Android Activity嵌套Fragment

问题：Activity嵌套4个Fragment。假如切换到后台。Activity被杀死。重新进入是否可以记录退出到后台时的页面。
使用：onSaveInstanceState()方法。

@BindView(R.id.fl_content)
FrameLayout flContent;
@BindView(R.id.btn_navigation)
BottomNavigationViewEx btnNavigation;
private BaseFragment[] mFragments = new BaseFragment[4];
private int currentTab = 0;
@Override
protected void onActivityCreate(Bundle savedInstanceState) {
    if (savedInstanceState == null) {
        mFragments[0] = TabHomeFragment.newInstance();
        mFragments[1] = TabMarketFragment.newInstance();
        mFragments[2] = TabTradeFragment.newInstance();
        mFragments[3] = MyPageFragment.newInstance();
        getSupportDelegate().loadMultipleRootFragment(R.id.fl_content, 0, mFragments);
    } else {
        currentTab = savedInstanceState.getInt(Constants.CURRENT_TAB);
        mFragments[0] = findFragment(TabHomeFragment.class);
        mFragments[1] = findFragment(TabMarketFragment.class);
        mFragments[2] = findFragment(TabTradeFragment.class);
        mFragments[3] = findFragment(TabHomeFragment.class);
        showHideFragment(mFragments[currentTab]);
    }
    btnNavigation.enableAnimation(false);
    btnNavigation.enableShiftingMode(false);
    btnNavigation.enableItemShiftingMode(false);
    btnNavigation.setTypeface(Typeface.DEFAULT_BOLD);
    btnNavigation.setOnNavigationItemSelectedListener(item -> {
        switch (item.getItemId()) {
            case R.id.menu_item_home:
                currentTab = 0;
                showHideFragment(mFragments[0]);
                return true;
            case R.id.menu_item_market:
                currentTab = 1;
                showHideFragment(mFragments[1]);
                return true;
            case R.id.menu_item_trade:
                currentTab = 2;
                showHideFragment(mFragments[2]);
                return true;
            case R.id.menu_item_mine:
                currentTab = 3;
                showHideFragment(mFragments[3]);
                return true;
            default:

        }
        return true;
    });
}

@Override
public void setStatusBar() {
    StatusBarUtil.setTransparentWithNoNavigation(this, true)；
}

@Override
public int getLayoutId() {
    return R.layout.activity_main;
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt(Constants.CURRENT_TAB, currentTab);
}

用到第三方

compile 'com.github.ittianyu:BottomNavigationViewEx:1.2.4'

xml

<?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="match_parent"
android:fitsSystemWindows="true"
android:orientation="vertical">

  <FrameLayout
    android:id="@+id/fl_content"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1" />

  <com.ittianyu.bottomnavigationviewex.BottomNavigationViewEx
    android:id="@+id/btn_navigation"
    android:layout_width="match_parent"
    android:layout_height="@dimen/dp50"
    android:background="@color/white"
    app:itemIconTint="@color/selector_bottom_item_color"
    app:itemTextColor="@color/selector_bottom_item_color"
    app:menu="@menu/bottom_navigation_menu" />
</LinearLayout>

或

/**
 * 初始化默认fragment
 *
 * @param savedInstanceState
 */
private void initDefaultFragment(Bundle savedInstanceState) {
    if (savedInstanceState != null) {
        LogUtil.e("-------------------------initAgain-------------------------------------->");
        //非空校验主要为了防止，置于后台activity被销毁，而fragment没有被销毁，再次add时页面重叠现象
        currentTab = savedInstanceState.getString(Constants.CURRENT_TAB);
        if (!TextUtils.isEmpty(currentTab)) {
            switchFragment(currentTab);
            LogUtil.e("--------------------------------------------------------------->" + currentTab);
        }
    } else {
        LogUtil.e("-------------------------init------------------------------------->");
        TabBuyQCFragment defaultFragment = TabBuyQCFragment.newInstance();
        getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.fl_content, defaultFragment, currentTab)
                .commit();
        currentFragment = defaultFragment;
    }
}