Question

我正在尝试使用ViewPager创建一个TabLayout，但每当我离开应用程序时（不要关闭它，只需切换到另一个应用程序并返回），这些片段就会被杀死。我肯定不知道，但片段需要重新创建，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

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。

任何人都知道如何解决这个问题？

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，为了遵循Commonsware的建议，GetDataForListViewTask只获取数据。它没有设置适用于ListViews的适配器。相反，设置ListAdapter是在Fragment本身完成的。

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

并且，不是＆＃34;到达＆＃34;从外部改变其小部件甚至交付数据，这段代码也尝试（非常广泛地）遵循另一条CommonsWare的指导：

通常，让片段外的代码尝试操作这个片段里面的小部件 - 就像你在这里做的那样 - 很糟糕想法。

在任何情况下，如果您使用此方法（与原始方法略有不同），您将要修改GetDataForListViewTask，以便从应用程序或网络中的其他位置获取数据。

<强> 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));
    }

}

<强> 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;
        }
    }

}

<强> 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);
            }
        }
    }

<强> activity_main.xml中

<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>

<强> 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>

build.gradle的选定编译语句

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onCreateView()之前，不会对片段调用

FragmentManager。如果在将fragmentOne添加到FragmentManager后的某个时间调用，则最终的代码段最多只会有效。否则，片段的ListView将不存在。

一般来说，让代码在之外的片段试图操纵里面的小部件那个片段 - 正如你在这里所做的那样 - 是个坏主意。

无法设置ListView的适配器，因为ListView具有空对象引用

2 个答案: