OO principles, patterns (THIẾT kế đối TƯỢNG SLIDE)

Overview Object-Oriented Design Principles and Design Patterns What goes wrong with software? • You start with a clear picture in your mind what the software should be The clarity of the design helps you to get to the first release • Then something goes wrong, the software starts to rot: – Changes and additions are harder and harder to make, – You are forced to let go of the original design, eventually even the simplest changes terrify you because of rippling unexpected effects, and you must redesign the whole software • What went wrong? Bad signs, symptoms of rotting design • Rigidity (Sự cứng nhắc)) – Code difficult to change (no continuity) – Changes propagate via dependencies • Fragility (Sự yếu ớt) – Even small changes can cause cascading effects – Code breaks in unexpected places having no conceptual relationship with the area that was changed (no protection) • Immobility (Sự không linh động) – Code is so tangled that it's impossible to reuse anything – No composability Bad signs, symptoms of rotting design • Viscosity – Design preserving ways of working are harder than hacks – Developement environment is slow and inefficient • Needless complexity (Sự phức tạp không cần thiết) – Design contains elements that are not currently useful – Results from too much anticipating future needs • Opacity (Sự tối nghĩa) – The tendency of a module to become more difficult to understand – A constant effort is needed to keep code readable Causes of rotting design • Inevitably changing requirements – "All systems change during their life-cycles This must be born in mind when developing systems expected to last longer than the first version." Ivar Jacobson, OOSE 1992 • Poor dependency management – Dependencies can be controlled • Tools: – OO-design principles, desing patterns – Guidelines: coupling, cohesion Object-Oriented Design Principles OO Design Principles • To create reusable, extensible and maintainable OO systems • Underlie the patterns, help you to cope changes • Design Principles – – – – – – – – Encapsulate what varies Favor Composition Over Inheritance Program To An Interface, Not An Implementation (SRP) The Single Responsibility Principle (OCP) The Open Closed Principle (LSP) The Liskov Substitution Principle (DIP) The Dependency Inversion Principle (ISP) The Interface Segregation Principle Design Patterns 49 What are Patterns? • The 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 (Alexander) • The pattern is at the same time a thing, which happens in the world, and the rule which tells us how to create that thing, and when we must create it (R Gabriel) • A pattern involves a general discription of a recurring solution to a recurring problem with various goal and constraints It identify more than a solution, it also explains why the solution is needed (J Coplien) 50 Learning Patterns • Successful solutions to many areas of human endeavor are deeply rooted in patterns – In fact, an important goal of education is transmitting patterns of learning from generation to generation • Learning to develop good software is similar to learning to play good chess! – Though the consequences of failure are often far less dramatic! 51 Becoming a Chess Master • First learn the rules – e.g., names of pieces, legal movements, chess board geometry and orientation etc • Then learn the principles – e.g., relative value of certain pieces, strategic value of center squares, power of a threat etc • However, to become a master of chess, one must study the games of other masters – These games contain patterns that must be understood, memorized and applied repeatedly • There are hundred of these patterns 52 Becoming a Software Design Master • First learn the rules – e.g., the algorithms, data structures and languages of software • Then learn the principles – e.g., structured programming, modular programming, object-oriented programming, generic programming, principles of OO etc • However, to become a master of software design, one must study the designs of other masters – These designs contain patterns that must be understood, memorized and applied repeatedly • There are hundred of these patterns 53 Overview of Patterns and Frameworks • Patterns support reuse of software architecture and design – Patterns capture the static and dynamic structures and collaborations of successful solutions to problems that arise when building applications in a particular domain • Frameworks support reuse of detailed design and code – A framework is an integrated set of components that collaborate to provide a reusable architecture for a family of related applications • Together design patterns and frameworks help to improve software quality and reduce development time – E.g reuses, extensibility, modularity, performance 54 Design Patterns • Design patterns represent solutions to problems that arise when developing software within a particular context – “Pattern == problem/solution pair in a context” • Patterns capture the static and dynamic structure and collaboration among key participants in software designs – They are particularly useful for articulating how and why to resolve non-functional forces • Patterns facilitate reuse of successful software architectures and designs 55 Design Pattern Space • Creational Patterns – Deal with initializing and configuring classes and objects • Structural Patterns – Deal with decoupling interface and implementation of classes and objects • Behavioral Patterns – Deal with interactions among classes and objects • Scope: what the pattern applies to – Class Patterns • Focus on the relationships between classes and their subclasses • Involve inheritance reuse – Object Patterns • Focus on relationships between objects • Involve composition reuse 56 Design Pattern Space 57 ... Jacobson, OOSE 1992 • Poor dependency management – Dependencies can be controlled • Tools: – OO- design principles, desing patterns – Guidelines: coupling, cohesion Object-Oriented Design Principles OO. .. Coplien) 50 Learning Patterns • Successful solutions to many areas of human endeavor are deeply rooted in patterns – In fact, an important goal of education is transmitting patterns of learning... repeatedly • There are hundred of these patterns 53 Overview of Patterns and Frameworks • Patterns support reuse of software architecture and design – Patterns capture the static and dynamic