E-Learning Material Web Application Design 2

Web Application Design Use cases Guidelines Exceptions Interaction Sequence diagrams Finding objects

What were use cases for? Illustrate how someone might use the app. High level functionality Dissociate implementation of app. from reason app. is required in first place Focus on what is truly important - meeting customer’s needs Customer can see functionality and agree to scope of application before work starts

Some simple guidelines Don’t model actor to actor communications – outside of scope Don’t show order of execution Use cases should be initiated by an actor (exceptions to follow!) Think of database as ‘underneath’ Enter info. In db using one use case Access info. From db using another use case Don’t draw arrows to show info. flow

So what are the exceptions? If we needed actors to initiate every use case, there would be a lot of duplication We can ‘break out’ common processes into a separate use case > relationship

> relationships checkout login Check account >

Another exception We might have a use case that normally works fine, but sometimes it needs to work differently The > relationship means that one use case always uses the functionality provided by another The > relationship provides optional functionality

> relationships loginregister > An actor initiates the login use case. A registered user has a password and succeeds. A new user doesn’t have a password and needs to register. The register use case > the functionality of the login use case only if a certain condition isn’t met.

A common example customerbrowser register Order product login >

But where’s the application? We have captured the user’s requirements We know what the application has to do We don’t have any code yet Where do we go now?

Interaction We need to examine the interactions that occur inside each of the use cases We can now look at process flow and order of precedence Two views of interaction: Sequence Collaboration

Sequence diagrams Shows step-by-step one of the flows through a use case Sequence diagrams ordered by time Represent ‘messages’ passed between ‘objects’

Example customer System The customer tells the system to checkout. The system looks at the contents of the shopping cart and produces an itemised list of all the items that are ready to be purchased. The customer confirms the order and tells the system to process the order. 1: Checkout 2: Examine cart 3: Display list 4: Confirm order objects messages time

Collaboration diagrams Show same information as sequence diagram Emphasis is on describing relationships (message passing) between objects Harder to read sequence of operations We shall stick with sequence diagrams

Remind me what objects are again? Something that has information and behaviour E.g. Bank Account Information: account number, account name, current balance Behaviour: balance increased when deposit made, balance reduced when cash withdrawn, charge made when overdrawn

More on objects The behaviours an object has are known as its operations Operations for the bank account are: Adjust balance for deposit Adjust balance for withdrawal Check if account overdrawn

How do we find the objects? Look for nouns in use cases Some will be actors, some objects, some attributes of objects Ask the question; Does it have any behaviour? If it has, it’s probably an object If not, it’s an attribute You shouldn’t need any help with the actors…

Recap We remove common use cases with the > relationship Optional functionality with > Sequence dia. explore each use case Messages passed over time from object to object Objects found lurking in use cases

Next time… Producing the final model