android - Fragment 的 GetTag 返回空对象引用

Question

使用其中使用了多个 fragment 的应用程序，并且以下代码用于获取 fragment 的选定位置。

private BaseFragment getSelectedFragment(FragmentManager fragmentManager)
    {
        int item = getModel().getSelectedItem();//0th position last
        String tag = String.valueOf(item);
        BaseFragment fragment = (BaseFragment) fragmentManager.findFragmentByTag(tag); //error shown at this line
        return fragment;
    }

并从

调用上面的方法

public boolean onBackPressed() 
{
   FragmentManager fragmentManager = activity.getFragmentManager();
   BaseController fragmentController =             getSelectedFragment(fragmentManager).getController();
}

由于以下错误而崩溃

java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“java.lang.String xxx.android.fwk.app.fragment.BaseFragment.getTag()”。

使用以下代码替换 fragment

private void displaySelectedFragment()
    {

        FragmentManager fragmentManager = getFragmentManager();
        if (fragmentManager.getBackStackEntryCount() > 0) {
            // pop any inner fragments that have been added.
            fragmentManager.popBackStackImmediate(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
        }

        // get the selected item position
        int selectedItem = model.getSelectedDrawerItem().getItemId();

        String tag = String.valueOf(selectedItem);
        Bundle extras = model.getExtras();

        BaseFragment newFragment = NomadFragmentManager.getInstance().getFragment(selectedItem, extras);
        if (newFragment != null) {
            FragmentTransaction ft = fragmentManager.beginTransaction();
            ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            ft.replace(R.id.container, newFragment, tag);
            ft.commit();
        }
    }

那么实际问题的原因是什么？深入研究这段代码。非常感谢任何帮助。提前致谢

Answer 1

引自Document

Replace an existing fragment that was added to a container. This is essentially the same as calling remove(Fragment) for all currently added fragments that were added with the same containerViewId and then add(int, Fragment, String) with the same arguments given here.

替换 Fragment 会将其从返回堆栈中完全移除。因此，如果您在与其他 Fragment 交易后尝试检索 Tag，则它将不可用。您应该使用以下方法将 Fragment 添加到返回堆栈addToBackStack(tagName)。