The Observer is a useful pattern to decouple producers and consumers by registering consumers with a producer. On specific events, the producer will call the API to send data to each consumer (observer). It’s similar to app notifications.

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TLM ports and exports depend heavily on the Observer pattern. Whenever connect is called, the export is added to the list of observers for ports. When a new object is generated, the ports call back all the observers.

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

// Observer interface
class Observer {
public:
    virtual void update(const std::string& message) = 0;
    virtual ~Observer() = default;
};

// Subject interface
class Subject {
private:
    std::vector<Observer*> observers;
    
public:
    void addObserver(Observer* observer) {
        observers.push_back(observer);
    }
    
    void removeObserver(Observer* observer) {
        observers.erase(
            std::remove(observers.begin(), observers.end(), observer),
            observers.end()
        );
    }
    
    void notifyObservers(const std::string& message) {
        for (auto observer : observers) {
            observer->update(message);
        }
    }
};

// Concrete Subject - Weather Station
class WeatherStation : public Subject {
private:
    float temperature;
    float humidity;
    
public:
    void setWeatherData(float temp, float hum) {
        temperature = temp;
        humidity = hum;
        
        std::string weatherUpdate = "Temperature: " + std::to_string(temperature) + 
                                  "°C, Humidity: " + std::to_string(humidity) + "%";
        notifyObservers(weatherUpdate);
    }
    
    float getTemperature() const { return temperature; }
    float getHumidity() const { return humidity; }
};

// Concrete Observer - Mobile App
class MobileApp : public Observer {
private:
    std::string appName;
    
public:
    MobileApp(const std::string& name) : appName(name) {}
    
    void update(const std::string& message) override {
        std::cout << "[" << appName << " App] Weather Update: " << message << std::endl;
    }
};

// Concrete Observer - Display Board
class DisplayBoard : public Observer {
private:
    std::string location;
    
public:
    DisplayBoard(const std::string& loc) : location(loc) {}
    
    void update(const std::string& message) override {
        std::cout << "[Display at " << location << "] " << message << std::endl;
    }
};

// Concrete Observer - Alert System
class AlertSystem : public Observer {
private:
    float alertThreshold;
    
public:
    AlertSystem(float threshold) : alertThreshold(threshold) {}
    
    void update(const std::string& message) override {
        std::cout << "[Alert System] " << message;
        
        // Simple alert logic (parse temperature from message)
        size_t pos = message.find("Temperature: ");
        if (pos != std::string::npos) {
            float temp = std::stof(message.substr(pos + 13));
            if (temp > alertThreshold) {
                std::cout << " ⚠️  HIGH TEMPERATURE ALERT!";
            }
        }
        std::cout << std::endl;
    }
};

int main() {
    std::cout << "=== Observer Pattern Demo ===" << std::endl;
    
    // Create weather station (subject)
    WeatherStation weatherStation;
    
    // Create observers
    MobileApp mobileApp("WeatherPro");
    DisplayBoard displayBoard("City Center");
    AlertSystem alertSystem(30.0f); // Alert if temp > 30°C
    
    // Register observers
    weatherStation.addObserver(&mobileApp);
    weatherStation.addObserver(&displayBoard);
    weatherStation.addObserver(&alertSystem);
    
    std::cout << "\n1. Initial weather update:" << std::endl;
    weatherStation.setWeatherData(25.5f, 60.0f);
    
    std::cout << "\n2. Hot weather update:" << std::endl;
    weatherStation.setWeatherData(35.0f, 45.0f);
    
    std::cout << "\n3. Removing mobile app observer:" << std::endl;
    weatherStation.removeObserver(&mobileApp);
    
    std::cout << "\n4. Weather update after removal:" << std::endl;
    weatherStation.setWeatherData(28.0f, 70.0f);
    
    return 0;
}