在显示它的内容之前在android中初始化一个片段

Question

我有一个简单的 Android 应用程序，主要用于练习。它包含 MainActivity 和 2 个片段，一个带有 ImageView，第二个带有 Listview。我尝试使用 ArrayAdapter 填充 ListView，但目前我遇到了这个错误：

FATAL EXCEPTION: main
Process: com.ikurek.pwr, PID: 2792
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
    at com.ikurek.pwr.Budynki.onCreateView(Budynki.java:40)
    at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
    at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)

如果我理解正确，这是由于我的应用程序试图在视图创建之前填充 ListView。在我的填充功能触发之前，我无法弄清楚如何创建视图。我该怎么做？

我的 MainActivity 代码：

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

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


}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();


    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")

@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    FragmentManager fragmentManager = getSupportFragmentManager();

    //Obsługa poszczególych opcji menu
    if (id == R.id.nav_Budynki) {

        Budynki fragment = null;
        fragment = new Budynki().newInstance();
        fragmentManager.beginTransaction().replace(R.id.startScreen, fragment).commit();

    } else if (id == R.id.nav_Mapa) {

        Mapa fragment = null;
        fragment = new Mapa().newInstance();
        fragmentManager.beginTransaction().replace(R.id.startScreen, fragment).commit();

    } else if (id == R.id.nav_slideshow) {

    } else if (id == R.id.nav_manage) {

    } else if (id == R.id.nav_share) {

    } else if (id == R.id.nav_send) {

    }



    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

还有我的ListView片段代码：

public class Budynki extends Fragment {

public Budynki() {
}


public static Budynki newInstance() {
    Budynki fragment = new Budynki();
    return fragment;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_budynki, container, false);
}

public void makeListViewArray()
{
    String[] arrayBudynki ={"A","B","C"};
    ArrayAdapter<String> adapter = new ArrayAdapter <String> (getActivity(), android.R.layout.simple_list_item_1, arrayBudynki);
    ListView lista= (ListView) getView().findViewById(R.id.listView);
    lista.setAdapter(adapter);
}

我试图自己找到解决方案，但恐怕我做不到。我尝试在 OnCreate() 和 OnCreateView() 中调用 makeListViewArray()，但没有成功。请帮忙：-D

更新：

我将片段更改为：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_budynki, container, false);
    // make changes to view and its children here
    String[] arrayBudynki ={"A","B","C"};
    ArrayAdapter<String> adapter = new ArrayAdapter <String> (getActivity(), android.R.layout.simple_list_item_1, arrayBudynki);
    ListView lista= (ListView) getView().findViewById(R.id.listView);
    lista.setAdapter(adapter);
    return view;
}

我仍然遇到同样的错误，请指教？

更新：

Budynki.xml 片段：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.ikurek.pwr.Budynki">

<ListView
    android:layout_width="wrap_content"
    android:layout_height="496dp"
    android:id="@+id/listView"
    android:layout_gravity="center_horizontal|top" />

Answer 1

您可以开始对片段中的视图层次结构进行更改的最快方法是在 onCreateView() 中。您可以遵循以下模式：

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_budynki, container, false);
    // make changes to view and its children here
    return view;
}

Answer 2

问题是getView() 返回null，因为您还没有设置片段的视图。 （您仍在onCreateView() 中创建它。）请注意，您已经有一个名为view 的View 引用。实际上，您只是将 XML 布局膨胀到这个 View 对象中。改用它：

ListView lista= (ListView) view.findViewById(R.id.listView);

Answer 3

您应该在片段中的onViewCreated() 中调用makeListViewArray()，此时视图已创建（您可能已经从方法名称中猜到了）

另外，在您的活动中，调用Budynki.newInstance(); 而不是new Budynki().newInstance();； newInstace 是一个静态方法，因此您无需实例化您的类即可调用它。同样适用于Mapa 片段