[iOS] Ensure the keyboard bottom inset is zero when keyboard dismiss.

[luckysmg]

This PR is to ensure the viewport metrics is correct when keyboard dismiss.

Related issue:
flutter/flutter#99951
flutter/flutter#100220

Demo code for first issue

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

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);
  
  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: (){
        FocusScope.of(context).unfocus();
      },
      child: Scaffold(
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Container(
          color: Colors.blue.shade100,
          child: Column(
            children: [
              const SizedBox(height: 48),
              const Padding(
                padding: EdgeInsets.symmetric(horizontal: 16),
                child: Text(
                  '1. Change iOS keyboard to Bitmoji or Slyder keyboard.\n'
                      '2. Tap on the button below.\n'
                      '3. Navigate back to previous route.\n'
                      '4. Check that shaded blue(Scaffold.body) not covering entire screen.\n'
                      '5. Compare the behavior with OS keyboard',
                  style: TextStyle(fontSize: 20),
                ),
              ),
              const TextField(autofocus: true),
              const Spacer(),
              ElevatedButton(
                onPressed: () {
                  FocusScope.of(context).unfocus();
                  Navigator.of(context).push(CupertinoPageRoute(
                    builder: (context) {
                      return const MyHomePage(title: 'Another Page');
                    },
                  ));
                },
                child: const Text('Hello', style: TextStyle(fontSize: 30)),
              )
            ],
          ),
        ),
        bottomNavigationBar: BottomNavigationBar(
          items: [Icons.abc, Icons.train]
              .map((e) => BottomNavigationBarItem(icon: Icon(e), label: 'hi'))
              .toList(),
        ),
      ),
    );
  }
}

Preview for flutter/flutter#99951

Before.MP4

After.mp4

Preview for flutter/flutter#100220

Before2.mp4

After2.mp4

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[coldstar96]

Just out of curiosity, why does keyboard with shorter height properly animates without this change?

[luckysmg]

@coldstar96
Reason: When custom keyboard is dismissed ,the keyboard frame of CGRect given by custom keyboard will intersect with screen....
So the targetBottomInset will be bottom * scale instead of 0.(I have debug the engine code and prove this)

engine/shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

Line 1173 in 56187fa

self.targetViewInsetBottom = bottom * scale;

I think maybe there are some issues in custom keyboard app. But making this change in engine side will make it more universal

[coldstar96]

It's unfortunate that iOS keyboard notification is not called with consistency. I've been through so many weird behaviors with iOS framework that this is no surprise. Thanks for the explanation!

[jmagman]

@coldstar96 were you able to confirm this fixes the issue since you can repro?

Very minor nits.
@gaaclarke can you give this a sanity check since you reviewed #29281 ?

[jmagman]

Use boxed literals

Suggested change

	@"UIKeyboardAnimationDurationUserInfoKey" : [NSNumber numberWithDouble:0.25],
	@"UIKeyboardIsLocalUserInfoKey" : [NSNumber numberWithBool:isLocal]
	@"UIKeyboardAnimationDurationUserInfoKey" : @(0.25),
	@"UIKeyboardIsLocalUserInfoKey" : @(isLocal)

[luckysmg]

done

[chinmaygarde]

This looks good to me. @jmagman can you LGTM this?

[jmagman]

@cyanglaz is reviewing.

[cyanglaz]

Did some debugging, It looks like the root cause of the issue is the keyboardFrame that we get from keyboardWillChangeFrame is a not CGRectZero when the keyboard is going to hide. This might be a bug from iOS.

With the above behavior, I think this fix is a good workaround.

Note: I cannot reproduce the issue on iPhone. I can confirm the iPad issue is fixed with the PR.

[luckysmg]

Hi @cyanglaz ,What's your iPhone version ? My iPhone can reproduce it.
iPhone 13 pro. iOS 15.4.With custom keyboard app Slyder

[cyanglaz]

LGTM % nits

[cyanglaz]

nits: This code is the same as in keyboardWillChangeFrame, maybe we can refactor it into a method.

[cyanglaz]

cc @jmagman or @chinmaygarde for a second LGTM

[jmagman]

Thanks @luckysmg and @cyanglaz!