Question

我创建了一个包含viewpager的HomeFragment，并将其添加到MainActivity中，它运行正常。但当我点击进入另一个片段如BucketFragment然后点击返回homefragment时，它崩溃了，抛出了一个nullpointer异常。

这是MainActivity

#include <iostream>
using namespace std;
int main() {
    int P=1000;
    int i=0.0275;
    int FV;
    int n;
    cout << "enter number of years:"<< endl;
    cin >> n;
    cout << "the future value is:"<< endl;
    FV = P*(1+(i*n));
    cout << FV << endl;
    return 0;
    }

这是HomeFragment

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener{
    private DrawerLayout mDrawer;
    private HomeFragment mHomeFragment;
    private LikeFragment mLikeFragment;
    private BucketFragment mBucketFragment;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    // add the fragment content
    FragmentManager fm = getSupportFragmentManager();
    Fragment fragment = fm.findFragmentById(R.id.fragment_container);

    if (fragment == null) {
        fragment = HomeFragment.newInstance();
        mHomeFragment = (HomeFragment) fragment;
        fm.beginTransaction()
                .add(R.id.fragment_container, fragment)
                .commit();
    }
}

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    Fragment currentFragment = null;
    Intent intent = null;

    switch (item.getItemId()) {
        case R.id.nav_home:
            if (mHomeFragment == null) {
                mHomeFragment = HomeFragment.newInstance();
            }
            currentFragment = mHomeFragment;
            break;
        case R.id.nav_like:
            if (mLikeFragment == null) {
                mLikeFragment = LikeFragment.newInstance();
            }
            currentFragment = mLikeFragment;
            break;
        case R.id.nav_bucket:
            if (mBucketFragment == null) {
                mBucketFragment = BucketFragment.newInstance();
            }
            currentFragment = mBucketFragment;
            break;
        case R.id.nav_help:
            intent = HelpActivity.newIntent(this);
            break;
        case R.id.nav_feedback:
            intent = FeedBackActivity.newIntent(this);
            break;
        default:break;
    }


    if (currentFragment != null) {
        FragmentManager fm = getSupportFragmentManager();
        fm.beginTransaction()
                .replace(R.id.fragment_container, currentFragment)
                .commit();
        setTitle(item.getTitle());
    } else {
        startActivity(intent);
    }

    mDrawer.closeDrawer(GravityCompat.START);
    return true;
}
}

和public class HomeFragment extends Fragment { private static final int NUM_CATOGARY = 2; private ViewPager mViewPager; public static HomeFragment newInstance() { return new HomeFragment(); } @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); } @Nullable @Override public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_home, container, false); mViewPager = (ViewPager) v.findViewById(R.id.view_pager); TabLayout tab = (TabLayout) v.findViewById(R.id.tab); tab.setupWithViewPager(mViewPager); FragmentManager fm = getChildFragmentManager(); mViewPager.setAdapter(new FragmentStatePagerAdapter(fm) { @Override public Fragment getItem(int position) { switch (position) { case 0: return PopularFragment.newInstance(); case 1: return RecentFragment.newInstance(); } return null; } @Override public int getCount() { return NUM_CATOGARY; } @Nullable @Override public CharSequence getPageTitle(int position) { switch (position) { case 0: return getResources().getText(R.string.tab_popular); case 1: return getResources().getText(R.string.tab_recent); } return null; } }); return v; } }

fragment_home.xml

当我点击返回homefragment时，它会抛出一个运行时异常

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
android:orientation="vertical">

<android.support.design.widget.TabLayout
    android:id="@+id/tab"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

<android.support.v4.view.ViewPager
    android:id="@+id/view_pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

Answer 1

问题在于这段代码

if (currentFragment != null) {
        FragmentManager fm = getSupportFragmentManager();
        fm.beginTransaction()
                .replace(R.id.fragment_container, currentFragment)
                .commit();
        setTitle(item.getTitle());
    }

替换方法用作删除+添加，这意味着您没有要返回的堆栈中的片段，因此您应该改为添加add或在.addToBackStack(null);之前添加此行.commit。它会将您的操作置于Backstack，然后您可以使用后退按钮

返回

Answer 2

如果您将addToBackStack(null)用于supportFragmentManager，则每个片段在开始时都会重新加载，因此fragment不会保留在堆栈中。

我已将childFragmentManager中的supportFragmentManager更改为android.support.v4.app.Fragment并开始工作。

用包含viewpager的片段替换片段时崩溃

2 个答案: