the next section, I discuss how this works when the parent object is an editable collection.
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Parent Editable Collection
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Editable collections are created by inheriting from BusinessListBase, as discussed in s 4 and 5. By definition, a collection is a parent because it contains the items in the collection. To a large degree, the interactions between parent and child objects discussed already in this chapter also apply when the parent is a collection. For example, the BusinessListBase class implements the IParent interface. And BusinessListBase calls SetParent() as each child object is added to the collection or when the collection is deserialized. The same benefits and features I ve already discussed apply to collections as well as editable objects. However, editable collections provide some different behaviors as well, most notably around how items are deleted from the collection and how child events are cascaded up as ListChanged events from the collection (especially after the deserialization of the collection).
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Parent-Child Interaction
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The BusinessListBase class provides the same set of behaviors in Table 9-1, earlier in the chapter. The primary difference in the implementation is that a collection has no intrinsic state of its own. Instead, its state comes from its child objects. In other words, the collection s IsValid and IsDirty properties simply reflect the underlying state of the child objects. For example, a collection is valid only if all the child objects it contains are valid. The one big difference between an editable object and an editable collection is in terms of how ChildChanged events are handled.
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ListChanged Event
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Whereas an editable object handles any PropertyChanged or ListChanged events from its child objects and raises a ChildChanged event, an editable collection works a little differently. An editable collection can only contain editable child objects so needs to worry only about those objects raising PropertyChanged and ChildChanged events. Any time a child object raises PropertyChanged, the collection raises a ListChanged event. This is automatic behavior provided by the BindingList(Of T) class from the System.ComponentModel namespace. Any time a child object raises a ChildChanged event, the collection raises its own ChildChanged event, effectively cascading the ChildChanged event up to each parent until it is raised by the editable root object. Unfortunately, BindingList(Of T) doesn t automatically handle the case where the collection is serialized and deserialized, which happens when the object is cloned or transferred over the network in a client/server scenario. When a collection is deserialized, the ListChanged event is no longer automatically raised in response to a child object s PropertyChanged event. To overcome this issue, BusinessListBase includes code to hook the PropertyChanged events from its child objects on deserialization and to raise the ListChanged event just like its base class did before serialization. When the collection is deserialized, the formatter invokes OnDeserializedHandler(), which is implemented in ExtendedBindingList and includes code to hook the PropertyChanged events from all child objects in the list:
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CHAPTER 9 PARE NT-C HILD RELATION SHIPS
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For Each item As T In Me OnAddEventHooksInternal(item) Next The OnAddEventHooksInternal() method includes code to hook a number of child object events, most notably PropertyChanged: Dim c As INotifyPropertyChanged = TryCast(item, INotifyPropertyChanged) If c IsNot Nothing Then AddHandler c.PropertyChanged, AddressOf Child_PropertyChanged End If The OnChildChangedInternal() method in BusinessListBase handles each child PropertyChanged event and raises a corresponding ListChanged event: Protected Friend Overrides Sub OnChildChangedInternal(ByVal sender As Object, _ ByVal e As ChildChangedEventArgs) If RaiseListChangedEvents AndAlso e.PropertyChangedArgs IsNot Nothing Then DeferredLoadIndexIfNotLoaded() If _indexSet.HasIndexFor(e.PropertyChangedArgs.PropertyName) Then ReIndexItem(DirectCast(sender, C), e.PropertyChangedArgs.PropertyName) End If Dim index As Integer = IndexOf(DirectCast(sender, C)) If index >= 0 Then Dim descriptor = _ GetPropertyDescriptor(e.PropertyChangedArgs.PropertyName) If descriptor IsNot Nothing Then OnListChanged(New ListChangedEventArgs( _ ListChangedType.ItemChanged, _ index, _ descriptor)) Else OnListChanged(New ListChangedEventArgs( _ ListChangedType.ItemChanged, index)) End If Return End If End If End Sub There s code here for LINQ to CSLA as well, which I discuss in 14. This method is invoked by the ExtendedBindingList base class, which contains the code to hook and unhook child object events as necessary. I ve highlighted the code relevant to the event discussion. You might expect that raising a ListChanged event would be easy, but it turns out to be quite complex. The reason is that the ListChanged event needs to provide both the index of the changed item and a PropertyDescriptor object for the child property that is changed. Once the index value has been found, a GetPropertyDescriptor() method is called to find the PropertyDescriptor for the changed child property. Here s that method:
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