viewpager默认只缓存3个fragment，当前和前后2个fragment，如果viewpager中有4个fragment，滑动到第三个fragment的时候，第一个fragment就会被销毁

解决办法是重写FragmentPagerAdapter适配器的instantiateItem、destroyItem方法

    @Override
    @NonNull
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        Fragment fragment = (Fragment)super.instantiateItem(container, position);
        fm.beginTransaction().show(fragment).commitAllowingStateLoss();
        return fragment;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        Fragment fragment = (Fragment)object;
        fm.beginTransaction().hide(fragment).commitAllowingStateLoss();
    }