Assigning Responsibilities by Larman

Transcription

1 Assigning Responsibilities by Larman Core design activity: The identification of objects and responsibilities and providing a solution in terms of an interaction diagram this is the creative part where the choices lead to good or bad designs cf. origins of O-O methodology; Smalltalk, Alan Kay

2 Assigning responsibility have an overview of responsibility for system operations (e.g. via contracts) need solution in terms of co-operating objects to achieve these design is identification of suitable objects assignment of responsibility

3 How to assign responsibility? Goal: to make a system that is easy to understand, maintain, reuse, extend exploration of design options/architectures CRC (Class-Responsibility-Collaborator) cards GRASP patterns...

4 CRC cards Due to Kent Beck, Ward Cunningham see original paper given as handout to encourage a different way of thinking about systems and design solutions identifying objects and assigning responsibility

5 Pattern name problem being solved solution Patterns codified solution principles, idioms no new ideas communication benefit: vocabulary; abstraction General info on software design patterns separately; here deal with Larman s take on design patterns

6 Patterns for responsibility by Larman General Responsibility Assignment Software Patterns (GRASP) (General Principles for Assigning Responsibility) GRASP 5 basic patterns Expert Creator High Cohesion Low Coupling Controler

7 Expert Most used pattern partial expert needs collaboration Benefits encapsulation; low coupling distributes behaviour across classes with required info `Do It Myself strategy --- Peter Coad

8 Part of Conceptual model date time Sale Sales LineItem quantity Contain s 1..* * Describedby Product Specification description price UPC Who should be responsible for knowing the grand total of the sale?

9 t := total() :Sale date time Sale New method total()

10 Collaboration Diagram to achieve this... t := total() :Sale 1*: [for each] sli := next() Sale 2: st := subtotal() date time total() sli:saleslineitem SalesLineItem :SalesLineItem SalesLineItem quantity New method subtotal()

11 t := total() :Sale 1*: [for each] sli := next() Sale 2: st := subtotal() date time total() 2.1: p := price() sli:saleslineitem SalesLineItem :SalesLineItem SalesLineItem quantity subtotal() :Product Specification Product Specification description price UPC New method price() Product Specification is the expert on price

12 Assigning responsibility by Expert Pattern Class Responsibility Sale SalesLineItem Knows sale total Knows line item subtotal ProductSpecification Knows product price

13 Creator Class B has responsibility to create instance of class A if B aggregates A objects B contains A objects B records instances of A objects B closely uses A objects B has the initialising data for A

14 Assigning Responsibility based on Creator Pattern makelineitem(quantity) :Sale Sale 1: create(quantity) New method :SalesLineItem date time makelineitem( total()

15 High Cohesion when the elements of a class (component) all work together to provide some wellbounded behaviour bad if a class is solely responsible for lots of different functions

16 Low Coupling Coupling A connected to B A has knowledge of B A relies on B Problems of high coupling changes ripple; difficult to understand alone, to reuse But excessive de-coupling: large, complex an evaluation pattern

17 Coupling typea has attribute or method that refers to an instance of typeb or typeb typea is a direct or indirect subclass of typeb typea is an interface; typeb implements the interface

18 Controller Assign responsibility for handling input system events to a class that represents the overall system represents the overall business or organisation (façade controller) represents an active entity in the real-world that does this (role controller) represents an artificial handler of all system events of a use case (use case controller)

19 Which class of object should be responsible for handling this system event message? It is a controller. enteritem(upc, quantity) :??? Controller Pattern choices: System --- POST Business or organisation --- Store Real-world controller --- Cashier Use Case handler --- BuyItemsHandler

20 enteritem(upc, quantity) :POST enteritem(upc, quantity) :Store enteritem(upc, quantity) :Cashier enteritem(upc, quantity) :BuyItemsHandler

21 Assigning System operations to one or more controller classes System endsale() enteritem() makepayment()... POST endsale() enteritem() makepayment() system operations discovered during system behavior analysis allocation of system operations during design using the Controller patte

22 Controller issues... UI objects and presentation layer should not have responsibility for fulfilling system events problem of bloated controllers single controller class handling all system events role-controllers (e.g. cashier object for handling makepayment) can be incohesive and avoid delegation

23 Object Store UPC Quantity Total Tendered Balance presses button Enter Item End Sale Make Payment Cashier onenteritem() Presentation Layer (Java applet) :POSTApplet system event message 1: enteritem(upc, qty) controller Domain Layer :POST 1.1: makelineitem(upc, qty) :Sale Desirable coupling.

24 Object Store UPC Quantity Total Tendered Balance presses button Enter Item End Sale Make Payment Cashier Presentation Layer (Java applet) :POSTApplet onenteritem() It is undesirable for a presentation layer objects such as a Java apple get involved in deciding how to ha domain processes. Business logic is embedded in the presentation layer, which is not us Domain Layer 1: makelineitem(upc, qty) :Sale POSTApplet should not send this message.