[webview_flutter_platform_interface] Provide flexible API for loading local HTML files (#9697)

[https://github.com/flutter/flutter/issues/136479](https://github.com/flutter/flutter/issues/136479)

These are the platform interface changes, after the main PR was approved.
[https://github.com/flutter/packages/pull/8787](https://github.com/flutter/packages/pull/8787)

What was changed:
* Added `LoadFileParams` to specify parameters for loading local HTML pages.
* `PlatformWebViewController` class extended with the new method `loadFileWithParams(LoadFileParams)`.
* Unit tests for the new functionality of the `PlatformWebViewController` were added.

## Pre-Review Checklist

Author: agavrilko

packages/webview_flutter/webview_flutter_platform_interface/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.14.0
+
+* Adds `PlatformWebViewController.loadFileWithParams(LoadFileParams)` to support loading local HTML files with platform-specific parameters.
+
 ## 2.13.1
 
 * Updates minimum supported SDK version to Flutter 3.27/Dart 3.6.

packages/webview_flutter/webview_flutter_platform_interface/lib/src/platform_webview_controller.dart

@@ -61,6 +61,20 @@ abstract class PlatformWebViewController extends PlatformInterface {
         'loadFile is not implemented on the current platform');
   }
 
+  /// Loads a local HTML file using the provided [params].
+  ///
+  /// The [params.absoluteFilePath] should contain the absolute path to the file
+  /// on the device.
+  /// For example: `/Users/username/Documents/www/index.html`.
+  ///
+  /// Platform-specific implementations may extend [LoadFileParams] to support
+  /// additional parameters, such as iOS/macOS-specific read access options.
+  Future<void> loadFileWithParams(
+    LoadFileParams params,
+  ) {
+    return loadFile(params.absoluteFilePath);
+  }
+
   /// Loads the Flutter asset specified in the pubspec.yaml file.
   ///
   /// Throws an ArgumentError if [key] is not part of the specified assets

packages/webview_flutter/webview_flutter_platform_interface/lib/src/types/load_file_params.dart

@@ -0,0 +1,38 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+
+/// Object specifying parameters for loading a local HTML file into a web view.
+///
+/// Platform-specific implementations can add additional fields by extending
+/// this class.
+///
+/// This example demonstrates how to extend [LoadFileParams] to provide
+/// additional platform-specific parameters.
+///
+/// When extending [LoadFileParams], additional parameters should always accept
+/// `null` or have a default value to prevent breaking changes.
+///
+/// ```dart
+/// class WebKitLoadFileParams extends LoadFileParams {
+///   const WebKitLoadFileParams({
+///     required super.absoluteFilePath,
+///     required this.readAccessPath,
+///   });
+///
+///   /// The directory to which the WebView is granted read access.
+///   final String readAccessPath;
+/// }
+/// ```
+@immutable
+base class LoadFileParams {
+  /// Creates a new [LoadFileParams] object.
+  const LoadFileParams({
+    required this.absoluteFilePath,
+  });
+
+  /// The path to the local HTML file to be loaded.
+  final String absoluteFilePath;
+}

packages/webview_flutter/webview_flutter_platform_interface/lib/src/types/types.dart

@@ -9,6 +9,7 @@ export 'javascript_dialog_request.dart';
 export 'javascript_log_level.dart';
 export 'javascript_message.dart';
 export 'javascript_mode.dart';
+export 'load_file_params.dart';
 export 'load_request_params.dart';
 export 'navigation_decision.dart';
 export 'navigation_request.dart';

packages/webview_flutter/webview_flutter_platform_interface/pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/flutter/packages/tree/main/packages/webview_flutt
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview_flutter%22
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 2.13.1
+version: 2.14.0
 
 environment:
   sdk: ^3.6.0

packages/webview_flutter/webview_flutter_platform_interface/test/platform_webview_controller_test.dart

@@ -69,6 +69,22 @@ void main() {
     );
   });
 
+  test('loadFileWithParams redirects to loadFile with correct path', () async {
+    final LoadFileSpyPlatformWebViewController controller =
+        LoadFileSpyPlatformWebViewController(
+      const PlatformWebViewControllerCreationParams(),
+    );
+
+    const String testPath = 'file:///test/index.html';
+    const LoadFileParams params = LoadFileParams(
+      absoluteFilePath: testPath,
+    );
+
+    await controller.loadFileWithParams(params);
+
+    expect(controller.loadFilePath, equals(testPath));
+  });
+
   test(
       'Default implementation of loadFlutterAsset should throw unimplemented error',
       () {
@@ -528,6 +544,17 @@ class ExtendsPlatformWebViewController extends PlatformWebViewController {
   ExtendsPlatformWebViewController(super.params) : super.implementation();
 }
 
+class LoadFileSpyPlatformWebViewController extends PlatformWebViewController {
+  LoadFileSpyPlatformWebViewController(super.params) : super.implementation();
+
+  String? loadFilePath;
+
+  @override
+  Future<void> loadFile(String absoluteFilePath) async {
+    loadFilePath = absoluteFilePath;
+  }
+}
+
 // ignore: must_be_immutable
 class MockLoadRequestParamsDelegate extends Mock
     with