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CHAPTER 2 FRAMEWORK DE SIGN
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<Serializable()> _ Public Class Customer Public Function Deserialize(ByVal state As Byte()) As Customer Using buffer As New MemoryStream(state) Dim formatter As BinaryFormatter = New BinaryFormatter() Return DirectCast(formatter.Deserialize(buffer), Customer) End Using End Function End Class Notice that this function returns a new customer object. It doesn t restore the existing object s state; it creates a new object. Somehow, you would have to tell any and all code that has a reference to the existing object to use this new object. In some cases, that might be easy to do, but it isn t always trivial. In complex applications, it s hard to guarantee that other code elsewhere in the application doesn t have a reference to the original object, and if you don t somehow get that code to update its reference to this new object, it will continue to use the old one. What s needed is some way to restore the object s state in place, so that all references to the current object remain valid but the object s state is restored. This is the purpose of the UndoableBase class.
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The BusinessBase class inherits from UndoableBase and thereby gains n-level undo capabilities. Because all business objects inherit from BusinessBase, they too gain n-level undo. Ultimately, the n-level undo capabilities are exposed to the business object and to UI developers via three methods: BeginEdit() tells the object to take a snapshot of its current state, in preparation for being edited. Each time BeginEdit() is called, a new snapshot is taken, allowing the state of the object to be trapped at various points during its life. The snapshot will be kept in memory so the data can be easily restored to the object if CancelEdit() is called. CancelEdit() tells the object to restore the object to the most recent snapshot. This effectively performs an undo operation, reversing one level of changes. If CancelEdit() is called the same number of times as BeginEdit(), the object will be restored to its original state. ApplyEdit() tells the object to discard the most recent snapshot, leaving the object s current state untouched. It accepts the most recent changes to the object. If ApplyEdit() is called the same number of times as BeginEdit(), all the snapshots will be discarded, essentially making any changes to the object s state permanent. Sequences of BeginEdit(), CancelEdit(), and ApplyEdit() calls can be combined to respond to the user s actions within a complex Windows Forms UI. Alternatively, you can totally ignore these methods, taking no snapshots of the object s state. In such a case, the object will incur no overhead from n-level undo, but it also won t have the ability to undo changes. This is common in web applications in which the user has no option to cancel changes. Instead, the user simply navigates away to perform some other action or view some other data. The Csla.Core.ISupportUndo interface exists to allow UI developers and framework authors to polymorphically invoke these three methods on any object that supports the concept. The BusinessBase and BusinessListBase classes already implement this interface.
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C HAPTE R 2 FRA MEWORK DES IGN
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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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