无法访问 Android 中的传感器

我正在编写一个指南针应用程序，但由于某种原因无法访问所需的传感器。 我已经实施了SensorEventListener以这种方式连接并注册我的传感器：

    sensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE);
    accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    magnetometer = sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);

In my onSensorChange() method:

public void onSensorChanged(SensorEvent event) {
    Toast t = Toast.makeText(this, "onSensorChanged", Toast.LENGTH_LONG);
    t.setGravity(Gravity.TOP, 0, 0);
    t.show();
    if(event.sensor.getType() == Sensor.TYPE_ACCELEROMETER){
        mGravity = event.values;
    }
    if(event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD){
        mGeomagnetic = event.values;
    }

    if(mGravity !=null && mGeomagnetic !=null){
        float [] R = new float[9];
        float [] I = new float [9];
        boolean success = sensorManager.getRotationMatrix(R, I, mGravity, mGeomagnetic);
        if(success){
            float [] orientation = new float [3];
              sensorManager.getOrientation(R, orientation);
            north = orientation[0];
        }
    }
}

The Toast消息不会显示在屏幕上，当我尝试将北值放入TextView输出是0。如果有人能向我解释为什么我无法访问该网站，那就太好了onSensorChanged()方法。清单中的权限：

<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
<uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION"/>
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

sensorManager.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
sensorManager.registerListener(this, magnetometer, SensorManager.SENSOR_DELAY_NORMAL);

不要忘记取消注册它们。

More: http://developer.android.com/reference/android/hardware/SensorManager.html