public class MainActivity extends AppCompatActivity implements View.OnClickListener {

public static final String TAG = "activity_main";
private static final String APP_PREFERENCES_PASSWORD = "Password";
public static final String APP_PREFERENCES_USER = "User";

private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListner;


private TextView email;
private TextView password;
private CheckBox remember_me;
private ProgressBar progressBar;


SharedPreferences mSettings;

EditSharedPreferences editSharedPreferences = new EditSharedPreferences();

ChangeStateProgressBar changeStateProgressBar = new ChangeStateProgressBar();

CheckInternetConnection checkInternetConnection = new CheckInternetConnection();

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(layout.activity_main);


    mSettings = PreferenceManager.getDefaultSharedPreferences(MainActivity.this);

    mAuth = FirebaseAuth.getInstance();     // get instance of fireBase

    if(mAuth.getCurrentUser()!=null){
        if(checkAdmin(mAuth.getCurrentUser().getEmail())){
            Intent intent = new Intent(MainActivity.this, admin_Activity.class);
            startActivity(intent);
        }else{
            Intent intent = new Intent(MainActivity.this, Categories.class);
            startActivity(intent);
        }
    }

    progressBar = (ProgressBar)findViewById(id.progressBar);
    email = (TextView) findViewById(id.email);
    password = (TextView) findViewById(id.password);
    remember_me = (CheckBox)findViewById(id.Remember_Me);


    // check file APP_PREFERENCES. If it consist
    // APP_PREFERENCES_USER code need to set email text view
    if(mSettings.contains(APP_PREFERENCES_USER)){
        email.setText(mSettings.getString(APP_PREFERENCES_USER, ""));
    }

    if(mSettings.contains(APP_PREFERENCES_PASSWORD)) {
        password.setText(mSettings.getString(APP_PREFERENCES_PASSWORD, ""));
    }

    // if user connect or disconnect listener recieves
    // an answer of instance
    mAuthListner = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser user = firebaseAuth.getCurrentUser();
            if (user != null) {
                Log.d(TAG, "Sign_in" + user.getUid());
            } else {
                Log.d(TAG, "Sign_out");
            }
        }
    };


    //find buttons
    findViewById(id.sign_in_Button).setOnClickListener(this);
    findViewById(id.registration_Button).setOnClickListener(this);

}


public void onStart() {
    super.onStart();
    mAuth.addAuthStateListener(mAuthListner);
}

public void onStop() {
    super.onStop();

    //must to remove of listener when
    // app closed
    if (mAuthListner != null) {
        mAuth.removeAuthStateListener(mAuthListner);
    }
}


@Override
public void onClick(View v) {
         if (v.getId() == id.sign_in_Button) {

             //check email and password rows for filling
             // if any rows an empty we take an
             // toast error
             signIn();
         }

         // if we press registration button

        if (v.getId() == id.registration_Button) {
        // show progress bar

            changeStateProgressBar.showProgressBar(progressBar);

            //start registration activity
            Intent intent = new Intent(MainActivity.this, RegistrationActivity.class);
            startActivity(intent);

            changeStateProgressBar.hideProgressBar(progressBar);

        }
}



private void signIn(){
    if ((!email.getText().toString().equals(null)
            && !email.getText().toString().equals(""))
            || (!password.getText().toString().equals(null)
            && !password.getText().toString().equals(""))) {

        //start progress bar visibility, when we do
        // hard piece of code
        changeStateProgressBar.showProgressBar(progressBar);

        // try to sign in with email and password

        mAuth.signInWithEmailAndPassword(email.getText().toString(), password.getText().toString())

                // need to add complete listener to track of successful doing
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {

                        //if task is successfull need to go to another intent
                        if (task.isSuccessful()) {
                            //delete from settings
                            editSharedPreferences.deletePreferences(mSettings, APP_PREFERENCES_PASSWORD);
                            editSharedPreferences.deletePreferences(mSettings,APP_PREFERENCES_USER);

                            if(remember_me.isChecked()) {
                                // remember me
                                editSharedPreferences.addPreferences(mSettings, APP_PREFERENCES_USER, email.getText().toString());
                                editSharedPreferences.addPreferences(mSettings, APP_PREFERENCES_PASSWORD, password.getText().toString());
                            }
                            // check email for administrator
                            // email, if administrator entered
                            // start entent Administrator activity
                            if (checkAdmin(email.getText().toString())) {

                                //start admin activity
                                Intent intent = new Intent(MainActivity.this, admin_Activity.class);
                                startActivity(intent);

                                // hide progress bar
                                changeStateProgressBar.hideProgressBar(progressBar);

                                email.setText("");
                                password.setText("");

                            } else{

                                // if not admin entered, start personal activity
                                Log.d(TAG, "Succesful enter");

                                //start new activity
                                Intent intent = new Intent(MainActivity.this, Categories.class);
                                startActivity(intent);

                                // HIDE PROGRESS bar
                                changeStateProgressBar.hideProgressBar(progressBar);

                                email.setText("");
                                password.setText("");
                            }
                        }
                    }
                })

                // if in process of entering
                // we have an errors we must
                // send toast with code of error
                .addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        // make toast message if there is anyobe exception
                        Toast.makeText(MainActivity.this, e.getMessage(), Toast.LENGTH_SHORT).show();
                        password.setText("");
                    }
                });
    }else{
        // make toast message if pass or email rows is empty
        Toast.makeText(MainActivity.this, "Password or email is empty", Toast.LENGTH_SHORT).show();
    }
}

private boolean checkAdmin(String email){
    if(email.equals("admin@rea.ru")){

        return true;
    }else{

        return false;
    }

}

public class Categories extends AppCompatActivity implements View.OnClickListener
{
private EditSharedPreferences editSharedPreferences;
private static final String APP_PREFERENCES_USER = "User";
private static final String APP_PREFERENCES_PASSWORD = "Password";
private static final String APP_PREFERENCES_DOESNOT_REMEMBER = "isChecked";

SharedPreferences mSettings;

Button problemsWithTeachers, signOut;
private static final String TAG = "CATEGORIES";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(layout.activity_categories);

    mSettings = PreferenceManager.getDefaultSharedPreferences(Categories.this);

    problemsWithTeachers = (Button)findViewById(id.SignOutBut);
    signOut = (Button)findViewById(id.problemsWithTeachersBut);
    signOut.setOnClickListener(this);
    problemsWithTeachers.setOnClickListener(this);

}

@Override
public void onClick(View v) {
    if(v.getId() == id.SignOutBut){
        logout();

    }else if(v.getId() == id.problemsWithTeachersBut){
        signOut.setVisibility(View.GONE);
    }

}


private void logout(){
    FirebaseAuth mAuth = FirebaseAuth.getInstance();
    mAuth.signOut();
    Intent intent = new Intent(Categories.this, MainActivity.class);
    startActivity(intent);
    }

}