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Type Alias Callback fired when a non-custodial signer needs the user to authenticate. 
typedef CrossmintSignerAuthRequiredCallback = Future<void> Function( String signerType, String signerLocator, bool needsAuth, CrossmintOtpSendCallback sendOtp, CrossmintOtpVerifyCallback verifyOtp, CrossmintOtpRejectCallback reject, )
The SDK passes:
  • signerType — "email" or "phone".
  • signerLocator — the identifier (email or phone number in E.164 form).
  • needsAuth — true when the challenge starts, false when it ends (e.g. after a successful verify).
  • sendOtp / verifyOtp / reject — callbacks that drive the flow.
Implementers show a UI prompt, call sendOtp to trigger delivery, then verifyOtp(code) on submission or reject() on cancel. The CrossmintWalletController.otp controller provides a ready-made implementation that marshalls the challenge through a ValueListenable — use that unless you need bespoke wiring.