CS 251 INTERMEDIATE SOFTWARE DESIGN SPRING Expression Tree Case Study : Prototype Pattern Reactor pattern

Transcription

1 CS 251 INTERMEDIATE SOFTWARE DESIGN SPRING 2011 Expression Tree Case Study : Prototype Pattern Reactor pattern

2 CLONING OBJECTS Cloning permits replicating an object C++ assignment operator implements a memberwise copy

3 CLONING OBJECTS Shallow copies make copies of only the objects references, not the actual objects. when the original object is changed, the new object also changes. Does not trace the references. Deep copies make copies of the actual data and objects referred too by the original object. When the original object is changed, the new object does not change. Traces the references and applies the copy to the referents.

4 CLONING OBJECTS Shallow Copy Object1 containedobject1 containedobject2 Object1 Deep Copy Object1 containedobject1 Object2 containedobject3 containedobject2 containedobject4

5 CLONING OBJECTS When the object being cloned only contains instances of data structures or primitives, then shallow and deep copying behave the same. When the object being cloned contains references (pointers) to data structures (objects) then A shallow copy will only duplicate the pointer. A deep copy will duplicate in memory the instance of the data structure or primitive.

6 TO CLONE OR NOT What type of copy is required? If both the original object and the clone are required to share their data members and data structures, then use a shallow copy. If two independent objects are required, then use a deep copy. Consequences If a shallow copy is used instead of a deep copy, then the application can have memory management issues.

7 CLONING IN C++ C++ does not provide a default clone method. The C++ assignment operation creates a copy using a memberwise copy, or a shallow copy. Must be aware of the data types, in particular immutable and mutable. If the object to be cloned is an aggregate object, then the aggregated objects have to clone themselves.

8 PROTOTYPE OBJECT CREATIONAL Intent Specify the kinds of objects to create using a prototypical instance and create new objects by copying this prototype Applicability when a system should be independent of how its products are created, composed, and represented when the classes to instantiate are specified at run-time; or Structure

9 PROTOTYPE OBJECT CREATIONAL Consequences + can add and remove classes at runtime by cloning them as needed + reduced subclassing minimizes/eliminates need for lexical dependencies at run-time every class that is used as a prototype must itself be instantiated classes that have circular references to other classes cannot really be cloned Implementation Use prototype manager Shallow vs. deep copies Initializing clone internal state within a uniform interface

10 PROTOTYPE OBJECT CREATIONAL Known Uses The first widely known application of the Prototype pattern in an object-oriented language was in ThingLab Coplien describes idioms related to the Prototype pattern for C++ and gives many examples and variations Etgdb debugger for ET++ The music editor example is based on the Unidraw drawing framework PrototypeExample.cpp

11 OVERVIEW OF APPLICATION STRUCTURE PATTERNS Reactor Singleton Reactor Event_Handler Options Expression_Tree_ Command_Factory Expression_Tree_ Event_Handler Expression_Tree_ Context << create >> Verbose_Expression_ Tree_Event_Handler Macro_Expression_ Tree_Event_Handler Expression_Tree_ Command Strategy

12 DRIVING THE APPLICATION EVENT FLOW Goals: Decouple expression tree application from the context in which it runs Support inversion of control Constraints/forces: Do not recode existing clients Add new event handles without recompiling Reactor NETWORKING STL algorithms EXPRESSION TREE FUNCTIONALITY CALL BACKS GUI INVOKES DATABASE

13 SOLUTION: SEPARATE EVENT HANDLING FROM EVENT INFRASTRUCTURE Create a reactor to detect input on various sources of events and then demux and dispatch the events to the appropriate event handlers Create concrete event handlers that perform the various operational modes of the expression tree application Register the concrete event handlers with the reactor Run the reactor s event loop to drive the application event flow Reactor register_handler() remove_handler() run_event_loop() end_event_loop()

14 REACTOR AND EVENT HANDLER An object-oriented event demultiplexor and dispatcher of event handler callback methods in response to various types of events Interface: virtual void ~Event_Handler (void) =0 virtual void delete_this (void) ~Reactor (void) uses virtual void handle_input (void)=0 void run_event_loop (void) void end_event_loop (void) void register_input_handler (Event_Handler *event_handler) void remove_input_handler (Event_Handler *event_handler) static Reactor * instance (void) Commonality: Provides a common interface for managing and processing events via callbacks to abstract event handlers Variability: Concrete implementations of the Reactor and Event_Handlers can be tailored to a wide range of operating system demultiplexing mechanisms and application-specific concrete event handling behaviors

15 REACTOR INTERACTIONS : Main Program : Exression_Tree Event Handler : Reactor : Synchronous Event Demultiplexer 1. Initialize phase Con. Event Handler Events register_handler() get_handle() 2. Event handling phase Handle handle_events() handle_event() service() Handles Handles select() event Observations Note inversion of control Also note how long-running event handlers can degrade the quality of service, since callbacks steal the reactor s thread! See main.cpp for example of using Reactor to drive event loop

16 REACTOR OBJECT BEHAVIORAL Intent allows event-driven applications to demultiplex and dispatch service requests that are delivered to an application from one or more clients Applicability Need to decouple event handling from event detecting/demuxing/dispatching When multiple sources of events must be handled in a single thread Structure Reactor handle_events() register_handler() remove_handler() <<uses>> handle set Synchronous Event Demuxer select () * Handle * notifies * dispatches owns Concrete Event Handler A handle_event () get_handle() Event Handler handle_event () get_handle() Concrete Event Handler B handle_event () get_handle()

17 REACTOR OBJECT BEHAVIORAL Consequences + Separation of concerns and portability + Simplify concurrency control Non-preemptive Implementation Decouple event demultiplexing mechanisms from event dispatching Handle many different types of events, e.g., input/output events, signals, timers, etc.

18 REACTOR OBJECT BEHAVIORAL Known Uses InterViews Kits ET++ WindowSystem AWT Toolkit X Windows Xt ACE & The ACE ORB (TAO) Event_Handler.h Event_Handler.cpp Expression_Tree_Event_Handler.h Expression_Tree_Event_Handler.cpp Main.cpp Observer.h Observer.cpp Reactor.h Reactor.cpp

19 SUMMARY OF APPLICATION STRUCTURE PATTERNS Reactor Singleton Reactor Event_Handler Options Expression_Tree_ Command_Factory Expression_Tree_ Event_Handler Expression_Tree_ Context << create >> Verbose_Expression_ Tree_Event_Handler Macro_Expression_ Tree_Event_Handler Expression_Tree_ Command Strategy