Make CollectionReference implement Query (#32)

Given that C++ types are exposed as structs and structs do not support inheritance in Swift, the fact that the C++ `CollectionReference` inherits from `Query` is not exposed on the Swift side. We then workaround that by defining a protocol that both `Query` and `CollectionReference` can conform to.

Also expose `CollectionReference.path()` and `FieldValue.serverTimestamp()`.

Sources/FirebaseFirestore/CollectionReference+Swift.swift

@@ -12,4 +12,14 @@ extension CollectionReference {
   public func document(_ path: String) -> DocumentReference {
     swift_firebase.swift_cxx_shims.firebase.firestore.collection_document(self, std.string(path))
   }
+
+  public var path: String {
+    String(swift_firebase.swift_cxx_shims.firebase.firestore.collection_path(self))
+  }
+}
+
+extension CollectionReference: QueryProtocol {
+  public var _asQuery: Query {
+    swift_firebase.swift_cxx_shims.firebase.firestore.collection_as_query(self)
+  }
 }

Sources/FirebaseFirestore/DocumentSnapshot+Swift.swift

@@ -14,7 +14,6 @@ private typealias MapFieldValue = firebase.firestore.MapFieldValue
 public typealias DocumentSnapshot = firebase.firestore.DocumentSnapshot
 public typealias ServerTimestampBehavior = firebase.firestore.DocumentSnapshot.ServerTimestampBehavior
 public typealias GeoPoint = firebase.firestore.GeoPoint
-public typealias FieldValue = firebase.firestore.FieldValue
 
 extension DocumentSnapshot {
   public var reference: DocumentReference {

Sources/FirebaseFirestore/FieldValue+Swift.swift

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: BSD-3-Clause
+
+@_exported
+import firebase
+
+public typealias FieldValue = firebase.firestore.FieldValue
+
+extension FieldValue {
+  public static func serverTimestamp() {
+    ServerTimestamp()
+  }
+}

Sources/FirebaseFirestore/Query+Swift.swift

@@ -10,14 +10,27 @@ import Foundation
 
 public typealias Query = firebase.firestore.Query
 
-extension Query {
+// Any types that extend from Query can conform to QueryProtocol to provide the
+// functionality of Query. This is needed since structs in Swift do not support
+// inheritance and C++ classes are exposed as structs to Swift.
+public protocol QueryProtocol {
+  // This is an internal means to expose `Query` when the conforming type is
+  // intended to be a subclass of `Query`.
+  var _asQuery: Query { get }
+}
+
+extension Query: QueryProtocol {
+  public var _asQuery: Query { self }
+}
+
+extension QueryProtocol {
   public var firestore: Firestore {
-    swift_firebase.swift_cxx_shims.firebase.firestore.query_firestore(self)
+    swift_firebase.swift_cxx_shims.firebase.firestore.query_firestore(_asQuery)
   }
 
   // This variant is provided for compatibility with the ObjC API.
   public func getDocuments(source: FirestoreSource = .default, completion: @escaping (QuerySnapshot?, Error?) -> Void) {
-    let future = swift_firebase.swift_cxx_shims.firebase.firestore.query_get(self, source)
+    let future = swift_firebase.swift_cxx_shims.firebase.firestore.query_get(_asQuery, source)
     future.setCompletion({
       let (snapshot, error) = future.resultAndError
       DispatchQueue.main.async {
@@ -28,7 +41,7 @@ extension Query {
 
   public func getDocuments(source: FirestoreSource = .default) async throws -> QuerySnapshot {
     try await withCheckedThrowingContinuation { continuation in
-      let future = swift_firebase.swift_cxx_shims.firebase.firestore.query_get(self, source)
+      let future = swift_firebase.swift_cxx_shims.firebase.firestore.query_get(_asQuery, source)
       future.setCompletion({
         let (snapshot, error) = future.resultAndError
         if let error {
@@ -48,7 +61,7 @@ extension Query {
     typealias ListenerCallback = (QuerySnapshot?, Error?) -> Void
     let boxed = Unmanaged.passRetained(listener as AnyObject)
     let instance = swift_firebase.swift_cxx_shims.firebase.firestore.query_add_snapshot_listener(
-      self, { snapshot, errorCode, errorMessage, pvListener in
+      _asQuery, { snapshot, errorCode, errorMessage, pvListener in
         let callback = Unmanaged<AnyObject>.fromOpaque(pvListener!).takeUnretainedValue() as! ListenerCallback
 
         let error = NSError.firestore(errorCode, errorMessage: errorMessage)
@@ -79,37 +92,37 @@ extension Query {
 
   public func whereField(_ field: String, isEqualTo value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_equal_to(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 
   public func whereField(_ field: String, isNotEqualTo value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_not_equal_to(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 
   public func whereField(_ field: String, isLessThan value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_less_than(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 
   public func whereField(_ field: String, isLessThanOrEqualTo value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_less_than_or_equal_to(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 
   public func whereField(_ field: String, isGreaterThan value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_greater_than(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 
   public func whereField(_ field: String, isGreaterThanOrEqualTo value: Any) -> Query {
     swift_firebase.swift_cxx_shims.firebase.firestore.query_where_greater_than_or_equal_to(
-      self, std.string(field), firestoreValueOrFail(value)
+      _asQuery, std.string(field), firestoreValueOrFail(value)
     )
   }
 

Sources/firebase/include/FirebaseFirestore.hh

@@ -151,12 +151,24 @@ document_snapshot_id(::firebase::firestore::DocumentSnapshot snapshot) {
   return snapshot.id();
 }
 
+// MARK: CollectionReference
+
 inline ::firebase::firestore::DocumentReference
 collection_document(::firebase::firestore::CollectionReference collection,
                     const ::std::string &document_path) {
   return collection.Document(document_path);
 }
 
+inline ::std::string
+collection_path(const ::firebase::firestore::CollectionReference& collection) {
+  return collection.path();
+}
+
+inline ::firebase::firestore::Query
+collection_as_query(::firebase::firestore::CollectionReference collection) {
+  return collection;
+}
+
 typedef void (*DocumentSnapshotListenerTypedCallback)(
     const ::firebase::firestore::DocumentSnapshot *snapshot,
     ::firebase::firestore::Error error_code, const char *error_message,