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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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The idea that a collection can be a root object or a child object is particularly important. It s fairly obvious that a collection can be a child object an Invoice root object will have a LineItems collection that contains LineItem objects, so the LineItems collection is itself a child object. However, collection objects can also be root objects. An application may have a root object called Categories, which contains a list of Category objects. It s quite possible that there s no root object to act as a parent for Categories it may simply be an editable list of objects. To support this concept, BusinessListBase, like BusinessBase itself, must support these two modes of operation. In root mode, some operations are legal while others are not; in child mode, the reverse is true. As in BusinessBase, the collection object needs to know whether it s a root or a child object: [NotUndoable()] private bool _isChild = false; protected bool IsChild { get { return _isChild; } } protected void MarkAsChild() { _isChild = true; } This functionality is the same in BusinessBase, and it allows the business developer to mark the object as a child object when it s first created. The IsChild property will be used in the rest of BusinessListBase to adjust the behavior of the object (such as exercising control over deletion) accordingly.
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As with a regular business object, a collection needs to support n-level undo. The functionality in BusinessListBase must integrate with UndoableBase. This means that BusinessListBase must implement the Csla.Core.IEditableCollection interface, which inherits from Csla.Core.IUndoableObject. Implementing the interface requires that the class implement CopyState(), UndoChanges(), and AcceptChanges() methods that store and restore the collection s state as appropriate. Because a collection can also be a root object, it needs public methods named BeginEdit(), CancelEdit(), and ApplyEdit(), like BusinessBase. In either scenario, the process of taking a snapshot of the collection s state is really a matter of having all the child objects take a snapshot of their individual states. The undo operation for a collection is where things start to get more complicated. Undoing all the child objects isn t too hard, since the collection can cascade the request to each child object. At the collection level, however, an undo means restoring any objects that were deleted and removing any objects that were added, so the collection s list of objects ends up the same as it was in the first place. There s a fair amount of code in BusinessListBase just to deal with deletion of child objects in order to support n-level undo. As with the rest of the framework, if n-level undo isn t used, then no overhead is incurred by these features.
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The hardest part of implementing n-level undo functionality is that not only can child objects be added or deleted, but they can also be undeleted or unadded in the case of an undo operation.
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Csla.Core.BusinessBase and UndoableBase use the concept of an edit level. The edit level allows the object to keep track of how many BeginEdit() calls have been made to take a snapshot of its state without corresponding CancelEdit() or ApplyEdit() calls. More specifically, it tells the object how many states have been stacked up for undo operations. BusinessListBase needs the same edit level tracking as in BusinessBase. However, a collection won t actually stack its states. Rather, it cascades the call to each of its child objects so that they can stack their own states. Because of this, the edit level can be tracked using a simple numeric counter. It merely counts how many unpaired BeginEdit() calls have been made: // keep track of how many edit levels we have int _editLevel; The implementations of CopyState(), UndoChanges(), and AcceptChanges() will alter this value accordingly.
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