问题
Assuming the you are implementing a user story that requires changes in all layers from UI (or service facade) to DB.
In what direction do you move?
- From UI to Business Layer to Repository to DB?
- From DB to Repository to Business Layer to UI?
- It depends. (On what ?)
回答1:
The best answer I've seen to this sort of question was supplied by the Atomic Object guys and their Presenter First pattern. Basically it is an implementation of the MVP pattern, in which (as the name suggests) you start working from the Presenter.
This provides you with a very light-weight object (since the presenter is basically there to marshal data from the Model to the View, and events from the View to the Model) that can directly model your set of user actions. When working on the Presenter, the View and Model are typically defined as interfaces, and mocked, so your initial focus is on defining how the user is interacting with your objects.
I generally like to work in this way, even if I'm not doing a strict MVP pattern. I find that focusing on user interaction helps me create business objects that are easier to interact with. We also use Fitnesse in house for integration testing, and I find that writing the fixtures for Fitnesse while building out my business objects helps keep things focused on the user's perspective of the story.
I have to say, though, that you end up with a pretty interesting TDD cycle when you start with a failing Fitnesse test, then create a failing Unit Test for that functionality, and work your way back up the stack. In some cases I'm also writing Database unit tests, so there is another layer of tests that get to be written, failed, and passed, before the Fitnesse tests pass.
回答2:
If change is likely, start in the front. You can get immediate feedback from shareholders. Who knows? Maybe they don't actually know what they want. Watch them use the interface (UI, service, or otherwise). Their actions might inspire you to view the problem in a new light. If you can catch changes before coding domain objects and database, you save a ton of time.
If requirements are rigid, it's not as important. Start in the layer that's likely to be the most difficult - address risk early. Ultimately, this is one of those "more an art than a science" issues. It's probably a delicate interplay between layer design that creates the best solution.
Cheers.
回答3:
I'd do it bottom up, since you'll have some working results fast (i. e. you can write unit tests without a user interface, but can't test the user interface until the model is done).
There are other opinions, though.
回答4:
I would start modeling the problem domain. Create relevant classes representing the entities of the system. Once I feel confident with that, I'd try to find a feasible mapping for persisting the entities to the database. If you put too much work into the UI before you have a model of the domain, there is a significant risk that you need to re-work the UI afterwards.
Thinking of it, you probably need to do some updates to all of the layers anyway... =)
来源：https://stackoverflow.com/questions/134094/developing-n-tier-app-in-what-direction