Add zookeeper support

include/cppkafka/zookeeper/zookeeper_watcher.h

@@ -33,6 +33,9 @@
 #include <memory>
 #include <string>
 #include <chrono>
+#include <map>
+#include <functional>
+#include <mutex>
 #include <zookeeper/zookeeper.h>
 
 namespace cppkafka {
@@ -41,21 +44,33 @@ class ZookeeperWatcher {
 public:
     static const std::chrono::milliseconds DEFAULT_RECEIVE_TIMEOUT;
 
+    using WatcherCallback = std::function<void(const std::string& brokers)>;
+
     ZookeeperWatcher(const std::string& endpoint);
     ZookeeperWatcher(const std::string& endpoint, std::chrono::milliseconds receive_timeout);
 
+    void setup_watcher();
+
+    std::string subscribe(WatcherCallback callback); 
+    void unsubscribe(const std::string& id);
 
     std::string get_brokers();
+    size_t get_subscriber_count() const;
 private:
-    using HandlePtr = std::unique_ptr<zhandle_t, decltype(&zookeeper_close)>;
-
     static const std::string BROKERS_PATH;
 
+    using HandlePtr = std::unique_ptr<zhandle_t, decltype(&zookeeper_close)>;
+    using CallbackMap = std::map<std::string, WatcherCallback>;
+
     static void handle_event_proxy(zhandle_t* zh, int type, int state, const char* path,
                                    void* ctx);
     void handle_event(int type, int state, const char* path);
+    std::string generate_id();
 
     HandlePtr handle_;
+    CallbackMap callbacks_;
+    mutable std::mutex callbacks_mutex_;
+    size_t id_counter_{0};
 };
 
 } // cppkafka