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10410248
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2012-05-02T08:33:53.783
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2012-05-02T09:43:59.740
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2012-05-02T09:43:59.740
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661519
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10409964
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2
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I think it is easier, and more scalable, to define a set of event types that each observer can listen to. Then you register the observer to listen to that particular event type. The observed then keeps a list of observers registered for each event, and notifies them if and when the event occurs. Using a combination of <code>std::function</code>, <code>std::bind</code> (or <code>boost</code> equivalents), it is easy to register callbacks for a given event type. You could put the callbacks in a map of event type to callback. For example, something along these lines (almost pseudo-code, has not been tested) <pre><code>class Publisher { public : void subscribe(const std::string& event, std::function<void(double)> callback) { m_subscribers[s].push_back(callback); } void publish(const std::string& event) const { for (auto& f : m_subscribers[event]) f( some double );} void event(const std::string& event) const { publish(event);} private: // map of event types (here simply strings) to list of callbacks std::map<std::string&, std::list<std::function<void(const std::string&)>>> m_subscribers; }; struct Foo { void foo(double x) { std::cout << "Foo received message: " << x << "\n"; } }; struct Bar { void bar(double x) { std::cout << "Bar received message: " << x << "\n"; } }; int main() { Publisher pub; Foo f0; Foo f1; Bar bar0; pub.subscribe("RED", std::bind(&Foo::foo, &foo0, _1)); pub.subscribe("GREEN", std::bind(&Foo::foo, &foo1, _1)); pub.subscribe("WHITE", std::bind(&Foo::foo, &foo1, _1)); pub.subscribe("RED", std::bind(&Bar::bar, &bar0, _1)); pub.subscribe("BLUE", std::bind(&Bar::bar, &bar0, _1)); pub.subscribe("MAGENTA", std::bind(&Bar::bar, &bar0, _1)); // trigger a "GREEN" event pub.event("GREEN"); } </code></pre> Here, the observers (or subscribers) register to some events, represented by strings here, and their registered callbacks get called when this event happens. In the example above I manually trigger an event to illustrate the mechanism. This event-callback mechanism allows to decouple the actual items from the callback action. The Observed (or publisher) knows what parameter to pass the callback for a given event, and which callbacks to call, so the observers are not dependent on the internal data of the observed object.
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1. POImplementing Observer pattern when observers wish to observe different items
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1. USjuanchopanza
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