12.2.3.3 extension Points

As expansions are optional native the perspective of the Base use Case, over there exists additional notation that helps in identifying when and also where come insert the extension. Us can affix a constraint come the «extend» arrow, describe the condition that must be true for the extension to be applied. We use the typical constraint notation, a keep in mind symbol, through the label “condition.”

We can additionally place within the expanded Use case a list of locations, dubbed extension points. This are locations where the expansion can be inserted if the problem is true. UML doesn’t have actually a standard method of identifying these locations, though typically they are the surname of a state in a state machine for the Base use Case.

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Both the extension points and condition space optional. You deserve to use neither, either, or both. The condition can describe an extension point to suggest where the condition will be tested. In Fig. 12.19, the base Use situation is “Perform ATM Transaction.” This Use case has one extension suggest “selection.” The extension Use Case, “On-line Help” will certainly be placed into carry out ATM Transaction in ~ the “selection” expansion point, when the problem “customer selected HELP” is true (Fig. 12.17).



There is an different format because that the use Case and extension points that supplies a divide rectangle. This format, shown in Fig. 12.18, is sometimes more useful when there are many extension clues or inheritance is involved. The Use situation oval in the upper right gives a visual indication that the classifier to represent a usage Case. The compartment surname is “extension points.”



Sanford Friedenthal, ... Stack Steiner, in Practical overview to SysML, 2008

Extension

A use case can extend a base use instance using the extension relationship. The extending use case is a fragment of functionality that is no considered part of the typical base use case functionality. It often explains some exceptional habits in the interaction, such as error handling, in between subject and also actors the does not add directly come the score of the base usage case.

To assistance extensions, a (base) use case defines a set of extension points the represent locations where it have the right to be extended. One extension allude can be referenced as part of the use situation description. Because that example, if the use instance had a textual description of a succession of steps, the extension suggest could be used to suggest at which action in the sequence an expanding use situation would be valid. An expansion has to reference an extension suggest to show where in the basic use situation it can occur. The conditions under which an expansion is valid can be further explained by a constraint that is evaluated once the extension point is reached to recognize whether the expanding use case occurs on this occasion. The existence of one extension allude does not imply that there will certainly be an extension related come it, and the basic use instance is unaware that whether over there is an extension.

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Unlike an contained use case, the base use situation does not depend on an prolonging use case. However, an prolonging use situation may it is in dependent on what is happening in that base usage case; because that example, that is likely to assume the some superior circumstance in the basic use situation has arisen. Over there is no implication that an actor linked with the base use situation participates in the extending use case, and also the expanded use case in truth may have actually entirely different participants, as demonstrated by the use instance Handle Camera Fault in number 11.4.