BaseActivity.java

package br.com.nglauber.livrosfirebase;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

public abstract class BaseActivity extends AppCompatActivity {
    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthListener;
    private boolean mCallInit;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mCallInit = true;
        performLogin();
    }

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

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

    private void performLogin() {
        mAuth = FirebaseAuth.getInstance();
        mAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    if (mCallInit) { // Call init just once
                        mCallInit = false;
                        init();
                    }
                } else {
                    finish();
                    Intent it = new Intent(BaseActivity.this, SignInActivity.class);
                    it.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                    it.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                    startActivity(it);
                }
            }
        };
    }

    protected final FirebaseAuth getAuth(){
        return mAuth;
    }

    protected abstract void init();
}