public class LoginActivity extends AppCompatActivity {

    private static final String TAG = "LoginActivity";

    @BindView(R.id.activity_login_et_email)
    EditText mEtEmail;

    @BindView(R.id.activity_login_et_password)
    EditText mEtPassword;

    private AlertDialog  mAlertDialog;
    private FirebaseAuth mAuth;
    private String       mEmail, mPassword;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        ButterKnife.bind(this);

        mAuth = FirebaseAuth.getInstance();
    }

    @OnClick(R.id.activity_login_btn_login)
    void onBtnLoginClick() {
        Log.d(TAG, "onBtnLoginClick: Clicked");
        mEmail = mEtEmail.getText().toString().trim();
        mPassword = mEtPassword.getText().toString().trim();

        if (mEmail.equals("") || mPassword.equals("")) {
            Toast.makeText(this, "Please fill out all fields. ", Toast.LENGTH_SHORT).show();
            return;
        }

        setProgressDialog();

        mAuth.signInWithEmailAndPassword(mEmail, mPassword)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            mAlertDialog.dismiss();
                            Toast.makeText(LoginActivity.this, "Authentication failed.",
                                    Toast.LENGTH_SHORT).show();
                        }
                    }
                });

    }

    private void setProgressDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this, R.style.alertDialogTheme);
        builder.setView(R.layout.alert_dialog_login_progress);
        mAlertDialog = builder.create();
        mAlertDialog.setCancelable(false);
        mAlertDialog.setCanceledOnTouchOutside(false);
        mAlertDialog.show();

    }

    @Override
    public void onStart() {
        super.onStart();
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = mAuth.getCurrentUser();
        //if user is signed in go to main
        if(currentUser != null){
            initMainActivity();
        }
    }
}