1.adapter:

public class ViewPager2FragmentAdapter extends FragmentStateAdapter {
    private List<Fragment> fragmentList;//管理所有的fragment

    public ViewPager2FragmentAdapter(@NonNull FragmentActivity fragmentActivity, List<Fragment> fragmentList) {
        super(fragmentActivity);
        this.fragmentList = fragmentList;
    }

    @NonNull
    @Override
    public Fragment createFragment(int position) {
        return fragmentList.get(position);
    }

    @Override
    public long getItemId(int position) {
        return fragmentList.get(position).hashCode();//唯一标识，可以用其他，具体看需求
    }

    /**
     * 异常问题：fragment already added 
     * @param itemId
     * @return
     */
    @Override
    public boolean containsItem(long itemId) {//是否已包含:true不添加，false则添加
        for(Fragment fragment:fragmentList){
            if(fragment.hashCode()==itemId){
                return true;
            }
        }
        return false;
    }

    @Override
    public int getItemCount() {
        return fragmentList != null ? fragmentList.size() : 0;
    }
}

2.添加到首位

fragmentList.add(0, LeftFragment.newInstance());
vp2Adapter.notifyItemInserted(0);