Fragment OnActivityCreated方法中的Android空指针异常

Question

我有一个相当简单的程序，我在其中玩弄 fragment 。它包含一个 Activity (ComponentsActivity)，在纵向 View 中附加一个 fragment (ComponentsFragment)。切换到横向时，ComponentsFragment 保留在屏幕左侧，ArticleFragment 添加到右侧。以纵向模式开始时一切正常，切换到横向模式也正常，但在处于横向模式后尝试切换回纵向模式会导致 ArticleFragment 的 OnActivityCreated 方法中出现空指针异常。我不确定为什么要调用此方法，因为 Activity 应该使用单个 ComponentsFragment 重新创建自身。

这是 Activity 代码:

package com.cab.apitesting;

import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.view.View;

public class ComponentsActivity extends FragmentActivity implements ComponentsFragment.OnArticleSelectedListener {
    boolean mDualPane;

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

        View articleContainer = findViewById(R.id.article_container);
        mDualPane = articleContainer != null && articleContainer.getVisibility() == View.VISIBLE;

        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.add(R.id.fragment_container, ComponentsFragment.newInstance());
        transaction.commit();

        if (mDualPane) {
            FragmentTransaction transaction2 = getSupportFragmentManager().beginTransaction();
            transaction2.add(R.id.article_container, ArticleFragment.newInstance(1));
            transaction2.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            transaction2.addToBackStack(null);
            transaction2.commit();
        }
    }

    @Override
    public void onArticleSelected(int id) {
        int articleIndex = 0;

        switch (id) {
            case R.id.button_article_one:
                articleIndex = 1;
                break;
            case R.id.button_article_two:
                articleIndex = 2;
                break;
            case R.id.button_article_three:
                articleIndex = 3;
                break;
            default:
                break;
        }

        if (mDualPane) {
            FragmentTransaction transaction3 = getSupportFragmentManager().beginTransaction();
            transaction3.add(R.id.article_container, ArticleFragment.newInstance(articleIndex));
            transaction3.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            transaction3.commit();
        } else {
            FragmentTransaction transaction3 = getSupportFragmentManager().beginTransaction();
            transaction3.replace(R.id.fragment_container, ArticleFragment.newInstance(articleIndex));
            transaction3.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
            transaction3.addToBackStack(null);
            transaction3.commit();
        }
    }
}

这是 ComponentsFragment 代码:

package com.cab.apitesting;

import android.app.Activity;
import android.content.ContentUris;
import android.content.Intent;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class ComponentsFragment extends Fragment { 
    Button artOneButton;
    Button artTwoButton;
    Button artThreeButton;

    OnArticleSelectedListener mArticleListener;

    public static ComponentsFragment newInstance() {
        ComponentsFragment f = new ComponentsFragment();
        Bundle args = new Bundle();
        f.setArguments(args);

        return f;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        try {
            mArticleListener = (OnArticleSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
        }
    }

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

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

        artOneButton = (Button)getView().findViewById(R.id.button_article_one);
        artOneButton.setOnClickListener(onClick);

        artTwoButton = (Button)getView().findViewById(R.id.button_article_two);
        artTwoButton.setOnClickListener(onClick);

        artThreeButton = (Button)getView().findViewById(R.id.button_article_three);
        artThreeButton.setOnClickListener(onClick);
    }

    private View.OnClickListener onClick = new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent;

            switch (view.getId()) {
                case R.id.button_article_one:
                case R.id.button_article_two:
                case R.id.button_article_three:
                    mArticleListener.onArticleSelected(view.getId());
                    break;
                default:
                    break;
            }
        }
    };

    public interface OnArticleSelectedListener {
        public void onArticleSelected(int id);
    }
}

这是 ArticleFragments 代码:

package com.cab.apitesting;

import android.content.res.Configuration;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class ArticleFragment extends Fragment {
    private static String[] articles = {
            "Bacon ipsum dolor sit amet ribeye nulla labore cow. Reprehenderit meatball pork belly non, laborum commodo tail. Aliquip nulla ad ham hock. Sausage swine jerky nulla consectetur in voluptate short loin ham hock tempor. Ball tip tenderloin excepteur strip steak in. Incididunt pork chop dolor, nisi short ribs ex mollit proident consequat.\n" +
                    "\n" +
                    "Velit sirloin doner brisket leberkas andouille elit rump. Cow est pariatur fugiat. Culpa shankle jowl andouille. Chuck beef ribs fatback ut, aliquip duis doner aute.\n" +
                    "\n" +
                    "T-bone consectetur filet mignon dolore beef ribs mollit, tenderloin esse tempor leberkas labore meatloaf ball tip. Jerky filet mignon meatloaf, officia pastrami voluptate beef drumstick capicola aute chuck shoulder venison short ribs. Fugiat ad enim venison, frankfurter rump beef ribs pancetta. Duis laborum in corned beef dolor in. Ball tip leberkas esse, pariatur commodo bacon jerky jowl pancetta bresaola tongue minim aliqua t-bone meatloaf. Ham sint tongue ut sunt pig culpa anim beef sirloin do. Irure jerky dolore tenderloin ball tip mollit ham hock, boudin eiusmod cow exercitation strip steak in filet mignon.\n" +
                    "\n" +
                    "Deserunt veniam meatloaf cupidatat meatball chicken sed qui cow consectetur filet mignon. Ad officia adipisicing, pig biltong nostrud nulla sirloin in sausage velit. Ut laborum meatloaf, tail exercitation consectetur officia fatback sirloin. Venison incididunt strip steak ham, sint ut pancetta cupidatat aliquip eiusmod in officia leberkas filet mignon. Beef ribs laborum kielbasa mollit boudin beef. Tri-tip ball tip pig nulla dolor ut pork chop aliqua corned beef sirloin t-bone. Ea adipisicing elit turkey velit id.\n" +
                    "\n" +
                    "Shank leberkas dolore pork chop nostrud qui short loin. Commodo ad ex aute proident elit filet mignon qui aliqua laborum kielbasa beef ribs ball tip labore. Voluptate laboris ut biltong incididunt corned beef. Ham hock kielbasa bresaola, shankle sint eu in reprehenderit minim excepteur frankfurter id sirloin commodo ut.",
            "Your bones don't break, mine do. That's clear. Your cells react to bacteria and viruses differently than mine. You don't get sick, I do. That's also clear. But for some reason, you and I react the exact same way to water. We swallow it too fast, we choke. We get some in our lungs, we drown. However unreal it may seem, we are connected, you and I. We're on the same curve, just on opposite ends.\n"
    };

    public static ArticleFragment newInstance(int index) {
        ArticleFragment f = new ArticleFragment();
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);

        return f;
    }

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

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

    TextView article = (TextView)getActivity().findViewById(R.id.textview_article);
    int index = getArguments().getInt("index");
    article.setText(articles[index - 1]);
}
}

错误日志如下:

08-20 15:35:58.214    1328-1328/com.cab.apitesting E/AndroidRuntime: FATAL EXCEPTION: main
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cab.apitesting/com.cab.apitesting.ComponentsActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
        at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3692)
        at android.app.ActivityThread.access$700(ActivityThread.java:141)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1240)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5041)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)
        Caused by: java.lang.NullPointerException
        at com.cab.apitesting.ArticleFragment.onActivityCreated(ArticleFragment.java:62)
        at android.support.v4.app.Fragment.performStart(Fragment.java:1484)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:941)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1070)
        at android.support.v4.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:1866)
        at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:568)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1164)
        at android.app.Activity.performStart(Activity.java:5114)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2153)
        ... 12 more

导致错误的行是:

article.setText(articles[index - 1]);

在 ArticleFragment 类中。我假设这是失败的，因为 article 变量为 null(应该是因为当应用程序处于纵向模式时 ArticleFragment 不应该存在)。但是为什么在一个根本不应该存在的 fragment 中调用 OnActivityCreated 方法？

Answer 1

不是这一行..

TextView article = (TextView)getActivity().findViewById(R.id.textview_article);

替换为..

TextView article = (TextView)getView().findViewById(R.id.textview_article);