创建片段时的 NPE

Question

我正在创建一个关于 Fragment 和 sensorEventlitener 的示例。 mainactivity 托管 fragment，如下面的代码所示，但在运行时我收到下面发布的 logcat 错误。

MainActivity:

public class MainActivity extends Activity {

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

    initViews();
}

private void initViews() {
    // TODO Auto-generated method stub

    View frag1 = findViewById(R.id.frag_1);

    FragmentManager fMgr = getFragmentManager();
    FragmentTransaction fragmentTransaction = fMgr.beginTransaction();
    fragmentTransaction.replace(R.id.frag_1, new Frag1());
    fragmentTransaction.commit();
}

}

Frag1:

public class Frag1 extends Fragment implements SensorEventListener {

private SensorManager sensorManager;
TextView tvAccX;
TextView tvAccY;
TextView tvAccZ;
Button btnSend;

@Override
public void onAttach(Activity activity) {
    // TODO Auto-generated method stub
    super.onAttach(activity);

    sensorManager = (SensorManager) getActivity().getSystemService(Context.SENSOR_SERVICE);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    View view = inflater.inflate(R.layout.frag_1, container, false);

    tvAccX = (TextView) view.findViewById(R.id.tv_accX_value);
    tvAccY = (TextView) view.findViewById(R.id.tv_accY_value);
    tvAccZ = (TextView) view.findViewById(R.id.tv_accZ_value);
    btnSend = (Button) view.findViewById(R.id.btn_send);

    return view;
}

@Override
public void onResume() {
    // TODO Auto-generated method stub
    super.onResume();
    Sensor accSensor = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    sensorManager.registerListener(this, accSensor, sensorManager.SENSOR_DELAY_FASTEST);
}

@Override
public void onPause() {
    // TODO Auto-generated method stub
    super.onPause();
    sensorManager.unregisterListener(this);
}

@Override
public void onSensorChanged(SensorEvent event) {
    // TODO Auto-generated method stub
    switch (event.sensor.getType()) {
    case Sensor.TYPE_ACCELEROMETER:
        showAccReadings(event);
        break;

    default:
        break;
    }
}

private void showAccReadings(SensorEvent event) {
    // TODO Auto-generated method stub
    float[] values = event.values;

    float x = values[0];
    float y = values[1];
    float z = values[2];

    tvAccX.setText(String.valueOf(x));
    tvAccY.setText(String.valueOf(y));
    tvAccZ.setText(String.valueOf(z));
}

@Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {
    // TODO Auto-generated method stub

}

}

mainActivity.xml:

<RelativeLayout 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="${relativePackage}.${activityClass}">

<fragment
    android:id="@+id/frag_1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"/>

<fragment
    android:id="@+id/frag_2"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="50dp"
    android:layout_centerHorizontal="true"
    android:layout_below="@+id/frag_1"/>

</RelativeLayout>

Frag1.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView 
    android:id="@+id/tv_accX_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentStart="true"
    android:text="Acc[X]: "/>
<TextView 
    android:id="@+id/tv_accX_value"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_toRightOf="@+id/tv_accX_label"/>

<TextView 
    android:id="@+id/tv_accY_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentStart="true"
    android:layout_below="@+id/tv_accX_label"
    android:text="Acc[Y]: "/>
<TextView 
    android:id="@+id/tv_accY_value"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_toRightOf="@+id/tv_accY_label"/>

<TextView 
    android:id="@+id/tv_accZ_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentStart="true"
    android:layout_below="@+id/tv_accY_label"
    android:text="Acc[Z]: "/>
<TextView 
    android:id="@+id/tv_accZ_value"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_toRightOf="@+id/tv_accZ_label"/>

<Button 
    android:id="@+id/btn_send"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/tv_accZ_label"
    android:gravity="center_vertical|center_horizontal"
    android:text="send to frag_2"/>
</RelativeLayout>

LogCat:

01-23 15:25:11.200: E/AndroidRuntime(7195): Process: com.example.fragmentcommunication_00, PID: 7195
01-23 15:25:11.200: E/AndroidRuntime(7195): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fragmentcommunication_00/com.example.fragmentcommunication_00.MainActivity}: android.view.InflateException: Binary XML file line #7: Error inflating class fragment
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2282)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2340)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread.access0(ActivityThread.java:157)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.os.Handler.dispatchMessage(Handler.java:102)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.os.Looper.loop(Looper.java:157)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread.main(ActivityThread.java:5293)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.reflect.Method.invokeNative(Native Method)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.reflect.Method.invoke(Method.java:515)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at dalvik.system.NativeStart.main(Native Method)
01-23 15:25:11.200: E/AndroidRuntime(7195): Caused by: android.view.InflateException: Binary XML file line #7: Error inflating class fragment
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:719)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.rInflate(LayoutInflater.java:761)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.inflate(LayoutInflater.java:498)
01-2    3 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.inflate(LayoutInflater.java:398)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.inflate(LayoutInflater.java:354)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:340)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Activity.setContentView(Activity.java:1973)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at com.example.fragmentcommunication_00.MainActivity.onCreate(MainActivity.java:17)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Activity.performCreate(Activity.java:5389)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2246)
01-23 15:25:11.200: E/AndroidRuntime(7195):     ... 11 more
01-23 15:25:11.200: E/AndroidRuntime(7195): Caused by: java.lang.NullPointerException: name == null
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.VMClassLoader.findLoadedClass(Native Method)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.ClassLoader.findLoadedClass(ClassLoader.java:350)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.ClassLoader.loadClass(ClassLoader.java:487)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at java.lang.ClassLoader.loadClass(ClassLoader.java:457)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Fragment.instantiate(Fragment.java:583)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Fragment.instantiate(Fragment.java:561)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.app.Activity.onCreateView(Activity.java:4927)
01-23 15:25:11.200: E/AndroidRuntime(7195):     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:695)

更新:

Frag1.xml:

 <fragment
    android:name="com.example.fragmentcommunication_00.Frag1"
    android:id="@+id/frag_1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"/>

Logact:

01-23 15:37:42.980: E/AndroidRuntime(8267): FATAL EXCEPTION: main
01-23 15:37:42.980: E/AndroidRuntime(8267): Process: com.example.fragmentcommunication_00, PID: 8267
01-23 15:37:42.980: E/AndroidRuntime(8267): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fragmentcommunication_00/com.example.fragmentcommunication_00.MainActivity}: android.view.InflateException: Binary XML file line #14: Error inflating class fragment
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2282)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2340)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread.access0(ActivityThread.java:157)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.os.Handler.dispatchMessage(Handler.java:102)
    01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.os.Looper.loop(Looper.java:157)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread.main(ActivityThread.java:5293)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.reflect.Method.invokeNative(Native Method)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.reflect.Method.invoke(Method.java:515)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at dalvik.system.NativeStart.main(Native Method)
01-23 15:37:42.980: E/AndroidRuntime(8267): Caused by: android.view.InflateException: Binary XML file line #14: Error inflating class fragment
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:719)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.rInflate(LayoutInflater.java:761)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.inflate(LayoutInflater.java:498)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.inflate(LayoutInflater.java:398)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.inflate(LayoutInflater.java:354)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:340)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Activity.setContentView(Activity.java:1973)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at com.example.fragmentcommunication_00.MainActivity.onCreate(MainActivity.java:17)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Activity.performCreate(Activity.java:5389)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2246)
01-23 15:37:42.980: E/AndroidRuntime(8267):     ... 11 more
01-23 15:37:42.980: E/AndroidRuntime(8267): Caused by: java.lang.NullPointerException: name == null
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.VMClassLoader.findLoadedClass(Native Method)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.ClassLoader.findLoadedClass(ClassLoader.java:350)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.ClassLoader.loadClass(ClassLoader.java:487)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at java.lang.ClassLoader.loadClass(ClassLoader.java:457)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Fragment.instantiate(Fragment.java:583)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Fragment.instantiate(Fragment.java:561)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.app.Activity.onCreateView(Activity.java:4927)
01-23 15:37:42.980: E/AndroidRuntime(8267):     at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:695)

Answer 1

您正在 xml

中声明 <fragment 标签

   <fragment
    android:id="@+id/frag_2"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="50dp"
    android:layout_centerHorizontal="true"
    android:layout_below="@+id/frag_1"/>

但是你没有指定android:name=属性，它告诉系统它必须实例化哪个片段。它必须包含要实例化的片段子类的完整路径。例如

android:name="path.to.Frag1"

Answer 2

您的 activity 布局文件中的 fragment 标签似乎缺少 name 属性。基本上有两种方法可以将 Fragment 扩展到您的视图层次结构中：

指定布局中的所有内容XML
在您的java代码中加载片段

看来你把这两种范式混在一起了，这就是给你带来麻烦的原因。

为了在布局 XML 中执行所有操作，您必须指定 name 属性。所以，像这样：

<fragment
    android:name="com.my.package.for.Frag1"
    android:id="@+id/frag_1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"/>

然后您可以从 initViews() 方法中删除代码，因为它不再需要了。

相反，如果您希望在代码中完成所有操作，那么您必须更改布局以指定一个空容器以放置片段。例如，使用 FrameLayout 而不是 fragment 标签，如下所示：

<FrameLayout
    android:id="@+id/frag_1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"/>

创建片段时的 NPE

NPE when creating a fragment

android

android-fragments