[iOS] Fixes selection of single line text input

[zhongwuzw]

Summary

UITextField don't have delegate to observe the selection changes of users(multiline text input UITextView have it), so we can add a KVO to observe selection changes.

@cpojer @shergin

Changelog

[iOS] [Fixed] - Fixes selection of single line text input

Test Plan

Create a single line textInput, then do some selection, onSelectionChange can be called.

[cpojer]

Thank you!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @zhongwuzw in fc8008e.

^{When will my fix make it into a release? | Upcoming Releases}

[JoshuaGross]

Hi @zhongwuzw, I'm reverting this because it's causing the following crash:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'An instance 0x7fb3453eda00 of class RCTUITextField was deallocated while key value observers were still registered with it.

I believe this is because we're adding a KVO observer to the UITextField but only have a weak reference to it, so it can be deallocated before the observer, causing this crash. Feel free to resubmit the PR with a fix for this crash as well.

[zhongwuzw]

@JoshuaGross Oops, I missing that weak declaration, I'll fix it :).

[banky]

Hey, @zhongwuzw I was wondering if this change will still be merged since it looks like you made a fix for the crashing problem. The issue from #21515 still exists

[zhongwuzw]

@BankyA c38f167, v0.61.2 fixes, please check.