Oberserver and Observable¶

To write output messages when desired by the user, the observer pattern is implemented in Ikarus. Four things are necessary to understand the implementation of observer patterns: Messages, IObservable, IObserver and Subscriptions.

Messages¶

A message class is a list of possible events that can happen and might be of interest. The messages that are used for nonlinear solvers are listed below as an example.

enum class NonLinearSolverMessages {
  BEGIN,
  INIT,
  ITERATION_STARTED,
  ITERATION_ENDED,
  RESIDUALNORM_UPDATED,
  CORRECTIONNORM_UPDATED,
  SOLUTION_CHANGED,
  FINISHED_SUCESSFULLY,
  END
};

IObservable¶

A class can be observable. The class then sends notifications when events are happening. To become observable, a class must inherit from IObservable<MessageType>, for example,

class NewtonRaphson : public IObservable<NonLinearSolverMessages> {...};

this->notify(NonLinearSolverMessages::SOLUTION_CHANGED);

IObserver¶

A class can be an observer. The class is then notified when events are happening and can perform actions. A very simple example is shown below. To become an observer, the class must inherit from IObserver<MessageType>, where MessageType is the enum of messages to use (see above).

class OurFirstObserver : public IObserver<NonLinearSolverMessages> {
public:
  void updateImpl(NonLinearSolverMessages message) override {
    if (message == NonLinearSolverMessages::ITERATION_STARTED) std::cout << "Iteration started.\n";
  }
};

To connect observer and observable, one has to call observalbe.subscribe(MessageType::Message,observer). Example:

Ikarus::NewtonRaphson nr(...);
auto ourSimpleObserver = std::make_shared<OurFirstObserver>();
nr.subscribe(NonLinearSolverMessages::ITERATION_STARTED, ourSimpleObserver);
};

Subscriptions¶

There are a couple of options for the subscription:

subscribe(MessageType::Message,observer) // (1)!
subscribeAll(observer) // (2)!
subscribeAll({observer1,observer2}) // (3)!
unSubscribe(...) // (4)!

1. Subscribes to one specific message.
2. Subscribes to all the messages in enum.
3. Multiple observers can subscribe at once.
4. Unsubscribe from specific messages or all messages.

To send a message together with data, the sender (observable) calls

this->notify(MessageType::Message,data);

void updateImpl(MessageType message, data) override {...}