Frameworks and Design Patterns

Frameworks and Design Patterns Reusability Revisited L156DP

A toolkit is a library of reusable classes designed to provide useful, general-purpose functionality. • E.g., Java APIs (awt, util, io, net, etc) • An application framework is a specific set of classes that cooperate closely with each other and together embody a reusable design for a category of problems. • E.g., Java APIs (Applet,Thread, etc) • E.g., MFC, JFC, etc. • A design pattern describes a general recurring problem in different domains, a solution, when to apply the solution, and its consequences. L156DP

A framework embodies a complete design of an application, while a pattern is an outline of a solution to a class of problems. • A framework dictates the architecture of an application and can be customized to get an application. (E.g., Java Applets) • When one uses a framework, one reuses the main body of the framework and writes the code it calls. • When one uses a toolkit, one writes the main body of the application that calls the code in the toolkit. (E.g., Java AWT) L156DP

Why catalog/learn design patterns? • Provides (an application-independent) vocabulary to communicate, document, and explore design alternatives. • Captures the experience of an expert (especially the rationale behind a design and the trade-offs involved) and codifies it in a form that is potentially “reusable”. • What, why, how, … • Example is not another way to teach, it is the only way to teach. -- Albert Einstein • (Cf. Vince Lombardi Quote) L156DP

Example : The Intermediary Pattern • A client interacts with an intermediary while the requested services are really carried out by the server/worker. • Proxy • Intermediary acts like a transmission agent. • E.g., rpc, rmi implementations. Server Client Proxy L156DP

Translator/Adapter • Intermediary acts like a translator between the client and the server. • E.g., Format/protocol conversions. Server Client Adapter L156DP

Facade • Intermediary acts like a focal point distributing work to other agents. • E.g., telnet, ftp, … --> web-browser. • E.g., local/network files, devices, ... -> UNIX files Server1 Facade Client Server2 Server3 L156DP

Bridge/Abstract Factory/Handle • Intermediary defines the interface but not the implementation. • E.g., Motif/Mac/Windows look and feel. • E.g., java.io.InputStream, java.io.OutputStream. Impl1 Impl2 Bridge Client Impl3 L156DP

Example : The Traversal Pattern • Task - Visit every element in an aggregate structure in a well-defined order and perform an action on each element. • Iterator • Defines a mechanism for enumerating elements of an aggregate without exposing the representation (by supporting first(), next(), item(), isDone(), etc.) • E.g., (pre-, in-) post-order traversals of tree. (Cf. Higher-order functions in Scheme.) L156DP

Model/View/Controller (Smalltalk) • Pattern for graphical interactive system • Model : Application Object • View : Screen Presentation • Controller : User interaction MVC pattern decouples these three different categories of objects to increase flexibility and reuse. This facilitates support for multiple views of the same information and multiple ways of interaction. L156DP

Java Support for MVC • The multiple views and the model communicate through a subscribe/notify protocol. • Java 1.1 Delegation-based Event Model • java.beansclasses • PropertyChangeEvent, PropertyChangeListener, PropertyChangeSupport, etc. • Controller specifies the way a view responds to user input. • Java AWT classes • Buttons, Pulldown menus, Pop-up menus, Keyboard shortcuts, etc. L156DP

Multi-Panel Interactive Systems Functional Decomposition vs Object-Oriented Decomposition L156DP

Session: sequence of states In each state: display panel seeking user input / new request read user input/query checking for consistency process user request update database transition to next state Example Airline Reservation States User Identification Enquiry on flights (for certain time) Display flights Enquiry on seats Reserve seat Help, Exit, ... Multi-Panel System : Pattern L156DP

Encoding FSM using go to Benquire_flights : display_panel(“Enquire_flights”); do { read_input(Input, okay); } while (!okay); Process(Input, Next); switch (Next) {... case 1: go to Bexit; case 2: go to Bhelp; case 3: go to Benquiry_on_seats; ...} • Each block encodes a state. “Spaghetti” code. • One module : Unsuitable to maintain and reuse. L156DP

Functional Top-Down Solution • Introduce transition function to localize gotos. • Turn each state into a module (routine). execute_session init transition execute_state final display read_input process L156DP

void execute_session() { int state, next; state = initial; do { execute_state(state,next); state = transition(state,next); } while (! is_final(state)); } void execute_state(int state,int next) { T input; int next; display(state); read_input(state); process(state,next); } L156DP

Limitations of functional decomposition • The various modules (routines: display, read_input, process) are tightly coupled via the input argument (state). • Thus, each module (routine) has information about all possible variants (states). • Remedy (Inversion) Instead of building modules around operations and distributing data structures between the resulting routines, use the data types as a basis for modularization, attaching each routine to the corresponding data structure. L156DP

Law of Inversion Illustrated(Functional vs Object-Oriented Decomposition) Data Structure Date Type Data1 Data1 Data2 Proc1 Data3 Data2 Routine Proc2 Proc1 Data 3 Proc 2 Proc3 Proc3 L156DP

Object-Oriented Architecture abstract class State { int next; T input; abstract void display(); abstract void read_input(); abstract void process(); void execute(next) { display(); read_input(); process(); }} L156DP

class Application { State[][] transition; State[] associated_state; Application(int n, int m) { transition = new State[n][m]; associated_state = new State[n]; } void put_state(State s, int i){…} void put_transition(State src, State dst, int choice){…} int initial; void choose_initial(int i){…} void execute { State s; int stn = initial; while ( !stn ) { s = associated_state(stn); s.execute(); stn = transition(stn,s.next) }}} L156DP

Building an Interactive Application Application airline_reservation = new new Application(num_states, num_choices); ... airline_reservation.put_state(s,i) ... airline_reservation.put_transition(s,d,c) ... airline_reservation.choose_initial(i) airline_reservation.execute_session() ... • Class Application is reusable in building other multi-panel interactive applications. L156DP

Undo-Redo Facility (using history) Inheritance, Dynamic Binding, Polymorphism L156DP

Requirements • Applicable to wide class of interactive applications. • Incremental w.r.t. command additions. • Use “reasonable” amount of storage. • Support arbitrary-level of undoing. • Practical Issues: • Part of the User Interface. • Some commands undoable. L156DP

abstract class Command { abstract void execute(); abstract void undo(); void redo() { execute(); } } • Commands can be undone/redone. • undo and redo are operations that cannot be undone/redone. • Each subclass of classCommand adds application specific details. L156DP

A History List isItem() item() count ... ... isFirst() isLast() prev next cursor ! isLast() L156DP

List History = new List(); Read_decode_request; if (request instanceOf Command) { if (! History.isLast()) History.removeAllItemsToRight(); History.addItem(request); request.execute(); } else if (requested instanceOf Undo) if (History.isItem()) { History.item.undo(); History.prev; } else ;// nothing to undo else if (requested instanceOf Redo) if (! History.isLast()) { History.next; History.item.redo(); } else ; // nothing to redo L156DP

Reusability and Extendibility • class Command can be subclassed to incorporate new commands. • class Command can be modified to incorporate additional functionality such as adding help documentation, monitoring statistics, etc, for each command. • This pattern can be used in an entirely new application, to support undo/redo capability. L156DP

Frameworks and Design Patterns