[UWP] Implement clipboard.

[moko256]

This PR will able to merge after:

• hasStrings on Windows #27749
• The hasString implementation for UWP.

This PR implements clipboard in platform handler in UWP.
I tested it by these steps:

• Selecting TextField in flutter uwp app.
• Typing some characters.
• Performing right click and checking Paste behavior in the context menu.

Clipboard content	Paste behavior
Text (copied from another app)	appeared, performing paste by clicking
Image (from Print Screen key)	disappeared

Fixes one item in flutter/flutter#70214.

No changes in flutter/tests.

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.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[chinmaygarde]

I attempted a review but I am not familiar with this component. Perhaps @cbracken @stuartmorgan

[chinmaygarde]

Not super familiar with these APIs, but, does this have an error handler as well that we may need to account for? Perhaps one that also gets invoked when the window is not in the foreground.

[moko256]

Before here I check window is in foreground, so I think this should throw no errors. I created an example and checked app doesn't crash from the engine. (I will post it on the comments.)

[moko256]

Reading clipboard every 1 sec to check no engine crash from background.

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

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Clipboard Loop',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Clipboard Loop'),
    );
  }
}

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> {
  _MyHomePageState() {
    startLoopingCheckClipboard().then((_) {});
  }

  Future<void> startLoopingCheckClipboard() async {
    while (true) {
      await Future.delayed(const Duration(seconds: 1));
      try {
        var text = (await Clipboard.getData("text/plain"))?.text;
        if (text != null) {
          _updateContentAndError(text, null);
        } else {
          _updateContentAndError(null, "clipboard is null");
        }
      } catch (e) {
        _updateContentAndError(null, e.toString());
      }
    }
  }

  String? _content;
  String? _error;

  void _updateContentAndError(String? content, String? error) {
    setState(() {
      _content = content;
      _error = error;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'Clipboard content',
            ),
            Text(
              _content ?? "",
              style: Theme.of(context).textTheme.headline4,
            ),
            Text(
              _error ?? "",
              style: Theme.of(context)
                  .textTheme
                  .headline4
                  ?.copyWith(color: Theme.of(context).errorColor),
            ),
          ],
        ),
      ),
    );
  }
}

[moko256]

With the example, I found that Clipboard.getData() throws PlatformException when app is not focused, in web, but here Clipboard.getData() send success. I will fix it.

[moko256]

done

[chinmaygarde]

cc @clarkezone. I am not sure about these APIs but they the implementation looks good to me modulo the error callback.

[clarkezone]

I'm blocked currently due to inability to build the UWP target on my machine.

[cbracken]

This conditional block is pretty huge; consider inverting the condition and writing this as a bailout-style check where if the window isn't in the foreground, you set the error and return. Similar comment for the inner case where the clipboard contains no text; that would also put the comment closer to the result->Success(empty document) handling of that case.

[moko256]

done

[cbracken]

LGTM other than the style nit above.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite Linux Framework Smoke Tests has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite build_and_test_linux_unopt_debug has failed. Please fix the issues identified (or deflake) before re-applying this label.
• The status or check suite Prebuild ci/docker/build/Dockerfile has failed. Please fix the issues identified (or deflake) before re-applying this label.

[cbracken]

The build_and_test_linux_unopt_debug failure was caused by a recent change in our cirrus API token. I've rebased to tip-of-tree to pick up the new API key introduced in #28584.