Sometimes it's necessary to issue requests to objects without knowing anything about the operation being requested or the receiver of the request.

Transcription

1 Cmmand 1 Intent Encapsulate a request as an bject, thereby letting yu parameterize clients with different requests, queue r lg requests, and supprt undable peratins. Als Knwn As Actin, Transactin Mtivatin Smetimes it's necessary t issue requests t bjects withut knwing anything abut the peratin being requested r the receiver f the request. The Cmmand pattern lets UI bjects make requests f unspecified applicatin bjects by turning the request itself int an bject. This bject can be stred and passed arund like ther bjects. The key t this pattern is an abstract Cmmand class, which declares an interface fr executing peratins. Cncrete Cmmand subclasses specify a receiver-actin pair by string the receiver as an instance variable and by implementing Execute t invke the request. The receiver has the knwledge required t carry ut the request.

2 2 The Cmmand pattern decuples the bject that invkes the peratin frm the ne having the knwledge t perfrm it. Applicability Use the Cmmand pattern when yu want t parameterize bjects by an actin t perfrm. specify, queue, and execute requests at different times. A Cmmand bject can have a lifetime independent f the riginal request. supprt und with Unexecute peratin. Executed cmmands are stred in a histry list. Unlimited-level und and red is achieved by traversing this list backwards and frwards calling Unexecute and Execute, respectively. supprt lgging changes s that they can be reapplied in case f a system crash with lad and stre peratins. structure a system arund high-level peratins built n primitives peratins (transactins).

3 Structure 3 Participants Cmmand declares an interface fr executing an peratin. CncreteCmmand (PasteCmmand, OpenCmmand) defines a binding between a Receiver bject and an actin. implements Execute by invking the crrespnding peratin(s) n Receiver. Client (Applicatin) creates a CncreteCmmand bject and sets its receiver. Invker (MenuItem) asks the cmmand t carry ut the request. Receiver (Dcument, Applicatin) knws hw t perfrm the peratins assciated with carrying ut a request. Any class may serve as a Receiver. Cllabratins The client creates a CncreteCmmand bject and specifies its receiver. An Invker bject stres the CncreteCmmand bject. The invker issues a request by calling Execute n the cmmand. When cmmands are undable, CncreteCmmand stres state fr unding the cmmand prir t invking Execute. The CncreteCmmand bject invkes peratins n its receiver t carry ut the request.

4 4 Cnsequences 1. Cmmand decuples the bject that invkes the peratin frm the ne that knws hw t perfrm it. 2. Cmmands are first-class bjects. They can be manipulated and extended like any ther bject. 3. Yu can assemble cmmands int a cmpsite cmmand. In general, cmpsite cmmands are an instance f the Cmpsite pattern. 4. It's easy t add new Cmmands, because yu dn't have t change existing classes. Implementatin 1. Hw intelligent shuld a cmmand be? It may merely define a binding between a receiver and the actins that carry ut the request. It may have enugh knwledge t find their receiver dynamically. It als can implement everything itself withut delegating t a receiver at all. 2. Supprting und and red. A CncreteCmmand class might need t stre additinal state t d s: the Receiver bject, which actually carries ut peratins in respnse t the request, the arguments t the peratin perfrmed n the receiver, and any riginal values in the receiver that can change as a result f handling the request. The receiver must prvide peratins that let the cmmand return the receiver t its prir state.

5 5 An applicatin needs a histry list f cmmands that have been executed. The histry list stres sequences f cmmands that have been executed. Traversing backward thrugh the list and reverse-executing cmmands cancels their effect; Traversing frward and executing cmmands reexecutes them. An undable cmmand might have t be cpied befre it can be placed n the histry list. Cmmands that must be cpied befre being placed n the histry list act as prttypes 3. Aviding errr accumulatin in the und prcess. Hysteresis can be a prblem in ensuring a reliable, semantics-preserving und/red mechanism. Stre mre infrmatin in the cmmand t ensure that bjects are restred t their riginal state (with Mement pattern) 4. Using C++ templates fr cmmands that (1) aren't undable and (2) dn't require arguments, aviding creating a Cmmand subclass fr every kind f actin and receiver. Sample Cde class Cmmand { virtual ~Cmmand(); virtual vid Execute() = 0; prtected: Cmmand(); ; class OpenCmmand : public Cmmand { OpenCmmand(Applicatin*); virtual vid Execute(); prtected: virtual cnst char* AskUser(); Applicatin* _applicatin; char* _respnse; ; OpenCmmand::OpenCmmand (Applicatin* a) { _applicatin = a; vid OpenCmmand::Execute () { cnst char* name = AskUser(); if (name!= 0) { Dcument* dcument = new Dcument(name); _applicatin->add(dcument); dcument->open();

6 class PasteCmmand : public Cmmand { PasteCmmand(Dcument*); 6 virtual vid Execute(); Dcument* _dcument; ; PasteCmmand::PasteCmmand (Dcument* dc) { _dcument = dc; vid PasteCmmand::Execute () { _dcument->paste(); template <class Receiver> class SimpleCmmand : public Cmmand { typedef vid (Receiver::* Actin)(); SimpleCmmand(Receiver* r, Actin a) : _receiver(r), _actin(a) { virtual vid Execute(); Actin _actin; Receiver* _receiver; ; template <class Receiver> vid SimpleCmmand<Receiver>::Execute () { (_receiver->*_actin)(); MyClass* receiver = new MyClass; Cmmand* acmmand = new SimpleCmmand<MyClass>(receiver, &MyClass::Actin); acmmand->execute(); class MacrCmmand : public Cmmand { MacrCmmand(); virtual ~MacrCmmand(); virtual vid Add(Cmmand*); virtual vid Remve(Cmmand*); virtual vid Execute(); List<Cmmand*>* _cmds; ;

7 vid MacrCmmand::Execute () { ListIteratr<Cmmand*> i(_cmds); 7 fr (i.first();!i.isdne(); i.next()) { Cmmand* c = i.currentitem(); c->execute(); vid MacrCmmand::Add (Cmmand* c) { _cmds->append(c); vid MacrCmmand::Remve (Cmmand* c) { _cmds->remve(c); Knwn Uses Unidraw A cmmand may be sent t anther bject fr interpretatin The result f the interpretatin varies with the receiving bject. The receiver may delegate the interpretatin t anther bject, as in a Chain f Respnsibility. The receiver f a Unidraw cmmand is thus cmputed rather than stred. Unidraw's interpretatin mechanism depends n run-time type infrmatin. Functrs in C++ Cmmand bjects are functins. Related Patterns Overlading the functin call peratr (peratr()). The Cmmand pattern fcus is n maintaining a binding between a receiver and a functin (i.e., actin). A Cmpsite can be used t implement MacrCmmands. A Mement can keep state the cmmand requires t und its effect. A cmmand that must be cpied befre being placed n the histry list acts as a Prttype.