CHAIN OF PESPONSIBILITY

When a request comes, chain the receiving objects and pass the request along the chain until an object handles it.

Scenario

• more than one object may handle a request
• set of objects that can handle the rest should be specified dynamically

Participants

• Handler

  • defines an interface for handling requests
  • (optional) implements the successor link, i.e.,

  class HelpHandler {
  public:
    virtual void HandleHelp();
  private:
    HelpHanlder* _successor;
  }
  
  void HelpHandler::HandleHelp() {
    if (_successor) {
      _successor->HandleHelp()
    }
  }
  

  if the subclass is not interested in handling the request, it can just leave the handle method default and it will gets passed to next successor.

• ConcreteHandler

  • if it can handle the request, it does so; otherwise, it forwards the request to its successor

• Clinet

  • initiates the request to ConcreteHandler object

Example

Application* application = new Application();
Dialog* dialog = new Dialog(application);
Button* button = new Button(dialog);

button->HandleHelp();

COMMAND

Bundle command, args, receiver into an object which can be executed later. It allows generic handling of different request, queuing those request and support undoable operations.

Scenario

• specify, queue and execute requests at different times
• support undo
• logging changes so they can be reapplied in case of system crash
• for transaction system

Participants

• Command
  • declares an interface for execution an operation - Execute, Undo, etc
• ConcreteCommand
  • defines a binding between a Receiver object and an action
  • implements Execute by invoking the corresponding operations on Receiver
• Client
  • create ConcreteCommand object and sets its receiver
• Invoker
  • ask command to execute the request

Example

class Command {
  ...
  virtual void Execute() = 0;
}

class OpenFileCommand: public Command {
 	OpenFileCommand(Application*);
  	virtual void Execute();
}

void OpenFileCommand::Execute() {
  char* name = AskUser();
  
  if (name != 0) {
    ....
    document->Open();
  }
}