重寫方法onAttachFragment

@Override
    public void onAttachFragment(Fragment fragment) {
        if (homeFragment == null && fragment instanceof MainFragment)
            homeFragment = (MainFragment) fragment;
        if (orderFragment == null && fragment instanceof OrderFragment)
            orderFragment = (OrderFragment)fragment;
        if (personFragment == null && fragment instanceof PersonFragment)
            personFragment = (PersonFragment) fragment;

    }