java - 放置在 FrameLayout 内的 Fragment View 会抛出 NullPointerException

Question

我正在尝试在我的一项 Activity 中使用 FrameLayout。我希望这个 FrameLayout 成为 Activity 中的占位符，并且我想在其中添加/替换/删除 fragment 。

我有一个简单的咖啡馆应用程序，用于学习 Android。为简单起见，我将发布蛋糕和面包类别的代码。

问题: NullPointerException。我不知道为什么我的 fragment View 为空的原因。我猜测 fragment 生命周期可能没有运行，因此 View 没有膨胀，因此访问 View 会抛出空指针？请帮助我理解为什么会抛出空指针。

应用程序流程

用户在 ListFragment 中选择特定的蛋糕或面包 > 调用 CakeAndBreadActivity > 显示蛋糕或面包的特定详细信息

我正在寻找: View 中 NullPointerException 背后的解释和原因以及如何在使用 FrameLayout 时将蛋糕和面包详细信息显示到 fragment 中。

空指针异常

--------- beginning of crash
10-25 01:41:21.550 2374-2374/com.cafe E/AndroidRuntime: FATAL EXCEPTION: main
                                                             Process: com.cafe, PID: 2374
                                                             java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cafe/com.cafe.CakeAndBreadActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                 at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                 at android.os.Looper.loop(Looper.java:148)
                                                                 at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                              Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                                                                 at com.cafe.CakeAndBreadDetailFragment.displayCakeAndBreadDetails(CakeAndBreadDetailFragment.java:97)
                                                                 at com.cafe.CakeAndBreadActivity.displayCakeAndBreadDetails(CakeAndBreadActivity.java:30)
                                                                 at com.cafe.CakeAndBreadActivity.onCreate(CakeAndBreadActivity.java:19)
                                                                 at android.app.Activity.performCreate(Activity.java:6237)
                                                                 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                 at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                 at android.os.Looper.loop(Looper.java:148) 
                                                                 at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                 at java.lang.reflect.Method.invoke(Native Method) 
                                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

蛋糕和面包 Activity

public class CakeAndBreadActivity extends Activity implements CakeAndBreadDetailFragment.onCakeAndBreadDetailFragmentListener {

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

    public void displayCakeAndBreadDetails(){

        Intent cakeAndBreadIntent = getIntent();
        int cakeAndBreadId = Integer.valueOf(cakeAndBreadIntent.getIntExtra("cakeAndBreadNumber", 0));

        CakeAndBreadDetailFragment cakeAndBreadDetail = new CakeAndBreadDetailFragment();

        FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();
        cakeAndBreadDetail.displayCakeAndBreadDetails(cakeAndBreadId);
        fragmentTransaction.replace(R.id.cake_and_bread_detail_frame_layout_id, cakeAndBreadDetail);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();

    }

    @Override
    public void onCakeAndBreadDetail(int id) {

    }
}

activity_cake_and_bread.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context="com.cafe.CakeAndBreadActivity">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:id="@+id/cake_and_bread_detail_frame_layout_id"/>

</LinearLayout>

CakeAndBreadDetailFragment

public class CakeAndBreadDetailFragment extends Fragment {

    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";
    private String mParam1;
    private String mParam2;

    private onCakeAndBreadDetailFragmentListener mListener;

    public CakeAndBreadDetailFragment() {
    }

    public static CakeAndBreadDetailFragment newInstance(String param1, String param2) {
        CakeAndBreadDetailFragment fragment = new CakeAndBreadDetailFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_cake_and_bread_detail, container, false);
    }

    public void onButtonPressed(int id) {
        if (mListener != null) {
            mListener.onCakeAndBreadDetail(id);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof onCakeAndBreadDetailFragmentListener) {
            mListener = (onCakeAndBreadDetailFragmentListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement onCakeAndBreadDetailFragmentListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    public interface onCakeAndBreadDetailFragmentListener {
        // TODO: Update argument type and name
        void onCakeAndBreadDetail(int id);
    }

    public void displayCakeAndBreadDetails(int id){
        CakeAndBread cakeAndBread = (CakeAndBread)CakeAndBread.getCakeAndBreadList().get(id);
        View cakeAndBreadView = getView();
        TextView cakeAndBreadTextView = (TextView)cakeAndBreadView.findViewById(R.id.cake_and_bread_detail_fragment_id);
        cakeAndBreadTextView.setText(cakeAndBread.getCakeAndBreadName() + " " + cakeAndBread.getCakeAndBreadNutritionFacts());
    }
}

fragment_cake_and_bread_detail.xml

<LinearLayout 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.cafe.CakeAndBreadDetailFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:id="@+id/cake_and_bread_detail_fragment_id"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
         /> 
</LinearLayout>

编辑:按照建议，当我尝试访问 View 时， fragment 未完全加载，因此引发了 nullpointerexception。现在我的问题是框架布局或 fragment 不显示我放置在 fragment 中的 TextView ，我该如何解决这个问题？我传递蛋糕和面包 id，然后在 TextView 的 setText(...) 方法中设置文本。

Answer 1

    cakeAndBreadDetail.displayCakeAndBreadDetails(cakeAndBreadId);
    fragmentTransaction.replace(R.id.cake_and_bread_detail_frame_layout_id, cakeAndBreadDetail);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();

您正在调用 displayCakeAndBreadDetails(..)，它又调用 displayCakeAndBreadDetails(..)，而 displayCakeAndBreadDetails(..) 又调用 getView()。此时您还没有插入 fragment (发生在 fragmentTransaction.commit(); 之后)，因此没有与该 fragment 关联的 View 。因此 getView() 返回 null。