网上有很多这样的教程，我找了一个，忘了链接在哪了（等找到补上），主要是
public interface FragmentBackHandler {
boolean onBackPressed();
}

public class BackHandlerHelper {
/**
* 将back事件分发给 FragmentManager 中管理的子Fragment，如果该 FragmentManager 中的所有Fragment都
* 没有处理back事件，则尝试 FragmentManager.popBackStack()
*
* @return 如果处理了back键则返回 <b>true</b>
* @see #handleBackPress(Fragment)
* @see #handleBackPress(FragmentActivity)
*/
public static boolean handleBackPress(FragmentManager fragmentManager) {
List<Fragment> fragments = fragmentManager.getFragments();

    for (int i = fragments.size() - 1; i >= 0; i--) {
        Fragment child = fragments.get(i);

        if (isFragmentBackHandled(child)) {
            return true;
        }
    }

    if (fragmentManager.getBackStackEntryCount() > 0) {
        fragmentManager.popBackStack();
        Logger.info("popBackStack ");
        return true;
    }
    Logger.info("handleBackPress false");
    return false;
}

public static boolean handleBackPress(Fragment fragment) {
    return handleBackPress(fragment.getChildFragmentManager());
}

public static boolean handleBackPress(FragmentActivity fragmentActivity) {
    return handleBackPress(fragmentActivity.getSupportFragmentManager());
}

/**
 * 判断Fragment是否处理了Back键
 *
 * @return 如果处理了back键则返回 <b>true</b>
 */
public static boolean isFragmentBackHandled(Fragment fragment) {
    return fragment != null && fragment.isVisible() && fragment.getUserVisibleHint()
            && fragment instanceof FragmentBackHandler && ((FragmentBackHandler) fragment).onBackPressed();
}

}

结果我一直无法捕获，安卓系统是12，难道是不兼容了？
后来调试发现，获取的fragment只有一个，但事实有三个，获取的是NavHostFragment类型，然后查了发现，这是个容器，我应该获取里面的fragment。于是在handleBackPress方法里加了

    if (fragments.size() == 1 && fragments.get(0) instanceof androidx.navigation.fragment.NavHostFragment) {
        androidx.navigation.fragment.NavHostFragment host = (androidx.navigation.fragment.NavHostFragment) fragments.get(0);
        FragmentManager fm2 = host.getChildFragmentManager();
        fragments = fm2.getFragments();
    }

好了，成功在fragment中获取到返回键了。