Add FirebaseFirestore.ListenForSnapshotsInSync()

This is the first in a series that implements a new pattern for managing listener callbacks. Previously listeners needed to register a callback with C++ and maintain a dictionary of callbacks keyed off of a unique id and cleaned up once listening stopped or the firestore instance was disposed. The pattern wasn't implemented properly and had to be redone.

This CL reimplements the pattern by:
1. Introducing a helper class, `ListenerRegistrationMap`, to keep a dictionary of all the callbacks. A static instance of this will get introduced for each `Listen*` method.
1. Updating `ListenerRegistration` to unregister callbacks when `Stop()` is called.
1. The C++ listener class that accepts the callback from C# will receive it in the native listen method instead of requiring a global callback registration.
1. In a future CL all the callbacks attached to a specific firestore instance will be pruned from its `ListenerRegistrationMap` instance.

Once this pattern has been approved in this CL it will get applied to other `Listen*` methods in the API.

PiperOrigin-RevId: 312713181

Author: Googler

firestore/src/include/firebase/csharp/snapshots_in_sync_listener.cc

@@ -0,0 +1,26 @@
+#include "firebase/csharp/snapshots_in_sync_listener.h"
+
+#include "app/src/assert.h"
+
+namespace firebase {
+namespace firestore {
+namespace csharp {
+
+void SnapshotsInSyncListener::OnEvent(Error error) {
+  firebase::callback::AddCallback(
+      firebase::callback::NewCallback(callback_, callback_id_));
+}
+
+/* static */
+ListenerRegistration SnapshotsInSyncListener::AddListenerTo(
+    Firestore* firestore, int32_t callback_id,
+    SnapshotsInSyncCallback callback) {
+  SnapshotsInSyncListener listener(callback_id, callback);
+
+  return firestore->AddSnapshotsInSyncListener(
+      [listener]() mutable { listener.OnEvent(Error::kOk); });
+}
+
+}  // namespace csharp
+}  // namespace firestore
+}  // namespace firebase

firestore/src/include/firebase/csharp/snapshots_in_sync_listener.h

@@ -0,0 +1,57 @@
+#ifndef FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_SNAPSHOTS_IN_SYNC_LISTENER_H_
+#define FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_SNAPSHOTS_IN_SYNC_LISTENER_H_
+
+#include <cstdint>
+
+#include "app/src/callback.h"
+#include "firebase/firestore.h"
+#include "firebase/firestore/event_listener.h"
+#include "firebase/firestore/listener_registration.h"
+#include "firebase/firestore/firestore_errors.h"
+
+namespace firebase {
+namespace firestore {
+namespace csharp {
+
+// Add this to make this header compile when SWIG is not involved.
+#ifndef SWIGSTDCALL
+#if !defined(SWIG) && \
+    (defined(_WIN32) || defined(__WIN32__) || defined(__CYGWIN__))
+#define SWIGSTDCALL __stdcall
+#else
+#define SWIGSTDCALL
+#endif
+#endif
+
+// The callbacks that are used by the listener, that need to reach back to C#
+// callbacks.
+typedef void(SWIGSTDCALL* SnapshotsInSyncCallback)(int callback_id);
+
+// Provides a C++ implementation of the EventListener for
+// ListenForSnapshotsInSync that can forward the calls back to the C# delegates.
+class SnapshotsInSyncListener : public EventListener<void> {
+ public:
+  SnapshotsInSyncListener(int32_t callback_id,
+                          SnapshotsInSyncCallback callback)
+      : callback_id_(callback_id), callback_(callback) {}
+
+  void OnEvent(Error error) override;
+
+  // This method is a proxy to Firestore::AddSnapshotsInSyncListener()
+  // that can be easily called from C#. It allows our C# wrapper to
+  // track user callbacks in a dictionary keyed off of a unique int
+  // for each user callback and then raise the correct one later.
+  static ListenerRegistration AddListenerTo(Firestore* firestore,
+                                            int32_t callback_id,
+                                            SnapshotsInSyncCallback callback);
+
+ private:
+  int32_t callback_id_;
+  SnapshotsInSyncCallback callback_;
+};
+
+}  // namespace csharp
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIREBASE_FIRESTORE_CLIENT_CPP_SRC_INCLUDE_FIREBASE_CSHARP_SNAPSHOTS_IN_SYNC_LISTENER_H_