[webview_flutter_wkwebview] add functions to display a system popup using Javascript in iOS

[jsharp83]

Description

• In the iOS Webview environment, there are cases where Web calls System Popup with javascript.
• At this time, the message comes in the WKUIDelegate part.
  • https://developer.apple.com/documentation/webkit/wkuidelegate/1537406-webview
  • https://developer.apple.com/documentation/webkit/wkuidelegate/1536489-webview
• In that part, I implemented a System Alert using UIAlertController.
  • Refer to ReactNative in the reference below.
• I'm wondering whether to expose this part as a System Popup or deliver the message to the Flutter.

Reference

• https://unpkg.com/browse/react-native-webview@10.2.3/apple/RNCWebView.m

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or this PR is test-exempt.
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[stuartmorgan]

Thanks for the contribution!

• I'm wondering whether to expose this part as a System Popup or deliver the message to the Flutter.

The latter; it looks like we'd never documented that as the desired implementation in flutter/flutter#30358, so I've added a comment to that effect.

• https://unpkg.com/browse/react-native-webview@10.2.3/apple/RNCWebView.m

You appear to have simply copy-pasted code from that file into our implementation, which does not satisfy the terms of that code's license. We cannot accept code that you have not actually authored; please refer to item 5 of the CLA you signed.

Closing, as this is not code that we can accept. Please feel free to open a new PR if you decide to implement the approach described in flutter/flutter#30358 (comment) with code that is your own original work.

[niumj]

@jsharp83
a2288e0

Thank you for your contents.But when I use it, system popup is working, the below error is also happened.
[[FWFUIDelegate webView:runJavaScriptConfirmPanelWithMessage:initiatedByFrame:completionHandler:] was called more than once']
After that the app is exited.
How can I do or any suggestions?