@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setTitle("first activity");
    setContentView(R.layout.activity_main);

    toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    drawer = findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    navigationView = findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
    Menu menu = navigationView.getMenu();
    v_camera = menu.findItem(R.id.nav_camera);
    v_contacts = menu.findItem(R.id.nav_contacts);
    v_parties = menu.findItem(R.id.nav_parties);
    v_gallery = menu.findItem(R.id.nav_gallery);
    v_logout = menu.findItem(R.id.nav_logout);
    v_manage = menu.findItem(R.id.nav_manage);
    v_share = menu.findItem(R.id.nav_share);
    v_send = menu.findItem(R.id.nav_send);
    vf = findViewById(R.id.viewflipper);
    vf.setDisplayedChild(vf.indexOfChild(findViewById(R.id.page1)));
    changeNavMenuItems(false);

    page1 = findViewById(R.id.page1);
    page2 = findViewById(R.id.page2);

    SharedPreferences.OnSharedPreferenceChangeListener sharedPref_listener =
            new SharedPreferences.OnSharedPreferenceChangeListener() {
                @Override
                public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
                    Log.e("SharedChanged",
                            "key= " + key + " " + sharedPreferences.getString(key, null));
                    if (sharedPreferences.getString(key, null) == null)
                        changeNavMenuItems(false);
                    else
                        changeNavMenuItems(true);
                }
            };
    G.sharedPref.registerOnSharedPreferenceChangeListener(sharedPref_listener);
    rq = Volley.newRequestQueue(MainActivity.this);
    onLayoutChangeListener = new View.OnLayoutChangeListener() {
        @Override
        public void onLayoutChange(View v, int left, int top, int right, int bottom, int oldLeft, int oldTop, int oldRight, int oldBottom) {
            Log.e("check view", v.toString());
            if (vf.getCurrentView() == page1) {

                Button signIn_btn = findViewById(R.id.signInBtn);
                signIn_btn.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        EditText userName = findViewById(R.id.username);
                        EditText passWord = findViewById(R.id.password);
                        str_username = userName.getText().toString();
                        str_password = passWord.getText().toString();

                        StringRequest JsonReq = new StringRequest(Request.Method.POST,
                                "https://www.ghasedakia.com/core/api/cc_login",
                                new Response.Listener<String>() {
                                    @Override
                                    public void onResponse(String response) {
                                        Log.e("response", response);
                                        String session_id = null;
                                        try {
                                            JSONObject jsonObj = new JSONObject(response);
                                            session_id = jsonObj.getString("session_id");
                                        } catch (JSONException e) {
                                            e.printStackTrace();
                                        }

                                        if (session_id != null) { ///Authorize successful
                                            Log.e("LOG", session_id);
                                            G.sharedPref.edit().putString("session_id", session_id).apply();

                                            vf.setDisplayedChild(vf.indexOfChild(findViewById(R.id.page2)));

                                        } else {    //// not authorized
                                            Log.e("LOG", "User Not found");
                                        }
                                    }
                                },
                                new Response.ErrorListener() {
                                    @Override
                                    public void onErrorResponse(VolleyError error) {
                                        Log.e("error", error.toString());
                                    }
                                }) {
                            Map<String, String> params = new HashMap<String, String>();

                            @Override
                            public Map<String, String> getParams() {

                                params.put("password", str_password);
                                params.put("username", str_username);

                                return params;
                            }
                        };
                        JsonReq.setRetryPolicy(new DefaultRetryPolicy(7000, DefaultRetryPolicy.DEFAULT_MAX_RETRIES, DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));
                        rq.add(JsonReq);
                    }
                });
            } else if (vf.getCurrentView() == page2) {
                Log.e("layout", "changed to page2");
                getContactsFromServer(G.sharedPref.getString("session_id", null));
            }
        }
    };
    vf.addOnLayoutChangeListener(onLayoutChangeListener);
}

<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:openDrawer="start">

<include
    layout="@layout/app_bar_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<android.support.design.widget.NavigationView
    android:id="@+id/nav_view"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:fitsSystemWindows="true"
    app:headerLayout="@layout/nav_header_main"
    app:menu="@menu/activity_main_drawer" />

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.ghasedakia.MainActivity">

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>
<ViewFlipper
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/viewflipper">
    <include
        android:id="@+id/page1" layout="@layout/content_main" />
    <include
        android:id="@+id/page2" layout="@layout/contacts_page" />
</ViewFlipper>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="@dimen/nav_header_height"
android:background="@drawable/bg"
android:gravity="bottom"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:theme="@style/ThemeOverlay.AppCompat.Dark">

<ImageView
    android:id="@+id/nav_image"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/nav_header_vertical_spacing"
    app:srcCompat="@mipmap/ic_launcher_round" />

<TextView
    android:id="@+id/nav_username"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/nav_header_vertical_spacing"
    android:text="Android Studio"
    android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
tools:showIn="navigation_view">

<group android:checkableBehavior="single">
    <item
        android:id="@+id/nav_contacts"
        android:icon="@drawable/ic_person_black"
        android:title="Contacts" />
    <item
        android:id="@+id/nav_parties"
        android:icon="@drawable/ic_cake_black"
        android:title="Parties" />
    <item
        android:id="@+id/nav_camera"
        android:icon="@drawable/ic_menu_camera"
        android:title="Import" />
    <item
        android:id="@+id/nav_gallery"
        android:icon="@drawable/ic_menu_gallery"
        android:title="Gallery" />
    <item
        android:id="@+id/nav_logout"
        android:icon="@drawable/ic_menu_logout"
        android:title="Log out" />
    <item
        android:id="@+id/nav_manage"
        android:icon="@drawable/ic_menu_manage"
        android:title="Tools" />
</group>

<item android:title="Communicate">
    <menu>
        <item
            android:id="@+id/nav_share"
            android:icon="@drawable/ic_menu_share"
            android:title="Share" />
        <item
            android:id="@+id/nav_send"
            android:icon="@drawable/ic_menu_send"
            android:title="Send" />
    </menu>
</item>