Android 我对恢复方法有困难

我正在开发一个实现SensorEventListener的应用程序，但onResume方法有问题 logcat告诉我有一个nullpointerexception，但我不明白为什么 这是我的活动

public class T11Ej2 extends Activity {

private SensorManager mSensorManager;
private Sensor mSensor;
private SampleView mView;
private float[] mValues;


private final SensorEventListener mListener = new SensorEventListener() {

    @Override
    public void onSensorChanged(SensorEvent event) {
        // TODO Auto-generated method stub
        mValues = event.values;
        if(mView != null){
            mView.invalidate();
        }

    }

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

        mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
        mSensorManager.registerListener(mListener, mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD), SensorManager.SENSOR_DELAY_NORMAL);            
    }
};
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    mView = new SampleView(this);
    setContentView(mView);
}






@Override
protected void onStop() {
    // TODO Auto-generated method stub
    super.onStop();
    mSensorManager.unregisterListener(mListener);

}




@Override
protected void onResume() {
    // TODO Auto-generated method stub
    super.onResume();
    mSensorManager.registerListener(mListener, 
            mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_NORMAL);

}



@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.t11_ej2, menu);
    return true;
}

// Código para pintar la brujula
private class SampleView extends View{

    private Paint mPaint = new Paint();
    private Path mPath = new Path();

    public SampleView(Context context) {
        super(context);
        // TODO Auto-generated constructor stub
        mPath.moveTo(0, -50);
        mPath.lineTo(-20, 60);
        mPath.lineTo(0, 50);
        mPath.lineTo(20, 60);
        mPath.close();


    }

    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub

            Paint paint = mPaint;
            canvas.drawColor(Color.WHITE);
            paint.setAntiAlias(true);
            paint.setColor(Color.BLACK);
            paint.setStyle(Paint.Style.FILL);

            int w = canvas.getWidth();
            int h = canvas.getHeight();
            int cx = w / 2;
            int cy = h / 2;
            canvas.translate(cx, cy);

            if (mValues != null) {
            canvas.rotate(-mValues[0]);
            }
            canvas.drawPath(mPath, mPaint);
    }   
}}

这是logcat

    07-29 01:51:27.758: E/AndroidRuntime(3236): FATAL EXCEPTION: main
    07-29 01:51:27.758: E/AndroidRuntime(3236): java.lang.RuntimeException: Unable to resume activity {com.example.t11ej2/com.example.t11ej2.T11Ej2}: java.lang.NullPointerException
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2742)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2771)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2235)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.os.Handler.dispatchMessage(Handler.java:99)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.os.Looper.loop(Looper.java:137)
    07-29 01:51:27.758: E/AndroidRuntime(3236):     at android.app.ActivityThread.main(ActivityThread.java:5041)

---

它看起来像是

mSensorManager

为

null

，因为在

onResume（）

中尝试使用它之前，您尚未初始化它

onResume（）

在

onCreate（）

之后调用，因此您需要在调用

onResume（）

之前对其进行初始化

我没有用过这个，所以我肯定不是专家，但这就是他们初始化它的方式

  public SensorActivity() {
     mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
     mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
 }

 protected void onResume() {
     super.onResume();
     mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);
 }

---

虽然初始化了mListener，但它的回调函数onSensorChanged和onAccuracyChanged实际上在侦听器注册之前都没有被调用。这意味着MSSensorManager仍然为空。小心点

首先，registerListener和unregisterListener方法不正确。您需要查看活动生命周期！因此，您应该将onResume（）方法更改为onStart（）。 第二，这种方法：

public SampleView(Context context) {
        super(context);
        // TODO Auto-generated constructor stub
        mPath.moveTo(0, -50);
        mPath.lineTo(-20, 60);
        mPath.lineTo(0, 50);
        mPath.lineTo(20, 60);
        mPath.close();
    }

，我认为你应该用onDraw方法来做这件事，在绘制完成之前，你不应该关闭mPath。 祝你好运