public class LoginActivity extends AppCompatActivity {

    private static final String TAG = "LoginActivity";

    private FirebaseAuth.AuthStateListener mAuthListener;

    private TextView mRegister;
    private EditText mEmail, mPassword;
    private Button mLogin;
    private ProgressBar mProgressBar;
    private ImageView mLogo;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_login);
        mRegister = findViewById(R.id.link_register);
        mEmail = findViewById(R.id.input_email);
        mPassword = findViewById(R.id.input_password);
        mLogin = findViewById(R.id.btn_login);
        mProgressBar = findViewById(R.id.progressBar);
        mLogo = findViewById(R.id.logo);

        initImageLoader();
        initProgressBar();
        setupFirebaseAuth();
        init();

    }

    private void init() {
        mLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //check if the fields are filled out
                if (!isEmpty(mEmail.getText().toString())
                        && !isEmpty(mPassword.getText().toString())) {
                    Log.d(TAG, "onClick: attempting to authenticate.");

                    showProgressBar();

                    FirebaseAuth.getInstance().signInWithEmailAndPassword(mEmail.getText().toString(),
                            mPassword.getText().toString())
                            .addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                                @Override
                                public void onComplete(@NonNull Task<AuthResult> task) {

                                    hideProgressBar();

                                }
                            }).addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            Toast.makeText(LoginActivity.this, "Authentication Failed", Toast.LENGTH_SHORT).show();
                            hideProgressBar();
                        }
                    });
                } else {
                    Toast.makeText(LoginActivity.this, "You didn't fill in all the fields.", Toast.LENGTH_SHORT).show();
                }
            }
        });

        mRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d(TAG, "onClick: Navigating to Register Screen");

                Intent intent = new Intent(LoginActivity.this, RegisterActivity.class);
                startActivity(intent);
            }
        });

        UniversalImageLoader.setImage("assets://shopping.png", mLogo);

        hideSoftKeyboard();
    }


    private void initImageLoader() {
        UniversalImageLoader imageLoader = new UniversalImageLoader(LoginActivity.this);
        ImageLoader.getInstance().init(imageLoader.getConfig());
    }

    private boolean isEmpty(String string) {
        return string.equals("");
    }

    private void showProgressBar() {
        mProgressBar.setVisibility(View.VISIBLE);

    }

    private void hideProgressBar() {
        if (mProgressBar.getVisibility() == View.VISIBLE) {
            mProgressBar.setVisibility(View.INVISIBLE);
        }
    }

    private void initProgressBar() {
        mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
        mProgressBar.setVisibility(View.INVISIBLE);
    }

    private void hideSoftKeyboard() {
        this.getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_HIDDEN);
    }


       /*----------------------------- Firebase  ---------------------------------*/

    private void setupFirebaseAuth() {
        Log.d(TAG, "setupFirebaseAuth: started");

        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {

                    //check if email is verified
                    if (user.isEmailVerified()) {
                        Log.d(TAG, "onAuthStateChanged: signed_in: " + user.getUid());
                        Toast.makeText(LoginActivity.this, "Authenticated with: " + user.getEmail(), Toast.LENGTH_SHORT).show();

                        Intent intent = new Intent(LoginActivity.this, SearchActivity.class);
                        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
                        startActivity(intent);
                        finish();

                    } else {
                        Toast.makeText(LoginActivity.this, "Email is not Verified\nCheck your Inbox", Toast.LENGTH_SHORT).show();
                        FirebaseAuth.getInstance().signOut();
                    }

                } else {
                    // User is signed out
                    Log.d(TAG, "onAuthStateChanged: signed_out");
                }
                // ...
            }
        };
    }

    @Override
    public void onStart() {
        super.onStart();
        FirebaseAuth.getInstance().addAuthStateListener(mAuthListener);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mAuthListener != null) {
            FirebaseAuth.getInstance().removeAuthStateListener(mAuthListener);
        }
    }