v1 using observer pattern

subscriber-notifs/include/app/inventory_manager.hpp

@@ -0,0 +1,20 @@
+
+#pragma once
+
+#include <string>
+#include <unordered_map>
+
+#include "core/subscribable.hpp"
+
+using ProductCount = int;
+using ProductId = std::string;
+
+// inventory manager is not generic ( works with the specific types ProductId
+// and ProductCount ) but the subscribable is generic
+class InventoryManager : public Subscribable<ProductId, ProductCount> {
+   private:
+    std::unordered_map<ProductId, int> inventory_;
+
+   public:
+    void setInventory(const ProductId& productId, ProductCount quantity);
+};

subscriber-notifs/include/app/services.hpp

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+#pragma once
+
+#include "app/inventory_manager.hpp"
+#include "core/subscriber.hpp"
+
+class SmsService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};
+
+class EmailService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};
+
+class LogService : public Subscriber<ProductId, ProductCount> {
+   public:
+    void onNotification(const ProductId& productId,
+                        const ProductCount& quantity) override;
+};

subscriber-notifs/include/core/subscribable.hpp

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <unordered_map>
+#include <vector>
+
+#include "subscriber.hpp"
+
+template <typename TEventId, typename TEventInfo>
+class Subscribable {
+    using TSubscriber = Subscriber<TEventId, TEventInfo>;
+
+   private:
+    std::unordered_map<TEventId, std::vector<TSubscriber*>> subscribersMap_;
+
+   public:
+    void subscribe(const TEventId& eventId, TSubscriber* subscriber) {
+        subscribersMap_[eventId].push_back(subscriber);
+    }
+
+    void unsubscribe(const TEventId& eventId, TSubscriber* subscriber) {
+        auto entry = subscribersMap_.find(eventId);
+        if (entry == subscribersMap_.end()) {
+            return;
+        }
+
+        std::erase(entry->second, subscriber);
+        if (entry->second.empty()) {
+            subscribersMap_.erase(entry);
+        }
+    }
+
+    void notify(const TEventId& eventId, const TEventInfo& eventInfo) {
+        auto entry = subscribersMap_.find(eventId);
+        if (entry == subscribersMap_.end()) {
+            return;
+        }
+
+        for (TSubscriber* subscriber : entry->second) {
+            subscriber->onNotification(eventId, eventInfo);
+        }
+    }
+};

subscriber-notifs/include/core/subscriber.hpp

@@ -0,0 +1,10 @@
+#pragma once
+
+template <typename TEventId, typename TEventInfo>
+class Subscriber {
+   public:
+    virtual void onNotification(const TEventId& eventId,
+                                const TEventInfo& eventInfo) = 0;
+
+    virtual ~Subscriber() = default;
+};