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Dim tmp As Object With lstObjects 'Transfer ownership to a local variable 'that will be released when this procedure 'goes out of scope VBoostAssign tmp, ItemData(Index) RemoveItem Index End With End Sub You now have a simple object-ownership mechanism that uses no storage object other than the ListBox You can use AssignAddRef in the lstObjects_Click event to get a strong reference to the underlying object Of course, the disadvantage if this method is that you have to write explicit clean-up code before the ListBox is cleared The ListBox is oblivious to the fact that it is now the proud owner of a set of objects you are responsible for clearing it explicitly on teardown
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Hierarchical Object Models
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Almost all object models are hierarchical, meaning that there is some kind of parent/child relationship among the different objects in the object model For example, a Word Document object has a Paragraphs property that returns a Paragraphs object Document is the parent of the Paragraphs collection object and the associated Paragraph objects, and, as expected, ParagraphsParent or Paragraphs(l)Parent both return the Parent document Similarly, a Paragraph object is the parent of the ParagraphFormat object, so Paragraph acts as both a parent and a child Hierarchical object models are highly cyclical Parent properties, whether they are called Parent, Container, Owner, or something similar, are almost always required for programming flexibility Hierarchical object models are a common design construct that you will use many times If you use them well, your components will have a professional programming model and feel When creating hierarchical object models in VB, you must first determine the parent/child relationships in the system If the parent object holds a strong reference to its children and the child object receives a weak reference to its parent object via a Friend function called by the parent, you have a solid model on
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which to build an object model You may think that the material shown so far is sufficient for you to create great hierarchical object models After all, you can support weak references to parent objects and rehydrate parent pointers when you need to call the parent or pass a parent reference to the outside world What more is there to do The remaining issues of using an object model fall under the category of creating robust objects In the context of object models, one of the requirements for robustness is that an object is fully functional even when it is the only external reference held against an object in the object model For example, this code should run correctly: Dim P As Parent Dim C As Child Set P = New Parent Set C = PChildren(1) Set P = Nothing Set P = CParent This code may seem diabolical when you view it sequentially But to be robust, objects must be prepared to handle such a situation Note that in the model shown above, Set P = Nothing releases the Parent object, which calls the SetParentPtr Friend function on the Child object with a 0 pointer Therefore, CParent returns Nothing Generally, Child objects are well-defined only in the context of their Parents, so this Child object is no longer fully functional An object in a less-thanfunctional state is said to be "zombied" its methods and properties do nothing or return errors because the context under which the object was created no longer exists A fully robust object should only "zombie" when absolutely necessary For example, it is valid for the Visual Basic IDE's extensibility model to be partially zombied during teardown (meaning not all methods work) because extensibility is simply a view of a set of objects, as opposed to an owner of a strong reference to those objects You need two versions of the child object to stop a Child object from reaching a zombied state The first version is internal This is the version of the object referenced by the Parent To ensure proper teardown, the internal version has a
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