英文:
Use-case diagram: When to decompose use-case and when not to?
问题
举个例子，假设我想为一个人工售货机绘制一个用例图，如下所示：
-
售货机有3个选项：
碳酸饮料、茶和纯净水。 -
用户只能选择上述选项中的一个进行订购：
- 如果用户选择
碳酸饮料，用户需要选择品牌：比如可口可乐或百事可乐。 - 如果用户选择
茶，则需要选择糖的数量。 - 如果用户选择
纯净水，则订购过程结束。
- 如果用户选择
-
在选择饮料后，用户需要选择饮料的数量（有点牵强）。然后机器会发送确认消息给用户进行确认。
我尝试了一下，但不太确定，不知道应该保留什么，添加什么或删除什么。。
以下是问题：
-
这里的两个步骤
选择饮料数量和确认应该作为独立的用例保留，还是应该与总的用例购买某种饮料合并？ -
如果至少要保留其中一个，是应该将它们绘制为
购买某种饮料的包含用例，还是应该作为独立的用例绘制？ -
我认为
购买碳酸饮料和购买茶这两个用例是购买某种饮料的具体化。对于购买碳酸饮料和购买茶，我应该保留包含用例选择品牌和选择糖的数量吗？还是也应该将它们删除？ -
更一般的问题是：在什么情况下，执行某个大用例的步骤应该作为一个用例保留，而在什么情况下不应该保留？ 这里有很多类似的问题，但答案并不一致。
英文:
For an example, say, I want to draw a use-case diagram for an artificial vending machine like so:
-
The vending machine have 3 options:
carbonated drinks,tea, andpure water. -
The user can choose only 1 of the above for an order:
- If the user chooses
carbonated drinks, the user needs to choose which brand: say Coke or Pepsi. - Else if the user chooses
tea, then he/she have to choose the amount of sugar. - Else if the user chooses
pure water, then that's it.
- If the user chooses
-
After choosing the drinks, the user then has to choose the number of drinks (a little bit contrived). The machine then sends out a confirmation message for the user to confirm.
I tried this but not really sure of it, don't know what to keep, what to add or what to remove..
The questions are:
-
Should the 2
Choose the number of drinksandConfirmhere be kept independently as use-cases, or should they be merge with the grand use-case:Buy a kind of drink? -
If at least 1 of them is to be kept, should I draw them as including use-case of
Buy a kind of drinkor should they be drawn as independent use-cases? -
The 3 use-cases
Buy carbonated,Buy tea,Buy pure waterI considered as the discretization ofBuy a kind of drink. ForBuy carbonatedandBuy tea, should I keep the including use-cases:Choose brandandChoose sugar amountor should I remove them also? -
More general question: When should a step of doing some big use-case should be kept as a use-case, and when shouldn't? There are a lot of questions on here that are the same, but the answers are non-consistent.
答案1
得分: 1
用例是关于“什么”而不是“如何”的。后者涉及到活动，并分为动作。用例只展示了系统在考虑中为其参与者带来的附加值。
所以唯一的附加值就是“销售饮料”，就是这样。
我建议阅读Bittner/Spence关于用例的内容。他们称之为“综合”，这是正确的词。不是分析，更不是分解。
奇怪的是，技术人员总是想要分解事物。嗯，我知道这很难学，因为我也曾为此苦恼过。这可能很复杂，因为它涉及到简单性。
英文:
Use cases are about the what and not about the how. The latter goes into Activity and is split to Actions. Use case only show the added value a system under consideration brings to its actors.
So the only added value is Sell drinkand that's it.
I recommend reading Bittner/Spence about use cases. They call it synthesis and that's the right word. Not analysis and even less decomposition.
Strange, that techies always want to decompose things. Well, I know it's hard to learn since I had been scratching my head too. It's probably so complicated because its about simplicity.
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