Patterns of learning

Transcription

1 Design patterns

2 Patterns of learning Suppose we want to learn how to play chess. First, we need to learn the basic rules. Then we need to learn the basic strategy/ principles (value of pieces, etc.). To become a master, however, we must study from other masters.

3 Fundamentals of Computing Learn the rules (Fund I & II): Language, basic data structures and algorithms Learn principles (labs and final projects): Modular, structured, object-oriented programming Become a master by studying designs of other masters (paradigms? lifetime?)

4 The challenge(s) Designing object-oriented software is not easy. It gets even harder when we design reusable software. The goal should be to avoid redesign, or at least minimize it as needed. Via experience / labs Via learning from others

5 What are design patterns? A widely accepted solution to a recurring design problem in OOP. Describes how to structure classes. Provides a general blueprint when implementing part of a program. Focuses on class relationships.

6 Don t reinvent the

7 More info Design patterns are useful for any language with classes and related tools. To be useful, the template must be useful in a variety of applications. To be substantive, it must make assumptions on the problem.

8 What design patterns are not Does not tell you how to structure the entire application. Data structures (i.e., linked lists). Does not describe a specific algorithm E.g., binary search, sorting

9 In the beginning Design Patterns got popularized by a book in 1995 written by Gamma, Helm, Johnson and Vlissides( Gang of four, or GoF) The OOP Four horseman J This book described 23 patterns divided into three main categories: creational, structural, and behavioral.

10 Quote The famous architect Christopher Alexander once said: 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. Some people argue gang of four just used architecture terms for old CS concepts

11 Bringing patterns to OOP Our programs use objects and interfaces instead of walls and windows. In general, we have four aspects: Pattern name, which describes a design Problem, which describes when to apply the pattern. Solution, how a general arrangement of elements solves a problem. Consequences, or the pros and cons of a given solution.

12 One sentence summary Design patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context. GHJV, 1995

13 Case study You have been hired by a biologist on campus to write a C++ program to convert a gene into a protein. You vaguely remember that: Triplets of DNA = 1 amino acid Proteins are made from chains of triplets ATG,TTT,CTA -> MLY You are given a file of gene sequences. Now what?

14 Iterator pattern We have used iterators for STL classes, but the concept is the same as any for loop. Similar applications are true for a lot of computational molecular biology.

15 The trick Expert designers do not solve every problem from first principles. Instead, they tend to use what worked in the past. Experience makes them experts. Analogy: writers and playwrights follow patterns such as romantic novel, buddy comedy, etc.

16 Benefits Learn from community wisdom Don t reinvent the wheel Faster implementation Makes code more accessible to others, as the general shape is well-understood

17 Case study #2 How would you use a linked list to implement a stack? How about a queue? (hint think to the buttercup lab)

18 Adapter pattern Transforms one class into a different class without changing the underlying parts. Examples: Stack and queue container adapters in the STL Stack<vector<int> >

19 How to reuse Inheritance lets us define the implementation of a given class in terms of another Called white-box reuse because the internals are visible to subclasses. Composition provides new functionality via combination Called black-box because internal details are not visible, just the interface.

20 One example: singleton pattern Suppose we want to make sure a class has only one instantiation and provide only a single access point in the program. One approach is to promise yourself to have only one object.

21

22 Self control, C++ style Class singleton { public: static Singleton* getinstance(); protected: // why do we need this? singleton(); Singleton& operator= (const Singleton&); private: static Singleton* instance; }

23 Another case study You have been asked to use your experience with Connect4 to develop a new Battleship video game. What is similar between these two games? What is different?

24 Model-View controller pattern

25 MVC pattern MVC is a way of dividing the I/O task from the rest of the application. One part maintains the state, one renders the screen, and the last one deals with user input. Might be useful for your projects.

26 Application Suppose we would like to develop a weather reporting application WeatherReport will store desired information. Two output classes: GraphicalReport (various icons like rain) TextReport (78 degrees, sunny)

27 Example: observer pattern Changes in the state of one object are broadcast to other objects The changing object is called the subject Others are the observers

28 Illustration

29 Bringing it all together Lets define a new class called Observable This will be a base class for objects such as WeatherReport Generic data: Observers() Add/removeObserver() NotifyObserver()

30 Conclusions Design patterns enable large-scale reuse Capture expert knowledge and design tradeoffs Improve developer communication