实际上我希望Accelerometer在后台运行并发出消息，但我得到一些例外。这是我的所有xml和java文件。请帮忙！
这是我的Service班级
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.os.IBinder;
import android.widget.Toast;
public class MyService extends Service implements AccelerometerListener
{
private static Context mycontext;
@Override
public IBinder onBind(Intent arg0) {
throw new UnsupportedOperationException("Not Implemented Yet");
}
@Override
public void onCreate() {
// TODO Auto-generated method stub
mycontext=MyService.this;
Toast.makeText(this,"Accelerometer is starting", Toast.LENGTH_LONG);
AccelerometerManager.startListening(this);
}
@Override
public void onShake(float force) {
// TODO Auto-generated method stub
Toast.makeText(this,"Motion detected", Toast.LENGTH_SHORT);
}
@Override
public void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
Toast.makeText(this,"Accelerometer is destroyed",Toast.LENGTH_LONG);
AccelerometerManager.stopListening();
}
public static Context getContext() {
return mycontext;
}
@Override
public void onAccelerationChanged(float x, float y, float z) {
// TODO Auto-generated method stub
}
}
MainActivity上课
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
// Start the service
public void startNewService(View view)
{
startService(new Intent(MainActivity.this, MyService.class));
}
// Stop the service
public void stopNewService(View view)
{
stopService(new Intent(MainActivity.this, MyService.class));
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
AccelerometerListener
public interface AccelerometerListener {
public void onAccelerationChanged(float x, float y, float z);
public void onShake(float force);
}
AccelerometerManager上课
import java.util.List;
import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.widget.Toast;
public class AccelerometerManager {
private static Context aContext=null;
/** Accuracy configuration */
private static float threshold = 15.0f;
private static int interval = 200;
private static Sensor sensor;
private static SensorManager sensorManager;
// you could use an OrientationListener array instead
// if you plans to use more than one listener
private static AccelerometerListener listener;
/** indicates whether or not Accelerometer Sensor is supported */
private static Boolean supported;
/** indicates whether or not Accelerometer Sensor is running */
private static boolean running = false;
/**
* Returns true if the manager is listening to orientation changes
*/
public static boolean isListening() {
return running;
}
/**
* Unregisters listeners
*/
public static void stopListening() {
running = false;
try {
if (sensorManager != null && sensorEventListener != null) {
sensorManager.unregisterListener(sensorEventListener);
}
} catch (Exception e) {}
}
/**
* Returns true if at least one Accelerometer sensor is available
*/
public static boolean isSupported(Context context) {
aContext = context;
if (supported == null) {
if (aContext != null) {
sensorManager = (SensorManager) aContext.
getSystemService(Context.SENSOR_SERVICE);
// Get all sensors in device
List<Sensor> sensors = sensorManager.getSensorList(
Sensor.TYPE_ACCELEROMETER);
supported = new Boolean(sensors.size() > 0);
} else {
supported = Boolean.FALSE;
}
}
return supported;
}
/**
* Configure the listener for shaking
* @param threshold
* minimum acceleration variation for considering shaking
* @param interval
* minimum interval between to shake events
*/
public static void configure(int threshold, int interval) {
AccelerometerManager.threshold = threshold;
AccelerometerManager.interval = interval;
}
/**
* Registers a listener and start listening
* @param accelerometerListener
* callback for accelerometer events
*/
public static void startListening( AccelerometerListener accelerometerListener )
{
sensorManager = (SensorManager) aContext.
getSystemService(Context.SENSOR_SERVICE);
// Take all sensors in device
List<Sensor> sensors = sensorManager.getSensorList(
Sensor.TYPE_ACCELEROMETER);
if (sensors.size() > 0) {
sensor = sensors.get(0);
// Register Accelerometer Listener
running = sensorManager.registerListener(
sensorEventListener, sensor,
SensorManager.SENSOR_DELAY_GAME);
listener = accelerometerListener;
}
}
/**
* Configures threshold and interval
* And registers a listener and start listening
* @param accelerometerListener
* callback for accelerometer events
* @param threshold
* minimum acceleration variation for considering shaking
* @param interval
* minimum interval between to shake events
*/
public static void startListening(
AccelerometerListener accelerometerListener,
int threshold, int interval) {
configure(threshold, interval);
startListening(accelerometerListener);
}
/**
* The listener that listen to events from the accelerometer listener
*/
private static SensorEventListener sensorEventListener =
new SensorEventListener() {
private long now = 0;
private long timeDiff = 0;
private long lastUpdate = 0;
private long lastShake = 0;
private float x = 0;
private float y = 0;
private float z = 0;
private float lastX = 0;
private float lastY = 0;
private float lastZ = 0;
private float force = 0;
public void onAccuracyChanged(Sensor sensor, int accuracy) {}
public void onSensorChanged(SensorEvent event) {
// use the event timestamp as reference
// so the manager precision won't depends
// on the AccelerometerListener implementation
// processing time
now = event.timestamp;
x = event.values[0];
y = event.values[1];
z = event.values[2];
// if not interesting in shake events
// just remove the whole if then else block
if (lastUpdate == 0) {
lastUpdate = now;
lastShake = now;
lastX = x;
lastY = y;
lastZ = z;
Toast.makeText(aContext,"No Motion detected",
Toast.LENGTH_SHORT).show();
} else {
timeDiff = now - lastUpdate;
if (timeDiff > 0) {
/*force = Math.abs(x + y + z - lastX - lastY - lastZ)
/ timeDiff;*/
force = Math.abs(x + y + z - lastX - lastY - lastZ);
if (Float.compare(force, threshold) >0 ) {
//Toast.makeText(Accelerometer.getContext(),
//(now-lastShake)+" >= "+interval, 1000).show();
if (now - lastShake >= interval) {
// trigger shake event
listener.onShake(force);
}
else
{
Toast.makeText(aContext,"No Motion detected",
Toast.LENGTH_SHORT).show();
}
lastShake = now;
}
lastX = x;
lastY = y;
lastZ = z;
lastUpdate = now;
}
else
{
Toast.makeText(aContext,"No Motion detected", Toast.LENGTH_SHORT).show();
}
}
// trigger change event
listener.onAccelerationChanged(x, y, z);
}
};
}
清单文件
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.secondapplication"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="18" />
<uses-permission android:name="android.permission.READ_PHONE_STATE"/>
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<service
android:name=".MyService"
/>
<activity
android:name="com.example.secondapplication.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
main.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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity" >
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="22dp"
android:layout_marginTop="44dp"
android:onClick="startNewService"
android:text="Start Activity"
/>
<Button
android:id="@+id/button2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/button1"
android:layout_alignParentRight="true"
android:layout_marginRight="14dp"
android:onClick="stopNewService"
android:text="Stop Activity"
/>
</RelativeLayout>
例外
FATAL EXCEPTION: main
12-30 17:28:12.729: E/AndroidRuntime(16553): java.lang.RuntimeException: Unable to create service com.example.secondapplication.MyService: java.lang.NullPointerException
12-30 17:28:12.729
答案 0 :(得分：1)
我认为当您在Service onCreate方法中调用AccelerometerManager.startListening(this);时，AccelerometerManager的aContext字段为空。
您应该分配传递给aContext
更改startListening(AccelerometerListener accelerometerListener)的签名
到startListening(Context accelerometerListener)
public static void startListening( Context accelerometerListener )
{
aContext = accelerometerLister;
sensorManager = (SensorManager) aContext.
getSystemService(Context.SENSOR_SERVICE);
.......
.........
listener = (AccelerometerListener) accelerometerListener;
}
}