Observer.hpp

#pragma once

#include <set>
#include <utility>

#include <amdis/common/ConceptsBase.hpp>
#include <amdis/common/Index.hpp>
#include <amdis/common/TypeTraits.hpp>

namespace AMDiS
{
  namespace event
  {
    /**
     * An event that is signaled before the actual adaption happens. Example: grid.preAdapt().
     * The \ref value might indicate whether any pre-processing is necessary.
     **/
    struct preAdapt { bool value = true; };

    /**
     * An event that is called directly of the adaption. Example: grid.adapt().
     * The \ref value indicates whether something is changed during adaption.
     **/
    struct adapt { bool value = true; };

    /**
     * An event that is called after adaption to indicate the start of a clean-up phase.
     **/
    struct postAdapt {};
  }


  template <class Event>
  class ObserverInterface
  {
  public:
    virtual ~ObserverInterface() = default;
    virtual void update(Event e) = 0;
    virtual void unset() = 0;
  };


  /// \brief Mixin for signaling of certain events.
  /**
   *  Derived classes T can emit a signal e by calling notify(e). This will send the signal to all
   *  classes U... deriving from Observer<S, Events...> if
   *  - the type of the event is included in Events,
   *  - T = S or S is an Observer of T (directly or indirectly via other Observers),
   *  - U called the Observer constructor with an instance of S that has direct or indirect access
   *      to the instance of T (see Observer)
   */
  template <class Event, class... Events>
  class Notifier
      : public Notifier<Event>
      , public Notifier<Events...>
  {
  public:
    using Notifier<Event>::notify;
    using Notifier<Events...>::notify;
  };

  template <class Event>
  class Notifier<Event>
  {
  public:
    virtual ~Notifier()
    {
      // Remove remaining pointers to this to avoid segfaults
      for (ObserverInterface<Event>* o : observers_)
        o->unset();
    }

    /// Call the \ref update method on all attached observers.
    void notify(Event const& e)
    {
      for (ObserverInterface<Event>* o : observers_)
        o->update(e);
    }

    /// Attach a new observer that gets called on \ref notify
    void attach(ObserverInterface<Event>* o)
    {
      observers_.insert(o);
    }

    /// Detaches the passed observer from the list, if stored.
    void detach(ObserverInterface<Event>* o)
    {
      observers_.erase(o);
    }

  private:
    std::set<ObserverInterface<Event>*> observers_;
  };


  /// Implementation of the \ref ObserverInterface
  template <class Event, class... Tags>
  class Observer
      : public ObserverInterface<Event>
  {
  public:
    template <class Notifier>
    Observer(Notifier const& notifier)
      : notifier_(const_cast<Notifier*>(&notifier))
    {
      notifier_->attach(this);
    }

    /// Destructor, detaches from the notifier
    virtual ~Observer()
    {
      if (notifier_)
        notifier_->detach(this);
    }

    /// Copy constructor. Attaches this to the copied notifier
    Observer(Observer const& other)
      : notifier_(other.notifier_)
    {
      notifier_->attach(this);
    }

    /// Copy-assignment operator, copies the notifier and attaches this.
    Observer& operator=(Observer const& other)
    {
      if (&other != this) {
        notifier_->detach(this);
        notifier_ = other.notifier_;
        notifier_->attach(this);
      }
      return *this;
    }

    /// Set the Notifier* to nullptr. Used by the Notifier to avoid segfaults when destruction occurs
    /// out of order.
    void unset() final
    {
      notifier_ = nullptr;
    }

    /// Implementation of the interface method \ref ObserverInterface::update.
    /// Redirects to the \ref updateImpl method with additional \ref Tags parameters
    void update(Event e) final
    {
      updateImpl(e, Tags{}...);
    }

  protected:
    /// \brief Implementation of the update method in derived class
    // NOTE: The additional `Tags...` arguments can be used to distinguish
    //       between multiple observers of the same event.
    virtual void updateImpl(Event e, Tags...) = 0;

  private:
    Notifier<Event>* notifier_ = nullptr;
  };


  namespace Impl
  {
    template <class Event, class Tags>
    class ObserverSequenceImpl;

    /// Combination of multiple observers of the same event but with different tags
    template <class Event, std::size_t... Is>
    class ObserverSequenceImpl<Event, std::index_sequence<Is...>>
        : private Observer<Event,index_t<Is>>...
    {
    public:
      template <class... Notifiers,
        REQUIRES(sizeof...(Notifiers) == sizeof...(Is))>
      ObserverSequenceImpl(Notifiers&&... notifiers)
        : Observer<Event,index_t<Is>>(FWD(notifiers))...
      {}
    };

  } // end namespace Impl

  template <class Event, std::size_t N>
  using ObserverSequence = Impl::ObserverSequenceImpl<Event, std::make_index_sequence<N>>;

} // end namespace AMDiS