1. Design ECE 417/617: Elements of Software Engineering Stan Birchfield Clemson University

2. From modeling to design Steps: 1.Analysis and modeling 2.Design 3.Construction (code generation and testing) Design involves making the analysis model more specific, to facilitate construction. Goal of design is quality.

3. FURPS Quality software is Functional – capabilities of program Usable – human factors, aesthetics Reliable – frequency and severity of failure Performance – response time, speed Supportable – is the code maintainable, extensible, testable, configurable, easy to install, etc. (Developed by Hewlett-Packard in 1980s)

4. Basic design principles Design is iterative: –Architecture is refined over time by successively filling in details –Refinement is a process of elaboration –Results in hierarchical model A good design exhibits –abstraction – details are provided in lower levels –modularity – divide-and-conquer via smaller independent components –refactoring – internal structure of software is improved without affecting external behavior

5. Design model Design model has three aspects: Architectural design Component-level (data) design Interface design We will consider these in turn, with UI covered in a separate lecture.

6. Architectural Design

7. Architectural design Architecture is a high-level representation of the S/W with the major components identified Architectural styles are templates, e.g., –Data-centered –Data-flow –Call and return –Object-oriented –Layered Architectural patterns define specific approach for handling some behavioral characteristic of system, e.g., –concurrency: use O/S features or provide task scheduler –persistence: storage and retrieval of data –distribution: communication of components with one another. Most common is broker – acts as middle man between client and server (CORBA). Style is like “Cape cod, A-frame”. Pattern is like “kitchen”.

8. Architectural Styles Data-centered – subsystems interact through single repository Model / View / Controller Call and return (Client / Server) Layered (three-tier, four-tier) Data-flow (pipe and filter)

9. Layers and Partitions Layer – group of related subsystems –Layer knows about layers below it, but not layers above it –Top layer: no one else knows about it –Closed architecture – can only access layer immediately below –Open architecture – can access any layer below Partition – peer subsystems, each with different responsibilities

10. Hierarchical decomposition Application Presentation Session Transport Network DataLink Physical Socket CORBA TCP/IP Object Ethernet Wire Level of abstraction Frame Packet Bit Connection Format Message Example: Open Systems Interconnection (OSI)

11. Mapping DFD into architecture Transform mapping –transform flow always exists; represents information flow within system; incoming flow passes through transform center, leads to outgoing flow –To map DFD with transform flow characteristics into specific architectural style, –review model –refine models –determine whether transform or transaction characteristics –isolate transform center –perform first and second level factoring –refine Transaction mapping –transaction flow occurs when one input gives rise to several outputs; transaction triggers data flow along one of many paths –To map DFD with transaction flow characteristics, –review model –refine models –determine whether transform or transaction characteristics –isolate transaction center –map to transform branch –factor and refine

12. Model / View / Controller (MVC) MVC: –Model subsystems maintain domain knowledge –View subsystems display it to the user –Controller subsystems manage sequence of interactions with user M doesn’t depend upon V or C Changes propagated via subscribe/notify protocol, using Observer design pattern Well-suited for interactive systems

13. MVC Details Controller Model subscriber notifier initiator * repository1 1 * View

14. MVC Example

15. MVC Example Details :Controller :InfoView :Model 2:enterNewFileName(file,newName) 3:setName(newName) :FolderView 1:subscribeToFileEvents(file) 5:getName() 4:notifySubscribedViews(file) 1:subscribeToFileEvents(file) 5:getName() 4:notifySubscribedViews(file)

16. GUI-Based Programming

17. Paradigms Compared Application Widgets The User draw outputinput callbacks output input Traditional command-lineGUI-based

18. Event/Message loop

19. Event loop – pseudocode WinMain() { while (1) { // loop forever, waiting for an event if (event_exists) {//there is an event, figure out what to do if (event == keydown_a) display(‘user pressed the A key’); else if (event == window_resize) display(‘window resized’); else if (event == repaint) display(‘need to repaint window’); else if (event == keydown_escape) exit_program(); } int main() { return WinMain(); }

20. Event loop – WinMain int WINAPI WinMain (HINSTANCE hInst, HINSTANCE hPrevInst, char * cmdParam, int cmdShow) { char className [] = "Winnie"; WinClass winClass (WindowProcedure, className, hInst); winClass.Register (); WinMaker win ("Hello Windows!", className, hInst); win.Show (cmdShow); MSG msg; int status; while ((status = ::GetMessage (& msg, 0, 0, 0)) != 0) { if (status == -1) return -1; ::DispatchMessage (& msg); } return msg.wParam; }

21. Event loop – WindowProc LRESULT CALLBACK WindowProcedure (HWND hwnd, unsigned int message, WPARAM wParam, LPARAM lParam) { switch (message) { case WM_DESTROY: ::PostQuitMessage (0); return 0; } return ::DefWindowProc (hwnd, message, wParam, lParam ); }

22. Event loop (cont.) int WINAPI WinMain(HINSTANCE hinstance, HINSTANCE hprev, PSTR cmdline, int ishow) { HWND hwnd; MSG msg; //initialization code goes here while(1) { // Get message(s) if there is one if(PeekMessage(&msg,hwnd,0,0,PM_REMOVE)) { if(msg.message == WM_QUIT) break; TranslateMessage(&msg); DispatchMessage(&msg); //this calls the CALLBACK function WinProc() } else { DrawScene(); //display the OpenGL/DirectX scene }

23. Event loop (cont.) LRESULT CALLBACK WinProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) { PAINTSTRUCT ps; // Depending on the message -- we'll do different stuff switch(message) { case WM_PAINT: Draw(); return 0; // ESC will quit program case WM_KEYDOWN: //user pressed a key if(GetAsyncKeyState(VK_ESCAPE)) //it was the escape key PostQuitMessage(0); //quit program return 0; case WM_DESTROY://windows wants the program to die case WM_CLOSE://or the user closed the window PostQuitMessage(0); //quit program return 0; } return DefWindowProc(hwnd, message, wparam, lparam); }

24. GUI Concepts Widget – graphic object with functionality; e.g., button, toolbar,... Window – holds widgets Child/parent – relationships between windows Event / message – how windows communicate

25. Anatomy of a Window title bar menu toolbar client area status bar

26. Microsoft Windows Programming History: –Win32 API: core library written in C –MFC: C++ wrappers around most common Win32 API functions Lots of macros Lots of legacy code Not free, not portable But it works (generally speaking)

27. Analyzing architectural designs Two approaches developed by SEI: Architecture trade-off analysis method (ATAM) –Collect scenarios and requirements –Evaluate quality attributes and their sensitivity –Critique candidate architectures Scenario-based architectural analysis (SAAM) –Uses scenarios to analyze architectures with respect to quality attributes Quality attributes: reliability, performance, security, maintainability, flexibility, testability, portability, reusability, interoperability

28. Component-level Design

29. Component-level design Occurs after the first iteration of architectural design Goal: translate the design model into operational software Component is set of collaborating classes Designing components –OCL –flow chart –tabular design notation Decision table has four quadrants specifying conditions and actions, as well as rules for both –PDL (pseudocode)

30. ... Decomposition System Subsystem1 Class1a SubsystemN Class1n... ClassNaClassNn...

31. Coupling and cohesion Coupling -- # of dependencies between subsystems Cohesion -- # of dependencies within subsystem Goal: low coupling, high cohesion 7 +/- 2 rule – keep number of concepts at any given layer of abstraction bounded

32. Additional design principles Single responsibility (SRP) A class should have only one reason to change Open-closed (OCP) Software entities should be open for extension but closed for modification (achieved via inheritance) Liskov substitution (LSP) Subclasses should be substitutable for their base classes Dependency inversion (DIP) Abstractions should not depend upon details Interface segregation (ISP) Many client-specific interfaces are better than one general purpose interface Release reuse equivalency (REP) Granule of reuse is granule of release Common closure (CCP) Classes that change together belong together Common reuse (CRP) Classes in a package are reused together

33. Data structures and flow Software system is composed of –data structures, and –data flow Which is more important? “Show me your code and conceal your data structures, and I shall continue to by mystified. Show me your data structures and I won’t usually need your code. It will be obvious.” – Fred Brooks

34. What is a Design Pattern? A design pattern –abstracts a recurring design structure –comprises class and/or object dependencies, structures, interactions, or conventions –distills design experience

35. Re-use Code re-use –Don’t reinvent the wheel –Requires clean, elegant, understandable, general, stable code –leverage previous work Design re-use –Don’t reinvent the wheel –Requires a precise understanding of common, recurring designs –leverage previous work

36. Some design patterns Abstract factory Adapter Bridge Command Composite Façade Subject / Observer Proxy Strategy

37. Subject-observer [from Vlissides]

38. Subject-observer (cont.) Subject Register(Observer) Unregister(Observer) NotifyAll() Observer OnUpdate() 1 * for all o in observers { o.OnUpdate() }

39. Subject-observer (cont.) Subject Register(Observer) Unregister(Observer) NotifyAll() Observer virtual OnUpdate() 1 * for all o in observers { o.OnUpdate() } ConcreteSubjectConcreteObserver virtual OnUpdate()

40. Model / view / controller (MVC) Controller Model View (displays data) (mediates) (holds data) { Model m; Controller c(&m); View v(&c); } calls Register() Main View Controller Model Create() Register() Set() OnUpdate()

41. MVC (cont.) Subject Register(Observer) Unregister(Observer) NotifyAll() Observer virtual OnUpdate() 1 * for all o in observers { o.OnUpdate() } ControllerView virtual OnUpdate()

42. MVC (cont.) class Observer { protected: virtual void OnUpdate(MsgId message_id) = 0; }; class Subject { public: enum MsgId {}; void RegisterObserver(Observer* obs); virtual void NotifyAllObservers(MsgId message_id) { for (int i=0 ; i<m_observers.size() ; i++) { m_observers[i]->OnUpdate(message_id); } private: std::vector m_observers; };

43. MVC (cont.) class Controller : public Subject { Controller(Data* d) : m_data(d) {} const Data* GetData() const; void AddSphere(const Sphere& s) { m_data->AddSphere(s); NotifyAllObservers(ADD_SPHERE); } private: Data* m_data; };

44. MVC (cont.) class MainWnd : public Observer, CWnd { public: MainWnd(Controller* c) : m_controller(c) { c.Register(this); } virtual void OnUpdate(int message_id) { switch (message_id) { case Subject::ADD_SPHERE:... } private: Controller* m_controller; };

45. Adapter You have –legacy code –current client Adapter changes interface of legacy code so client can use it Adapter fills the gap b/w two interfaces No changes needed for either –legacy code, or –client

46. Adapter (cont.) class NewTime { public: int GetTime() { return m_oldtime.get_time() * 1000 + 8; } private: OldTime m_oldtime; };

47. Command You have commands that need to be –executed, –undone, or –queued Command design pattern separates –Receiver from Invoker from Commands All commands derive from Command and implement do(), undo(), and redo()

48. Implementing ‘Undo/Redo’ Multi-level undo/redo requires two classes: –Command –CommandManager class Command { public: virtual bool Execute() = 0; virtual bool Unexecute() = 0; virtual ~Command() { } }; class CommandManager { private: typedef list CommandList; CommandList m_undoList; CommandList m_redoList; public: void DoCommand(Command* com); void Undo(); void Redo(); }; http://www.codeproject.com/KB/cpp/undoredo_cpp.aspx

49. Facade You –have a set of related classes –want to shield the rest of the system from these details Facade provides a simplified interface Encapsulates a subsystem

50. Composite You want uniformly to treat –items (atomic elements), and –groups (containing items or other groups) Composite interface specifies operations that are shared between items and groups Examples: hierarchy of files and directories, groups of drawable elements

51. Composite (cont.) GroupItem Composite

52. Proxy You want to –delay expensive computations, –use memory only when needed, or –check access before loading an object into memory Proxy –has same interface as Real object –stores subset of attributes –does lazy evaluation

53. Strategy You want to –use different algorithms depending upon the context –avoid having to change the context or client Strategy –decouples interface from implementation –shields client from implementations –Context is not aware which strategy is being used; Client configures the Context –strategies can be substituted at runtime –example: interface to wired and wireless networks

54. Strategy (cont.) Client Strategy Concrete StrategyA Concrete StrategyB Context Policy

55. Strategy (cont.) Lots of switch statements is evidence of “code smell”: Polymorphism, using strategy pattern, cleans this up: http://codebetter.com/blogs/jeremy.miller/archive/2006/04/11/142665.aspx

56. Bridge You –have several different implementations –need to choose one, possibly at run time Bridge –decouples interface from implementation –shields client from implementations –Abstraction creates and initializes the ConcreteImplementations –Example: stub code, slow code, optimized code

57. Bridge (cont.) Client Implementor Refined Abstraction Concrete ImplementorA Concrete ImplementorB Abstraction

58. Design pattern space [from Vlissides]