observable.h

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#ifndef SENSESP_SYSTEM_OBSERVABLE_H
#define SENSESP_SYSTEM_OBSERVABLE_H
 
#include <forward_list>
#include <functional>
#include <vector>
 
namespace sensesp {

class Observable {
 public:
  struct ObserverEntry {
    int id;
    std::function<void()> callback;
  };
 
  Observable() {}

  Observable(Observable&& other)
      : observers_{std::move(other.observers_)},
        next_id_{other.next_id_} {}
 
  void notify() {
    for (const ObserverEntry& entry : observers_) {
      entry.callback();
    }
  }

  int attach(std::function<void()> observer) {
    int id = next_id_++;
    // Iterate to the last element
    auto before_end = observers_.before_begin();
    for (auto& _ : observers_) {
      ++before_end;
    }
    // Insert the new observer
    observers_.insert_after(before_end, ObserverEntry{id, observer});
    return id;
  }

  void detach(int id) {
    observers_.remove_if(
        [id](const ObserverEntry& entry) { return entry.id == id; });
  }
 
 private:
  std::forward_list<ObserverEntry> observers_;
  int next_id_ = 0;
};
 
}  // namespace sensesp
 
#endif