Fix sms code entry using certain custom keyboards

[0thernet]

r? @jflinter

tl;dr, When the delete key is pressed, some custom keyboards will call textDocumentProxy.insertText("") if the field is empty, rather than textDocumentProxy.deleteBackward(). This seems entirely at the discretion of the developer, so we should support both possibilities.

I tested entering SMS codes with a few custom keyboards:

• Using the system number pad or Swype, when the delete key is pressed, deleteBackward is called.
• Using Gboard or SwiftKey, deleteBackward is not called – instead, shouldChangeCharactersInRange: is called with an empty string.

A more heavy-handed solution would be to use UIKeyboardTypePhonePad, but that doesn't seem ideal.

Your custom keyboard is also ineligible to type into so-called phone pad objects, such as the phone number fields in Contacts. These input objects are exclusively for strings built from a small set of alphanumeric characters specified by telecommunications carriers and are identified by having one or another of the following two keyboard type traits: UIKeyboardTypePhonePad, UIKeyboardTypeNamePhonePad

https://developer.apple.com/library/prerelease/content/documentation/General/Conceptual/ExtensibilityPG/CustomKeyboard.html

[jflinter]

Hey @benzguo sorry it took me so long to get to this.

Honestly, I think we should just use UIKeyboardTypePhonePad here (and on STPPaymentCardTextField, etc). I'm getting worried about the amount of magic keyboard logic we're maintaining. The only difference is the "+*#" button in the lower-left, and we already ignore those characters if the user taps them (which they won't). Thoughts?

[0thernet]

That sounds reasonable to me – I've updated this PR to replace all occurrences of UIKeyboardTypeNumberPad with UIKeyboardTypePhonePad.

[jflinter]

thanks, lgtm in that case!