无法设置 ListView 的适配器，因为 ListView 有一个空对象引用

Question

我正在尝试使用 ViewPager 制作 TabLayout，但是每当我离开应用程序（不要关闭它，只需切换到另一个应用程序并返回）时，片段就会被杀死。我不确定，但是片段需要重新创建，然后失败。

创建TabLayout、TabLayoutAdapter和ViewPager的代码：

    TabLayout tabLayout = (TabLayout) findViewById(R.id.tablayout);     
    MyTabLayoutAdapter tabLayoutAdapter;
    ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);

    tabLayout.addTab(tabLayout.newTab().setText("-2"));
    tabLayout.addTab(tabLayout.newTab().setText("-1"));
    tabLayout.addTab(tabLayout.newTab().setText("0"));
    tabLayout.addTab(tabLayout.newTab().setText("1"));
    tabLayout.addTab(tabLayout.newTab().setText("2"));

    tabLayoutAdapter = new MyTabLayoutAdapter(getSupportFragmentManager(), tabLayout.getTabCount());
    viewPager.setAdapter(tabLayoutAdapter);
    viewPager.setOffscreenPageLimit(3);
    tabLayout.setupWithViewPager(viewPager);
    viewPager.setCurrentItem(2);

我的 TabLayoutAdapter 的代码：

public class MyTabLayoutAdapter extends FragmentStatePagerAdapter {
    private MyFragment fragmentOne;
    private MyFragment fragmentTwo;
    private MyFragment fragmentDefault;
    private MyFragment fragmentFour;
    private MyFragment fragmentFive;
    private int numberOfTabs;

    public MyTabLayoutAdapter(FragmentManager fragmentManager, int numberOfTabs) {
        super(fragmentManager);
        fragmentOne = new MyFragment();
        fragmentTwo = new MyFragment();
        fragmentDefault = new MyFragment();
        fragmentFour = new MyFragment();
        fragmentFive = new MyFragment();
        this.numberOfTabs = numberOfTabs;
    }

    @Override
    public MyFragment getItem(int position) {
        switch (position) {
            case 0:
                return fragmentOne;
            case 1:
                return fragmentTwo;
            case 2:
                return fragmentDefault;
            case 3:
                return fragmentFour;
            case 4:
                return fragmentFive;
            default:
                return fragmentDefault;
        }
    }

    @Override
    public int getCount() {
        return numberOfTabs;
    }
}

MyFragment 片段：

public class MyFragment extends Fragment {
    private View currentView;
    private ListView listView;

    public MyFragment() {}

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        currentView = LayoutInflater.from(getContext()).inflate(R.layout.my_fragment_layout, container, false);
        listView = (ListView) currentView.findViewById(R.id.listview);
        return currentView;
    }

    //This line throws a NullPointer when I resume the app
    public void populateListView(MyListAdapter myListAdapter) {
        listView.setAdapter(myListAdapter);
    }
}

每当我打电话时

MyListAdapter myListAdapter = new MyListAdapter(this, R.layout.my_item, justSomeStringArray);
tabLayoutAdapter.getItem(0).populateListView(myListAdapter);

我得到一个 NullPointerException 说它无法在空对象引用上设置适配器。

有人知道我该如何解决这个问题吗？

Answer 1

这是一种让 Fragment 实例通过使用 AsyncTask 实现 (GetDataForListViewTask) 来驱动数据检索的方法。

但是，按照 Commonsware 的建议，GetDataForListViewTask 只是获取数据。它不会设置适配器以在 ListViews 上使用。相反，设置 ListAdapter 是在 Fragment 本身中完成的。

GetDataForListViewTask 使用 EventBus 将数据传回 Fragment 实例。这使 GetDataForListViewTask 不必维护对 ListView、Activity、Context 或任何有风险的东西的引用。

而且，通过不从外部“进入”片段来更改其小部件甚至传递数据，此代码还尝试（非常广泛地）遵循 CommonsWare 的另一条指导：

一般来说，让片段外部的代码尝试操作 该片段中的小部件——正如你在这里所做的那样——是一个坏的 想法。

无论如何，如果您使用这种方法，这与您原来的方法有点不同，您需要修改 GetDataForListViewTask 以便它从您的应用程序或网络等其他地方获取数据。

GetDataForListViewTask.java

import android.os.AsyncTask;
import org.greenrobot.eventbus.EventBus;
import java.util.ArrayList;


class GetDataForListViewTask extends AsyncTask<Void, Void, ArrayList<String>> {

    @Override
    protected ArrayList<String> doInBackground(Void... ignored) {
        // since this method runs on a worker thread, you may
        // replace this code with a network call or a background computation
        // from another part of the app is ready
        ArrayList<String> listItems = new ArrayList<>();
        for (int i=0; i<10; i++) {
            listItems.add("item " + i);
        }

        return listItems;
    }

    @Override
    protected void onPostExecute(ArrayList<String> result) {
        super.onPostExecute(result);

        // post event so that Fragment can use the data to populate its ListView
        EventBus.getDefault().post(new MyFragment.ListViewDataReadyEvent(result));
    }

}

MainActivity.java

import android.os.Bundle;
import android.support.design.widget.TabLayout;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

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

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tablayout);
        ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);

        MyAdapter tabLayoutAdapter = new MyAdapter(getSupportFragmentManager(), 5);
        viewPager.setAdapter(tabLayoutAdapter);
        viewPager.setOffscreenPageLimit(3);
        tabLayout.setupWithViewPager(viewPager);
        viewPager.setCurrentItem(2);
    }

    class MyAdapter extends FragmentStatePagerAdapter {

        private final int count;

        public MyAdapter(FragmentManager fm, int count) {
            super(fm);
            this.count = count;
        }

        @Override
        public MyFragment getItem(int position) {
            return MyFragment.newInstance();
        }

        @Override
        public int getCount() {
            return this.count;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return "Tab " + position;
        }
    }

}

MyFragment.java

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import org.greenrobot.eventbus.EventBus;
import org.greenrobot.eventbus.Subscribe;
import org.greenrobot.eventbus.ThreadMode;

import java.util.ArrayList;


public class MyFragment extends Fragment {

        private ListView listView;
        private GetDataForListViewTask getDataForListViewTask;

        public static MyFragment newInstance() {
            return new MyFragment();
        }

        @Override
        public void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setRetainInstance(true); // to support use of GetDataForListViewTask
            EventBus.getDefault().register(this);
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            listView = (ListView) LayoutInflater.from(getContext()).inflate(
                    R.layout.my_fragment_layout, container, false);
            getDataForListViewTask = new GetDataForListViewTask();
            getDataForListViewTask.execute();
            return listView;
        }

        @Override
        public void onDestroy() {
            if (getDataForListViewTask != null) {
                getDataForListViewTask.cancel(false);
            }
            EventBus.getDefault().unregister(this);
            super.onDestroy();
        }

        static class ListViewDataReadyEvent {

            ArrayList<String> data;
            ListViewDataReadyEvent(ArrayList<String> data) {
                this.data = data;
            }
        }

        @SuppressWarnings("unused")
        @Subscribe(threadMode = ThreadMode.MAIN)
        public void onMessageEvent(ListViewDataReadyEvent event) {
            if (listView != null) {
                ArrayAdapter<String> listAdapter = new ArrayAdapter<>(listView.getContext(),
                        android.R.layout.simple_list_item_1, event.data.toArray(new String[]{}));
                listView.setAdapter(listAdapter);
            }
        }
    }

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical" android:padding="4dip"
    android:gravity="center_horizontal"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <android.support.design.widget.TabLayout
        android:id="@+id/tablayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabMode="fixed"
        app:tabGravity="fill" />

    <android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/viewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </android.support.v4.view.ViewPager>

</LinearLayout>

my_fragment_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<ListView android:id="@+id/listview"
    android:layout_alignParentTop="true"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
</ListView>

为 build.gradle 选择的编译语句

compile 'org.greenrobot:eventbus:3.0.0'
compile 'com.android.support:design:25.3.1'
compile 'com.android.support:appcompat-v7:25.3.1'

Answer 2

在将片段添加到FragmentManager 之前，不会在片段上调用onCreateView()。您的最终代码 sn-p 充其量只能在将 fragmentOne 添加到 FragmentManager 之后的某个时间调用时才有效。否则，fragment 的ListView 将不存在。

一般来说，让片段外部的代码试图操纵片段内部的小部件——就像你在这里所做的那样——是个坏主意。