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Supporting Child Objects
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As it traces through a business object to take a snapshot of the object s state, UndoableBase may encounter child objects. For n-level undo to work for complex objects as well as simple objects, any snapshot of object state must extend down through all child objects as well as the parent object. I discussed this earlier with the InvoiceEdit and LineItem example. When BeginEdit() is called on an InvoiceEdit, it must also take snapshots of the states of all its LineItem objects because they re technically part of the state of the InvoiceEdit object itself. To do this while preserving encapsulation, each individual object takes a snapshot of its own state so that no object data is ever made available outside the object, thus preserving encapsulation for each object. However, to complicate matters, a BeginEdit() call on a parent object does not cascade to its child objects when BeginEdit() is called through the IEditableObject interface. This is because data binding, which uses IEditableObject, gets confused if that happens and the child objects will end up out of sync with the parent, resulting in very hard to debug issues with data bound interfaces. In that case, UndoableBase simply calls a method on the child object to cascade the BeginEdit(), CancelEdit(), or ApplyEdit() call to that object. It is then up to the individual child object to take a snapshot of its own data. In other words, each object is responsible for managing its own state, including taking a snapshot and potentially restoring itself to that snapshot later. UndoableBase implements Core.IUndoableObject, which simplifies the code in the class. This interface defines the methods required by UndoableBase during the undo process. A child object could also be a collection derived from BusinessListBase. Notice that BusinessListBase implements the Core.IEditableCollection interface, which inherits from the Core.IUndoableObject interface.
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The final concept to discuss regarding n-level undo is the idea that some data might not be subject to being in a snapshot. Taking a snapshot of an object s data takes time and consumes memory; there s no reason to take a snapshot if the object includes read-only values. Because the values can t be changed, there s no benefit in restoring them to the same value in the course of an undo operation.
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CHAPTER 2 FRAMEWORK DE SIGN
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To accommodate this scenario, the framework includes a custom attribute named NotUndoableAttribute, which you can apply to fields within your business classes, as follows: [NotUndoable] private string _readonlyData; The code in UndoableBase simply ignores any fields marked with this attribute as the snapshot is created or restored, so the field will always retain its value regardless of any calls to BeginEdit(), CancelEdit(), or ApplyEdit() on the object. You should be aware that the n-level undo implementation doesn t handle circular references, so if you have a field that references another object in a way that would cause a circular reference, you must mark the field as NotUndoable to break the circle.
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Data Binding Support
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As I discuss earlier in the chapter, the .NET data binding infrastructure directly supports the concept of data binding to objects and collections. However, an object can provide more complete behaviors by implementing a few interfaces in the framework base classes. Table 2-3 lists the interfaces and their purposes. Table 2-3. .NET Data Binding Interfaces
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Interface
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Purpose
Defines data binding behaviors for collections, including change notification, sorting, and filtering (implemented by BindingList<T>) Defines data binding behaviors for collections to allow data binding to cancel addition of a new child object (implemented by BindingList<T>) Indicates that a collection object will raise a ListChanged event to show that one of its child objects has raised a PropertyChanged event (implemented by BindingList<T>) Defines a CollectionChanged event to be raised by a list when the list or its items have changed (implemented by ObservableCollection<T>) Defines single-level undo behavior for a business object, allowing the object to behave properly with in-place editing in a DataGridView Defines an event allowing an object to notify data binding when a property has been changed Defines an event allowing an object to notify listeners when a property is about to be changed Defines properties used by the DataGridView and ErrorProvider controls to automatically show descriptions of broken validation rules within the object
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