public class LoginActivity extends AppCompatActivity implements View.OnClickListener {

    private Button buttonSignIn;
    private EditText editTextEmail;
    private EditText editTextPassword;
    private TextView textViewSignUp;
    private ProgressDialog progressDialog;
    private FirebaseAuth firebaseAuth;
    private CallbackManager mCallbackManager;
    private static final String TAG = "FACELOG";

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

        firebaseAuth = FirebaseAuth.getInstance();

        if(firebaseAuth.getCurrentUser() != null) {
            //profile activity here (user already logged in)
            finish();
            startActivity(new Intent(getApplicationContext(), ProfileActivity.class));
        }

        progressDialog = new ProgressDialog(this);
        editTextEmail = (EditText) findViewById(R.id.editTextEmail);
        editTextPassword = (EditText) findViewById(R.id.editTextPassword);
        buttonSignIn = (Button) findViewById(R.id.buttonSignIn);
        textViewSignUp = (TextView) findViewById(R.id.textViewSignUp);

        buttonSignIn.setOnClickListener(this);
        textViewSignUp.setOnClickListener(this);

        // Initialize Facebook Login button
        mCallbackManager = CallbackManager.Factory.create();
        LoginButton loginButton = findViewById(R.id.login_button);
        loginButton.setReadPermissions("email", "public_profile");
        loginButton.registerCallback(mCallbackManager, new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                Log.d(TAG, "facebook:onSuccess:" + loginResult);
                handleFacebookAccessToken(loginResult.getAccessToken());
            }

            @Override
            public void onCancel() {
                Log.d(TAG, "facebook:onCancel");
            }

            @Override
            public void onError(FacebookException error) {
                Log.d(TAG, "facebook:onError", error);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        // Pass the activity result back to the Facebook SDK
        mCallbackManager.onActivityResult(requestCode, resultCode, data);
    }

    private void userLogin() {
        String email = editTextEmail.getText().toString().trim();
        String password = editTextPassword.getText().toString().trim();

        if(TextUtils.isEmpty(email)) { //email is empty
            Toast.makeText(this, "Please enter email", Toast.LENGTH_SHORT).show();
            return;
        }
        if(TextUtils.isEmpty(password)) { //password is empty
            Toast.makeText(this, "Please enter password", Toast.LENGTH_SHORT).show();
        }

        //if validation is okay
        //we will first show a progress bar
        progressDialog.setMessage("Logging In...");
        progressDialog.show();

        firebaseAuth.signInWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        progressDialog.dismiss();
                        if(task.isSuccessful()) {
                            //start profile activity
                            finish();
                            startActivity(new Intent(getApplicationContext(), ProfileActivity.class));
                        }
                    }
                });
    }

    @Override
    public void onClick(View view) {
        if(view == buttonSignIn) { userLogin(); }
        if(view == textViewSignUp) {
            finish();
            startActivity(new Intent(this, SignupActivity.class));
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = firebaseAuth.getCurrentUser();

        if(currentUser != null) {

            updateUI();
        }
    }

    private void handleFacebookAccessToken(AccessToken token) {
        Log.d(TAG, "handleFacebookAccessToken:" + token);

        AuthCredential credential = FacebookAuthProvider.getCredential(token.getToken());
        firebaseAuth.signInWithCredential(credential);
                /*.addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            // Sign in success, update UI with the signed-in user's information
                            Log.d(TAG, "signInWithCredential:success");
                            FirebaseUser user = firebaseAuth.getCurrentUser();
                            updateUI();
                        } else {
                            // If sign in fails, display a message to the user.
                            Log.w(TAG, "signInWithCredential:failure", task.getException());
                            Toast.makeText(LoginActivity.this, "Authentication failed.",
                                    Toast.LENGTH_SHORT).show();
                            //updateUI();
                        }

                        // ...
                    }
                });*/
                updateUI();
    }

    private void updateUI() {
        Toast.makeText(LoginActivity.this, "You logged in with FB.",
                Toast.LENGTH_LONG).show();
        finish();
        startActivity(new Intent(getApplicationContext(), ProfileActivity.class));
    }
}