Observer Design Pattern in Detail: Software Architecture

1. COMP2110 Software Design in 2004lecture 14 Patterns(1) /Detailed Design • detailed design contrasted with high level design • introducing the Observer Design Pattern • information resources: • lecture 6 2004: How to design and Tetris example • Braude Software Design • chapters 4 & 5 – background • chapter 6: Design Patterns • chapter 9: Observer pattern • Gamma (book on reserve, chap 5 Behavioral Patterns on eReserve – see website Resources)

2. Requirements Analysis Design Framework Architecture Detailed Design Implementation Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. Process Phase for this module

3. Recap from lecture 6: Design (1) Module structure One way to describe the program's module structure is called a "module guide" • defines the name for each module and the design responsibility for a module by stating the design decisions/ areas of design responsibility that will be found within it(not the functional responsibility) • This is a decomposition of the solution • decompose the solution into modules,each module may consist of submodules • the document should reflect a tree structure,dividing the system into a small number of modulesand treating each module in the same wayuntil all modules are "quite small"

4. Module guide ctd. • we also call this the system architecture or high-level design • there are other ways to choose and describe the architecture

5. Design (1) recap Module structure/architecture Architecture: • The system is event driven in a computational loop that generates and drives a series of falling bricks in soft real time,using interrupt events to process user command keystrokes. The system includes a reusable generic playing field for games with coloured tiles. The system design requires a generic GUI library.

6. Module relationships The “uses” relationship... ...between modules ...between classes

7. Class relationships and interfaces

8. Class relationships – control sequence

9. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. Key Concept: Flexibility  Good design? We design flexibly, introducing parts, because change and reuse are likely.

10. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. Making a Method Re-usable • Specify the method completely • Preconditions etc (see Braude section 1.2.2) • Avoid unnecessary coupling with the enclosing class • Make static if this is feasible • Include parameterization • make the method functional • But limit the number of parameters • Make the names expressive • Understandability promotes re-usability • Explain the algorithm • Re-users need to know how the algorithm works

11. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. Making a Class Re-usable • Describe the class completely • Make the class name and functionality match a real world concept • Define a useful abstraction • attain broad applicability • Reduce dependencies on other classes • Elevate dependencies in the hierarchy alternatives

12. Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. Example: design of a Command Line Calculator-requirements CommandLineCalculator begins by asking the user how many accounts he wants to open. It then establishes the desired number, each with zero balance. CommandLineCalculator asks the user which of these accounts he wants to deal with. When the user has selected an account, CommandLineCalculator allows the user to add whole numbers of dollars to, or subtract them from the account for as long as he requires. When the user is done with an account, he is permitted to quit, or to pick another account to process. (Braude chapter 1.4.1)

13. Calculator solicitNumAccounts() CalcDisplay display() CommandLineCalculator main() executeAdditions() solicitNumberAccounts() getAnInputFromUser() interactWithUser() CalcOperation execute() Add Multiply Divide Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission. A More Flexible Design forCalculator Application New Design Existing Design

14. Two kinds of reuse • actual software (packages, classes) can be reused if designed well • but it is difficult to actually do design this well • existing ideas, concepts, general ways of putting components togethercan be adapted by the designer to new projects • Design Patterns are an excellent way of describing sets of well-known ideas for designers to reuse

15. Design Patterns • Design Patterns have names:other designers can recognise what you are doing and playing variations on • Design Patterns have documented properties:where and how they work, what performance problems, what other restrictions Consequences: • Design patterns can save lots of time learning by (bad) experience: pre-packaged (good) experiences • thinking in design patterns pushes us to think generally, more abstractly, more productively

16. The point of studying design patterns Patterns • enable you to re-use great design ideas • create a common vocabulary for talking about design with designers, reviewers, programmers. • give you a higher-level perspective on design, • allow you to talk and think at a higher level of abstraction (Forest vs. trees) • The solutions in the standard patterns embody some important design principles • in particular many of them produce code that is much easier to modify than a morestraightforward, simple-minded solution.

17. Patterns: from their inventor, a bricks-and-mortar Architect Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice. Christopher Alexander

18. Describing Software Design Patterns Item Description Name A unique name that identifies this pattern Intent The purpose of this pattern Problem The problem the pattern tries to solve Solution How the pattern provides a solution to the problem Participant and Collaborators The entities (usually classes) involved in the pattern Consequences The consequences of using this pattern; discussion of the forces at play Implementation How to implement it GoF Page number in the Gang of Four book (Gamma et al)

19. Observer pattern • see also Braude 9.5, GoF chap 5 Problem:Changes to one object require changes to others that depend on it, but you don't know how many objects will need to be changed. Examples: • a networked file system • a spreadsheet program with multiple views: charts and graphs, cells and formulae • a set of different views (render, edit, window) over a single complex structured text-like file (e.g. HTML)

20. Observer (1) Here is the "standard" pattern outline [GoF pp 293-303] • Name Observer • Intent Define a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically. • Problem You need to notify a varying list of objects that an event has occurred. • Solution The observers delegate the responsibility for monitoring for an event to a central object: the subject (or Source- [Braude])

21. Observer (2) • Participants and Collaborators The subject knows its observers because they register with it. The subject must notify the observers when the event occurs. The observers are responsible both for registering with the subject and for getting the information they need from the subject when notified. • Consequences Subjects may tell observers about events they do not need to know about, if some observers are only interested in a subset of events. Extra communication is needed when the observers ask the subject for more information.

22. Observer (3) • Implementation Have observers register with the subject. The subject is responsible for monitoring for (or generating) the interesting events. Observers need to keep a reference to the subject. The subject needs to keep a list of observers and to add observers or remove them from the list on request. When an interesting event occurs, the subject goes through its list and tells each observer to update itself.

23. Observer implementation – in UML see separate diagrams: • Observer class diagram • Observer sequence diagram