TextInput component fix maxLength property when entering characters at the beginning.

[MadeinFrance]

Fix TextInput component and the maxLength property when entering characters at the beginning. (#21639).

Improvements: In TextInputTest.js We could place the native cursor at the beginning of the input in the function componentDidMount. Not sure how to do that from the JavaScript.

Summary:

• In RCTBaseTextInputView.m the ivar _predictedText is not updated correctly when inserting at the beginning.
• The actual fix is to update the value before the checking the text mismatch from backedTextInputView.

Test Plan:

• When entering 1 character at the beginning of the TextInput, we expect to have only 1 character inserted. However we experience that the character is inserted and the current value is duplicated.
• Insert 0 character before the value 123 with: <TextInput maxLength={5} />.
  

Release Notes:

[IOS] [BUGFIX] [TextInput] - Fix maxLength property when entering characters at the beginning.

Result:

[analysis-bot]

Code analysis results:

• eslint found some issues.

[analysis-bot]

Code analysis results:

• eslint found some issues.
• flow found some issues.

[analysis-bot]

Code analysis results:

• eslint found some issues.
• flow found some issues.

[analysis-bot]

no-undef: 'updateText' is not defined.

[analysis-bot]

Cannot resolve name updateText.

[shergin]

@MadeinFrance Thank you!
Could you please describe why exactly your fix fixes things (doing them right)?

[MadeinFrance]

Thank you for the feedback, I appreciate that.

Lets use the TestCase I described in the description (<TextInput maxLength={5} /> and try to insert 0 at the beginning in order to have 0123 as value.)

---

What happen when inserting 0:

1. RCTBackedTextInputDelegateAdapter.shouldChangeCharactersInRange triggers and unfortunately override the ivar _predictedText to 0.
2. At the same time, from BackedInputDelegateAdapter, textFieldDidChange delegate is called.

The bug is that the BaseTextInputView (line) will find a mismatch between 0123 and 0 and try to add a replacementText of 123 at NSRange: location=1, length=0.

3. The error result will be 01123.

---

• My current implementation keeps the ivar _predictedText to the correct value.
• Another possible implementation including the clean up of ([0.54] [iOS] Wrong 'Backspace' event fired onKeyPress after clearing TextInput #18374) could be: L-336

if (_predictedText) {
    _predictedText = [_predictedText stringByReplacingCharactersInRange:range withString:text];
}

[MadeinFrance]

Fixed in a later commit. Closing.