activity_login.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="contract.manager.login.LoginActivity">

    <FrameLayout
        android:id="@+id/login_conteiner"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</android.support.constraint.ConstraintLayout>

LoginActivity.java

package contract.manager.login;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;

import contract.manager.R;

public class LoginActivity extends AppCompatActivity {
    private Contracts.Presenter presenter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        LoginFragmentView view = LoginFragmentView.newInstance();
        presenter = new LoginPresenter(view);
        getSupportFragmentManager().beginTransaction().add(R.id.login_conteiner, view, "login").commit();
    }

    @Override
    protected void onStart() {
        super.onStart();
        presenter.start();
    }

    @Override
    protected void onStop() {
        super.onStop();
        presenter.stop();
    }
}

LoginFragmentView.java

package contract.manager.login;


import android.os.Bundle;
import android.support.design.widget.Snackbar;
import android.support.design.widget.TextInputLayout;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

import com.google.firebase.auth.FirebaseAuth;

import butterknife.BindView;
import butterknife.ButterKnife;
import butterknife.OnClick;
import contract.manager.R;

public class LoginFragmentView extends Fragment implements Contracts.View {
    private static final String TAG = "LoginFragmentView";
    @BindView(R.id.login_til_email)
    TextInputLayout email;
    @BindView(R.id.login_til_senha)
    TextInputLayout senha;
    @BindView(R.id.login_tv_esqueceu_senha)
    TextView esqueceu;
    @BindView(R.id.login_pb_carregando)
    ProgressBar carregando;
    @BindView(R.id.login_bt_login)
    Button login;

    private Contracts.Presenter presenter;

    public LoginFragmentView() {
    }

    public static LoginFragmentView newInstance() {
        LoginFragmentView fragment = new LoginFragmentView();
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_login_fragment_view, container, false);
        ButterKnife.bind(this, v);
        return v;
    }

    @Override
    public void setPresenter(Contracts.Presenter presenter) {
        this.presenter = presenter;
    }

    @OnClick(R.id.login_bt_login)
    void login() {
        limparErros();
        presenter.autenticarUsuario(
                email.getEditText().getText().toString(),
                senha.getEditText().getText().toString());
    }

    @Override
    public void mostrarEmailErro(boolean mostrar) {
        if (mostrar) {
            email.setError("Email inválido");
        } else {
            email.setErrorEnabled(false);
        }
    }

    @Override
    public void mostrarSenhaErro(boolean mostrar) {
        if (mostrar) {
            senha.setError("Senha inválida");
        } else {
            senha.setErrorEnabled(false);
        }
    }

    @Override
    public void mostrarEmailNaoCadastradoErro(boolean mostrar) {
        if (mostrar) {
            email.setError("Email não está cadastrado");
        } else {
            email.setErrorEnabled(false);
        }
    }

    @Override
    public void mostrarHorarioErro(boolean mostrar) {
        SnackErro("Fora do horário de trabalho.");
    }

    @Override
    public void mostrarInativoErro(boolean mostrar) {
        SnackErro("Usuario inativo.");
    }

    private void SnackErro(String msg) {
        if (getView() == null) return;
        Snackbar.make(getView(), msg, Snackbar.LENGTH_INDEFINITE).setAction("Fechar", new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getActivity().finish();
            }
        }).show();
    }

    @Override
    public void mostrarCarregando(boolean mostrar) {
        carregando.setVisibility(mostrar ? View.VISIBLE : View.GONE);
        login.setEnabled(!mostrar);
        email.setEnabled(!mostrar);
        senha.setEnabled(!mostrar);
    }

    @Override
    public void mostrarEsqueceuSenha(boolean mostrar) {
        senha.setError("Confira sua senha e tente novamente.");
        esqueceu.setVisibility(mostrar ? View.VISIBLE : View.INVISIBLE);
    }

    @Override
    public void mostrarTelaPrincipal() {
//        startActivity(new Intent(getContext(),PrincipalActivity.class));
        Toast.makeText(getContext(), "Logado com: " + FirebaseAuth.getInstance().getCurrentUser().getEmail(), Toast.LENGTH_SHORT).show();
    }

    private void limparErros() {
        TextInputLayout inputLayout[] = {email, senha};
        for (TextInputLayout inp : inputLayout) {
            inp.setErrorEnabled(false);
        }
        esqueceu.setVisibility(View.INVISIBLE);
        login.setEnabled(true);
    }
}

manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="contract.manager">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".splash.SplashActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".login.LoginActivity">
            <intent-filter>
                <action android:name="android.intent.action.QUICK_VIEW" />
            </intent-filter>
        </activity>
        <activity
            android:name=".principal.MainActivity"
            android:label="@string/title_activity_main"
            android:theme="@style/AppTheme.NoActionBar"></activity>
    </application>

</manifest>