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diff --git a/Firestore/core/include/firebase/firestore/event_listener.h b/Firestore/core/include/firebase/firestore/event_listener.h
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+++ b/Firestore/core/include/firebase/firestore/event_listener.h
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+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// TODO(rsgowman): This file isn't intended to be used just yet. It's just an
+// outline of what the API might eventually look like. Most of this was
+// shamelessly stolen and modified from rtdb's header file, melded with the
+// (java) firestore api.
+
+#ifndef FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_EVENT_LISTENER_H_
+#define FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_EVENT_LISTENER_H_
+
+namespace firebase {
+namespace firestore {
+
+// TODO(rsgowman): replace these forward decl's with appropriate includes (once
+// they exist)
+class Error;
+
+/**
+ * @brief An interface for event listeners.
+ */
+template <typename T>
+class EventListener {
+ public:
+  /**
+   * @brief OnEvent will be called with the new value or the error if an error
+   * occurred.
+   *
+   * It's guaranteed that exactly one of value or error will be non-null.
+   *
+   * @param value The value of the event. null if there was an error.
+   * @param error The error if there was error. null otherwise.
+   */
+  void OnEvent(const T* value, const Error* error);
+};
+
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_EVENT_LISTENER_H_