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It is important to realize that the n-level undo capability implemented in the framework is optional and is designed to incur no overhead if it is not used.
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Consider the case of an Invoice object that contains a collection of LineItem objects. The Invoice itself contains data that the user can edit plus data that s derived from the collection. The TotalAmount property of an Invoice, for instance, is calculated by summing up the individual Amount properties of its LineItem objects. Figure 2-2 illustrates this arrangement.
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Figure 2-2. Relationship between the Invoice, LineItems, and LineItem classes The UI may allow the user to edit the LineItem objects and then press Enter to accept the changes to the item or Esc to undo them. However, even if the user chooses to accept changes to some LineItem objects, she can still choose to cancel the changes on the Invoice itself. Of course, the only way to reset the Invoice object to its original state is to restore the states of the LineItem objects as well, including any changes to specific LineItem objects that might have been accepted earlier. As if this isn t enough, many applications have more complex hierarchies of objects and subobjects (which I ll call child objects). Perhaps the individual LineItem objects each has a collection of Component objects beneath it. Each Component object represents one of the components sold to the customer that makes up the specific line item, as shown in Figure 2-3.
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CHAPTER 2 FRAMEWORK DE SIGN
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Figure 2-3. Class diagram showing a more complex set of class relationships Now things get even more complicated. If the user edits a Component object, the changes ultimately impact the state of the Invoice object itself. Of course, changing a Component also changes the state of the LineItem object that owns the Component. The user might accept changes to a Component but cancel the changes to its parent LineItem object, thereby forcing an undo operation to reverse accepted changes to the Component. Or in an even more complex scenario, the user may accept the changes to a Component and its parent LineItem only to cancel the Invoice. This would force an undo operation that reverses all those changes to the child objects. Implementing an undo mechanism to support such n-level scenarios isn t trivial. The application must implement code to take a snapshot of the state of each object before it s edited so that changes can be reversed later on. The application might even need to take more than one snapshot of an object s state at different points in the editing process so that the object can revert to the appropriate point, based on when the user chooses to accept or cancel any edits.
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This multilevel undo capability flows from the user s expectations. Consider a typical word processor, where the user can undo multiple times to restore the content to ever earlier states.
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C HAPTE R 2 FRA MEWORK DES IGN
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And the collection objects are every bit as complex as the business objects themselves. The application must handle the simple case in which a user edits an existing LineItem, but it must also handle the case in which a user adds a new LineItem and then cancels changes to the parent or grandparent, resulting in the new LineItem being discarded. Equally, it must handle the case in which the user deletes a LineItem and then cancels changes to the parent or grandparent, thereby causing that deleted object to be restored to the collection as though nothing had ever happened. Things get even more complex if you consider that the framework keeps a list of broken validation rules for each object. If the user changes an object s data so that the object becomes invalid but then cancels the changes, the original state of the object must be restored. The reverse is true as well: an object may start out invalid (perhaps because a required field is blank), so the user must edit data until it becomes valid. If the user later cancels the object (or its parent, grandparent, etc.), the object must become invalid once again because it will be restored to its original invalid state. Fortunately, this is easily handled by treating the broken rules and validity of each object as part of that object s state. When an undo operation occurs, not only is the object s core state restored but so is the list of broken rules associated with that state. The object and its rules are restored together. N-level undo is a perfect example of complex code that shouldn t be written into every business object. Instead, this functionality should be written once, so that all business objects support the concept and behave the way we want them to. This functionality will be incorporated directly into the business object framework but at the same time, the framework must be sensitive to the different environments in which the objects will be used. Although n-level undo is of high importance when building sophisticated Windows user experiences, it s virtually useless in a typical web environment. In web-based applications, users typically don t have a Cancel button. They either accept the changes or navigate away to another task, allowing the application to simply discard the changed object. In this regard, the web environment is much simpler, so if n-level undo isn t useful to the web UI developer, it shouldn t incur any overhead if it isn t used. The framework design takes into account that some UI types will use the concept while others will simply ignore it.
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