Require explicit typing for DocumentSnapshot decoding. DocumentReference decoding.

Firestore/Swift/Source/Codable/CodableErrors.swift

@@ -17,6 +17,7 @@
 public enum FirestoreDecodingError: Error {
   case decodingIsNotSupported(String)
   case fieldNameConflict(String)
+  case `internal`
 }
 
 public enum FirestoreEncodingError: Error {

Firestore/Swift/Source/Codable/DocumentReference+ReadDecodable.swift

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2021 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.
+ */
+
+import Foundation
+import FirebaseFirestore
+
+extension DocumentReference {
+  /// Encodes an instance of `Encodable` and overwrites the encoded data
+  /// to the document referred by this `DocumentReference`. If no document exists,
+  /// it is created. If a document already exists, it is overwritten.
+  ///
+  /// See `Firestore.Encoder` for more details about the encoding process.
+  ///
+  /// - Parameters:
+  ///   - value: An instance of `Encodable` to be encoded to a document.
+  ///   - encoder: An encoder instance to use to run the encoding.
+  ///   - completion: A block to execute once the document has been successfully
+  ///                 written to the server. This block will not be called while
+  ///                 the client is offline, though local changes will be visible
+  ///                 immediately.
+
+  public func getDocument<T: Decodable>(as type: T.Type,
+                                        with serverTimestampBehavior: ServerTimestampBehavior =
+                                          .none,
+                                        decoder: Firestore.Decoder? = nil,
+                                        completion: @escaping (Result<T, Error>) -> Void) {
+    getDocument { snapshot, error in
+      guard let snapshot = snapshot else {
+        completion(.failure(error ?? FirestoreDecodingError.internal))
+        return
+      }
+      let result = Result {
+        try snapshot.data(as: T.self,
+                          with: serverTimestampBehavior,
+                          decoder: decoder)
+      }
+      completion(result)
+    }
+  }
+}

Firestore/Swift/Source/Codable/DocumentSnapshot+ReadDecodable.swift

@@ -35,11 +35,9 @@ extension DocumentSnapshot {
   ///     default decoder.
   public func data<T: Decodable>(as type: T.Type,
                                  with serverTimestampBehavior: ServerTimestampBehavior = .none,
-                                 decoder: Firestore.Decoder? = nil) throws -> T? {
+                                 decoder: Firestore.Decoder? = nil) throws -> T {
     let d = decoder ?? Firestore.Decoder()
-    if let data = data(with: serverTimestampBehavior) {
-      return try d.decode(T.self, from: data, in: reference)
-    }
-    return nil
+    let data: Any = data(with: serverTimestampBehavior) ?? NSNull()
+    return try d.decode(T.self, from: data, in: reference)
   }
 }

Firestore/third_party/FirestoreEncoder/FirestoreDecoder.swift

@@ -38,7 +38,7 @@ extension Firestore {
     ///             decoded.
     /// - Returns: An instance of specified type by the first parameter.
     public func decode<T: Decodable>(_: T.Type,
-                                     from container: [String: Any],
+                                     from container: Any,
                                      in document: DocumentReference? = nil) throws -> T {
       let decoder = _FirestoreDecoder(referencing: container)
       if let doc = document {