CallbackProxy.h

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#ifndef Framework_CallbackProxy_h
#define Framework_CallbackProxy_h
// -*- C++ -*-
//
// Package:     Framework
// Class  :     CallbackProxy
// 
//
// Author:      Chris Jones
// Created:     Fri Apr  8 11:50:21 CDT 2005
//

// system include files
#include <cassert>
#include <memory>

// user include files
#include "FWCore/Framework/interface/DataProxy.h"
#include "FWCore/Framework/interface/EventSetupRecord.h"

#include "FWCore/Framework/interface/produce_helpers.h"
#include "FWCore/Utilities/interface/propagate_const.h"

// forward declarations
namespace edm {
   namespace eventsetup {

      template<class CallbackT, class RecordT, class DataT>
      class CallbackProxy : public DataProxy {
         
      public:
         typedef  typename produce::smart_pointer_traits<DataT>::type value_type;
         typedef  RecordT record_type;
         
         CallbackProxy(std::shared_ptr<CallbackT>& iCallback) :
         data_(),
         callback_(iCallback) { 
            //The callback fills the data directly.  This is done so that the callback does not have to
            //  hold onto a temporary copy of the result of the callback since the callback is allowed
            //  to return multiple items where only one item is needed by this Proxy
            iCallback->holdOntoPointer(&data_) ; }
         ~CallbackProxy() override {
            DataT* dummy(nullptr);
            callback_->holdOntoPointer(dummy) ;
         }
         // ---------- const member functions ---------------------
         
         // ---------- static member functions --------------------
         
         // ---------- member functions ---------------------------
         const void* getImpl(const EventSetupRecordImpl& iRecord, const DataKey&) override {
            assert(iRecord.key() == RecordT::keyForClass());
            record_type rec;
            rec.setImpl(&iRecord);
            (*callback_)(rec);
            return &(*data_);
         }
         
         void invalidateCache() override {
            data_ = DataT();
            callback_->newRecordComing();
         }
      private:
         CallbackProxy(const CallbackProxy&) = delete; // stop default
         
         const CallbackProxy& operator=(const CallbackProxy&) = delete; // stop default
         
         // ---------- member data --------------------------------
         DataT data_;
         edm::propagate_const<std::shared_ptr<CallbackT>> callback_;
      };
      
   }
}

#endif