FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
//Сперва пользователь зарегистрируется
registration = (Button)findViewById(R.id.registration);
mail = (EditText)findViewById(R.id.txtEmailRegistration);
        password = (EditText)findViewById(R.id.txtPasswordRegistration);
registration.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                    final ProgressDialog progressDialog = ProgressDialog.show(Registration.this, getResources().getString(R.string.loading), getResources().getString(R.string.wait), true);
                    firebaseAuth.createUserWithEmailAndPassword(mail.getText().toString(), password.getText().toString()).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            progressDialog.dismiss();
                            if (task.isSuccessful()) {
                                Toast toast = Toast.makeText(getApplicationContext(), getResources().getString(R.string.successful), Toast.LENGTH_LONG);
                                toast.show();
                                Intent intent = new Intent(Registration.this, Login.class);
                                startActivity(intent);
                                DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();
                                FirebaseUser user = firebaseAuth.getCurrentUser();
                                assert user != null;
                                user.sendEmailVerification();//После успешной регистрации, отправляется подтверждение на почту
                            } else {
                                Toast toast = Toast.makeText(getApplicationContext(), getResources().getString(R.string.error), Toast.LENGTH_LONG);
                                toast.show();
                            }
                        }
                    });
                }

            }
        });

firebaseAuth = FirebaseAuth.getInstance();
        email = (EditText)findViewById(R.id.txtEmailLogin);
        password = (EditText)findViewById(R.id.txtPasswordLogin);
        login = (Button)findViewById(R.id.btnLogin);
login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                    final ProgressDialog progressDialog = ProgressDialog.show(Login.this, getResources().getString(R.string.loading), getResources().getString(R.string.wait), true);
                    firebaseAuth.signInWithEmailAndPassword(email.getText().toString(), password.getText().toString()).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            if (task.isSuccessful()) {
                                    progressDialog.dismiss();
                                    Toast toast = Toast.makeText(getApplicationContext(), getResources().getString(R.string.successful), Toast.LENGTH_LONG);
                                    toast.show();
                                    Intent intent = new Intent(Login.this, MainActivity.class);
                                    startActivity(intent);
                                if(!user.isEmailVerified()){
                                    final AlertDialog.Builder builder = new AlertDialog.Builder(Login.this);
                                    builder.setTitle("Подтверждение")
                                            .setCancelable(false)
                                            .setMessage("Пожалуйста, подтвердите аккаунт. Затем только нажмите на кнопку потдвердил")
                                            .setNegativeButton("Подтвердил",
                                                    new DialogInterface.OnClickListener() {
                                                        public void onClick(DialogInterface dialog, int id) {
                                                            if(!user.isEmailVerified()){
                                                                AlertDialog alert = builder.create();
                                                                alert.show();
                                                            }else{

                                                            }
                                                        }
                                                    });
                                    AlertDialog alert = builder.create();
                                    alert.show();
                                }else{
                                    Toast.makeText(getApplicationContext(), "Аккаунт подтвержден", Toast.LENGTH_LONG).show();
                                }
                            } else {
                                progressDialog.dismiss();
                                Toast toast = Toast.makeText(getApplicationContext(), getResources().getString(R.string.error), Toast.LENGTH_LONG);
                                toast.show();
                            }
                        }
                    });


            }
            });