用例中的前置条件和后置条件

A preconditions section definesall the conditions that must be true (i.e., describes the state ofthe system) for the trigger to meaningfully cause the initiation ofthe use case

Post-conditions are thestates the system can be in after the use case ends，such as systemstate or persistent data that is left when the use case iscomplete

A post-condition for a use case should be trueregardless of which alternative flows were executed; it should notbe true only for the main flow. If something could fail, you wouldcover that in the post-condition by saying “The action iscompleted, or if something failed, the action is not performed,”rather than just “The action is completed.”

eg.

Post-condition for the Control Light Use Case

On leaving this use case, the system remembers the currentbrightness level for the selected On/Off/Dim button