feat(cli): add --firebase-out flag for custom firebase.json path

packages/flutterfire_cli/lib/src/commands/config.dart

@@ -188,6 +188,12 @@ class ConfigCommand extends FlutterFireCommand {
           'Where to write the `google-services.json` file to be written for android platform. Useful for different flavors',
     );
 
+    argParser.addOption(
+      kFirebaseOutFlag,
+      valueHelp: 'filePath',
+      help: 'The output file path of the `firebase.json` file that will be generated or updated.',
+    );
+
     argParser.addFlag(
       kOverwriteFirebaseOptionsFlag,
       abbr: 'f',
@@ -345,6 +351,12 @@ class ConfigCommand extends FlutterFireCommand {
     return argResults!['out'] as String;
   }
 
+  String get firebaseJsonPath {
+    final customPath = argResults![kFirebaseOutFlag] as String?;
+    if (customPath != null) return customPath;
+    return path.join(flutterApp!.package.path, 'firebase.json');
+  }
+
   bool get overwriteFirebaseOptions {
     if (argResults!['overwrite-firebase-options'] == null) {
       return false;
@@ -536,8 +548,6 @@ class ConfigCommand extends FlutterFireCommand {
   }
 
   Future<bool> checkIfUserRequiresReconfigure() async {
-    final firebaseJsonPath =
-        path.join(flutterApp!.package.path, 'firebase.json');
     final file = File(firebaseJsonPath);
 
     if (file.existsSync()) {
@@ -550,7 +560,7 @@ class ConfigCommand extends FlutterFireCommand {
             );
 
         if (reuseFirebaseJsonValues) {
-          final reconfigure = Reconfigure(flutterApp, token: testAccessToken);
+          final reconfigure = Reconfigure(flutterApp, token: testAccessToken, firebaseJsonPath: firebaseJsonPath);
           reconfigure.logger = logger;
           await reconfigure.run();
           return true;
@@ -705,12 +715,11 @@ class ConfigCommand extends FlutterFireCommand {
         firebaseJsonWrites.add(firebaseJsonWrite);
       }
 
-      // 5. Writes for "firebase.json" file in root of project
+      // 5. Writes for "firebase.json" file
       if (firebaseJsonWrites.isNotEmpty) {
         await writeToFirebaseJson(
           listOfWrites: firebaseJsonWrites,
-          firebaseJsonPath:
-              path.join(flutterApp!.package.path, 'firebase.json'),
+          firebaseJsonPath: firebaseJsonPath,
         );
       }
 

packages/flutterfire_cli/lib/src/commands/reconfigure.dart

@@ -56,18 +56,26 @@ class ConfigFileWrite {
 }
 
 class Reconfigure extends FlutterFireCommand {
-  Reconfigure(FlutterApp? flutterApp, {String? token}) : super(flutterApp) {
+  Reconfigure(FlutterApp? flutterApp, {String? token, String? firebaseJsonPath}) : super(flutterApp) {
     setupDefaultFirebaseCliOptions();
     _accessToken = token;
+    _firebaseJsonPath = firebaseJsonPath;
     argParser.addOption(
       'ci-access-token',
       valueHelp: 'ciAccessToken',
       hide: true,
       help:
           'Set the access token for making Firebase API requests. Required for CI environment.',
     );
+    argParser.addOption(
+      kFirebaseOutFlag,
+      valueHelp: 'filePath',
+      help: 'The path to the `firebase.json` file.',
+    );
   }
 
+  String? _firebaseJsonPath;
+
   @override
   final String description =
       'Updates the configurations for all build variants included in the "firebase.json" added by running `flutterfire configure`.';
@@ -378,11 +386,14 @@ class Reconfigure extends FlutterFireCommand {
   Future<void> run() async {
     try {
       commandRequiresFlutterApp();
+      final customPath = argResults != null ? argResults![kFirebaseOutFlag] as String? : null;
       final firebaseJson = File(
-        path.join(
-          flutterApp!.package.path,
-          'firebase.json',
-        ),
+        _firebaseJsonPath ??
+            customPath ??
+            path.join(
+              flutterApp!.package.path,
+              'firebase.json',
+            ),
       );
 
       if (!firebaseJson.existsSync()) {

packages/flutterfire_cli/lib/src/common/utils.dart

@@ -79,6 +79,7 @@ const String kMacosTargetFlag = 'macos-target';
 const String kIosOutFlag = 'ios-out';
 const String kMacosOutFlag = 'macos-out';
 const String kAndroidOutFlag = 'android-out';
+const String kFirebaseOutFlag = 'firebase-out';
 const String kOverwriteFirebaseOptionsFlag = 'overwrite-firebase-options';
 const String kTestAccessTokenFlag = 'test-access-token';
 

packages/flutterfire_cli/test/configure_test.dart

@@ -1740,4 +1740,52 @@ void main() {
       Duration(minutes: 3),
     ),
   );
+
+  test('flutterfire configure: --firebase-out flag should dictate where firebase.json is written', () async {
+    // Install flutterfire_cli from local path
+    final installDevDependency = Process.runSync(
+      'flutter',
+      [
+        'pub',
+        'add',
+        '--dev',
+        'flutterfire_cli',
+        '--path=${Directory.current.path}',
+      ],
+      workingDirectory: projectPath,
+    );
+
+    if (installDevDependency.exitCode != 0) {
+      fail(installDevDependency.stderr as String);
+    }
+
+    const customFirebaseJsonPath = 'custom/firebase.json';
+    final result = Process.runSync(
+      'dart',
+      [
+        'run',
+        'flutterfire_cli:flutterfire',
+        'configure',
+        '--yes',
+        '--project=$firebaseProjectId',
+        '--platforms=android',
+        '--firebase-out=$customFirebaseJsonPath',
+      ],
+      workingDirectory: projectPath,
+      runInShell: true,
+    );
+
+    if (result.exitCode != 0) {
+      fail(result.stderr as String);
+    }
+
+    // check custom "firebase.json" was created and has correct content
+    final firebaseJsonFile = p.join(projectPath!, 'custom', 'firebase.json');
+    expect(File(firebaseJsonFile).existsSync(), true);
+
+    final firebaseJsonFileContent = await File(firebaseJsonFile).readAsString();
+    final decodedFirebaseJson = jsonDecode(firebaseJsonFileContent) as Map<String, dynamic>;
+
+    expect(decodedFirebaseJson[kFlutter], isNotNull);
+  });
 }