UiKitView should participate in the Flutter focus tree

[amirh]

Right now selecting a text field inside a WebView doesn't change the framework's focus state so e.g a previously focused Flutter text field is still focused(and will show the caret animation).

On Android we've added added a focus node for AndroidViews which stays in sync with the platform view's focus state.

This is what it currently looks like on iOS when focus is moved from a Flutter text field to a WebView text field:

[amirh]

cc @cyanglaz

[markusaksli-nc]

Reproducible on the latest master 1.21.0-6.0.pre.223 using webview_flutter: ^0.3.22+1. On Android this does not repro but #34169 does instead.

Minimal reproducible code sample

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Sample'),
        ),
        body: SingleChildScrollView(
          child: Column(
            children: [
              TextField(),
              TextField(),
              Container(
                height: 400,
                child: WebView(initialUrl: "https://www.google.com/"),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

flutter doctor -v

[✓] Flutter (Channel master, 1.21.0-6.0.pre.223, on Mac OS X 10.15.6 19G73, locale en-GB)
    • Flutter version 1.21.0-6.0.pre.223 at /Users/markus/development/flutter_master
    • Framework revision 6d487355a3 (7 hours ago), 2020-08-05 20:21:02 -0700
    • Engine revision d728f691f4
    • Dart version 2.10.0 (build 2.10.0-3.0.dev d5ff386abe)

 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.1)
    • Android SDK at /Users/markus/Library/Android/sdk
    • Platform android-30, build-tools 30.0.1
    • Java binary at: /Users/markus/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.6, Build version 11E708
    • CocoaPods version 1.9.3

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.0)
    • Android Studio at /Users/markus/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android Studio.app/Contents
    • Flutter plugin version 48.0.2
    • Dart plugin version 193.7361
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] Connected device (4 available)
    • Nevercode’s iPhone (mobile) • b668e524315069f3db3661ac11ff1f66afafebdb • ios            • iOS 13.6
    • macOS (desktop)             • macos                                    • darwin-x64     • Mac OS X 10.15.6 19G73
    • Web Server (web)            • web-server                               • web-javascript • Flutter Tools
    • Chrome (web)                • chrome                                   • web-javascript • Google Chrome 84.0.4147.105

• No issues found!