classPhoneVerifier:publicPhoneAuthProvider::Listener{public:PhoneVerifier(constchar*phone_number,PhoneAuthProvider*phone_auth_provider):display_message_("Sending SMS with verification code"),display_verification_code_input_box_(false),display_resend_sms_button_(false),phone_auth_provider_(phone_auth_provider),phone_number_(phone_number){SendSms();}~PhoneVerifier()override{}voidOnVerificationCompleted(Credentialcredential)override{// Grab `mutex_` for the scope of `lock`. Callbacks can be called on// other threads, so this mutex ensures data access is atomic.MutexLocklock(mutex_);credential_=credential;}voidOnVerificationFailed(conststd::string&error)override{MutexLocklock(mutex_);display_message_="Verification failed with error: "+error;}voidOnCodeSent(conststd::string&verification_id,constPhoneAuthProvider::ForceResendingToken&force_resending_token)override{MutexLocklock(mutex_);verification_id_=verification_id;force_resending_token_=force_resending_token;display_verification_code_input_box_=true;display_message_="Waiting for SMS";}voidOnCodeAutoRetrievalTimeOut(conststd::string&verification_id)override{MutexLocklock(mutex_);display_resend_sms_button_=true;}// Draw the verification GUI on screen and process input events.voidDraw(){MutexLocklock(mutex_);// Draw an informative message describing what's currently happening.ShowTextBox(display_message_.c_str());// Once the time out expires, display a button to resend the SMS.// If the button is pressed, call VerifyPhoneNumber again using the// force_resending_token_.if(display_resend_sms_button_&&!verification_id_.empty()){constboolresend_sms=ShowTextButton("Resend SMS");if(resend_sms){SendSms();}}// Once the SMS has been sent, allow the user to enter the SMS// verification code into a text box. When the user has completed// entering it, call GetCredential() to complete the flow.if(display_verification_code_input_box_){conststd::stringverification_code=ShowInputBox("Verification code");if(!verification_code.empty()){credential_=phone_auth_provider_->GetCredential(verification_id_.c_str(),verification_code.c_str());}}}// The phone number verification flow is complete when this returns// non-NULL.Credential*credential(){MutexLocklock(mutex_);returncredential_.is_valid()?&credential_:nullptr;}private:voidSendSms(){staticconstuint32_tkAutoVerifyTimeOut=2000;MutexLocklock(mutex_);phone_auth_provider_->VerifyPhoneNumber(phone_number_.c_str(),kAutoVerifyTimeOut,&force_resending_token_,this);display_resend_sms_button_=false;}// GUI-related variables.std::stringdisplay_message_;booldisplay_verification_code_input_box_;booldisplay_resend_sms_button_;// Phone flow related variables.PhoneAuthProvider*phone_auth_provider_;std::stringphone_number_;std::stringverification_id_;PhoneAuthProvider::ForceResendingTokenforce_resending_token_;Credentialcredential_;// Callbacks can be called on other threads, so guard them with a mutex.Mutexmutex_;};