[webview_flutter] Add listener for content offset (Platform Interface) (

flutter#5427)

Adds the platform interface implementation for content offset listener

This PR is part of a series of PRs that aim to close flutter/flutter#31027.

The PR that contains all changes can be found at flutter#3444.

packages/webview_flutter/webview_flutter_platform_interface/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.8.0
+
+* Adds support to track scroll position changes. See `PlatformWebViewController.setOnScrollPositionChange`.
+
 ## 2.7.0
 
 * Adds `WebResourceRequest` and `WebResourceResponse` to `HttpResponseError`.

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

@@ -286,6 +286,14 @@ abstract class PlatformWebViewController extends PlatformInterface {
       'setOnConsoleMessage is not implemented on the current platform',
     );
   }
+
+  /// Sets the listener for content offset changes.
+  Future<void> setOnScrollPositionChange(
+      void Function(ScrollPositionChange scrollPositionChange)?
+          onScrollPositionChange) {
+    throw UnimplementedError(
+        'setOnScrollPositionChange is not implemented on the current platform');
+  }
 }
 
 /// Describes the parameters necessary for registering a JavaScript channel.

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

@@ -0,0 +1,17 @@
+// 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.
+
+/// Defines the parameters of the scroll position change callback.
+class ScrollPositionChange {
+  /// Creates a [ScrollPositionChange].
+  const ScrollPositionChange(this.x, this.y);
+
+  /// The value of the horizontal offset with the origin being at the leftmost
+  /// of the `WebView`.
+  final double x;
+
+  /// The value of the vertical offset with the origin being at the topmost of
+  /// the `WebView`.
+  final double y;
+}

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

@@ -16,6 +16,7 @@ export 'platform_webview_controller_creation_params.dart';
 export 'platform_webview_cookie_manager_creation_params.dart';
 export 'platform_webview_permission_request.dart';
 export 'platform_webview_widget_creation_params.dart';
+export 'scroll_position_change.dart';
 export 'url_change.dart';
 export 'web_resource_error.dart';
 export 'webview_cookie.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: 2.7.0
+version: 2.8.0
 
 environment:
   sdk: ">=3.0.0 <4.0.0"