etl/docs/patterns/observer.md

title
observer

A set of template classes to enable the easy creation of objects using the Observer Pattern.

The observer pattern is a software design pattern in which an object maintains a list of observers, and notifies of any changes. It is often used to implement event handling systems.

There are two templated classes:-

observer

Derive observers from this class.

C++03
Up to eight different notification messages may be defined.

template <typename T1,        typename T2 = void, typename T3 = void, typename T4 = void,
          typename T5 = void, typename T6 = void, typename T7 = void, typename T8 = void>
class observer

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C++11 and above
Any number of notification messages may be defined.

template <typename... Types>
class observer

---

observer defines a pure virtual notification member function for each defined type, taking the type as a parameter.
The parameter type may be void (Since 20.39.0).
Derived classes are forced to implement these functions.

Example

// An observer type that handles int (by value), std::string (by reference)
// and std::complex (by const reference).
typedef etl::observer<int, std::string&, const std::complex&> observer_type;

class my_observer : public observer_type
{
public:
  void notification(int i);
  void notification(std::string& s);
  void notification(const std::complex);
};

observable

This class is observable from a class derived from observer.
Derive observable classes from this.

template <typename TObserver, const size_t MAX_OBSERVERS>
class observable

void add_observer(TObserver& observer)

Description Adds an observer to the list. If the maximum number of observers have already been added then an etl::observer_list_full is emitted.

---

void remove_observer(TObserver& observer)

Remove an observer from the list. If the observer is not in the list then there is no change.
Do not call this from within a notification override.

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void enable_observer(TObserver& observer, bool state = true)

Enables / disables an observer according to the parameter value.
The default is true.
May be called from a notification override.

---

void disable_observer(TObserver& observer)

Disables an observer from being called from notify_observers.
May be called from a notification override.

---

void clear_observers()

Removes all observers from the list.

---

size_type number_of_observers() const

Returns the number of observers currently in the list.

---

template <typename TNotification>
void notify_observers(TNotification data)

By default, the parameter is passed by as the actual data.
To pass by reference or rvalue reference, add the type as a template parameter.

notify_observers<const MyData&>(data);