android - 如果该 fragment 已打开，如何防止从抽屉导航中打开该 fragment

Question

如果 fragment 已经打开，如何防止从抽屉导航中打开 fragment ，当 fragment 打开并单击 NavigationDrawer 项目时， fragment 会再次重新创建，那么如何检查 fragment 是否已经打开？

 public boolean onNavigationItemSelected(MenuItem item) {
    int id = item.getItemId();
    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

    if (id == R.id.fragmentA) {
         {   FragmentA fragment = new FragmentA();
             transaction.add(R.id.main_screen, fragment1, "MainFrag").addToBackStack(null);
         }

    } else if (id == R.id.fragmentB) {
        FragmentB fragment = new FragmentB();
        transaction.add(R.id.main_screen, fragment).addToBackStack(null);

    } else if (id == R.id.fragmentC) {
        FragmentC fragment = new FragmentC();
        transaction.replace(R.id.main_screen, fragment).addToBackStack(null);

    }

Answer 1

我是这样解决的:

public boolean onNavigationItemSelected(MenuItem item) {

    int id = item.getItemId();

    Fragment f = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
    if (id == R.id.n_1 && !(f instanceof MainFragment)) {
        MainFragment fragment = new MainFragment();
        android.support.v4.app.FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.replace(R.id.fragment_container, fragment);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    } }