我希望在用户玩游戏时播放背景音乐。当用户启动应用程序时音乐开始,当他们离开应用程序时暂停,并在返回应用程序时恢复。
我尝试使用this method,我编辑了一下:
public class MainActivity extends Activity {
private boolean bounded;
private BackgroundSoundService backgroundSoundService;
ServiceConnection connection = new ServiceConnection() {
@Override
public void onServiceDisconnected( ComponentName name ) {
bounded = false;
backgroundSoundService = null;
}
@Override
public void onServiceConnected( ComponentName name, IBinder service ) {
bounded = true;
BackgroundSoundService.LocalBinder localBinder = (BackgroundSoundService.LocalBinder) service;
backgroundSoundService = localBinder.getServiceInstance();
}
};
@Override
public void onCreate( Bundle savedInstanceState ) {
super.onCreate(savedInstanceState);
// (code that's not necessary)
backgroundSoundService.start(); // this is where the error is thrown
}
@Override
public void onPause() {
super.onPause();
backgroundSoundService.pause();
}
@Override
public void onResume() {
super.onResume();
backgroundSoundService.resume();
}
@Override
public void onStop() {
super.onStop();
backgroundSoundService.pause();
}
@Override
public void onStart() {
super.onStart();
Intent intent = new Intent(this, BackgroundSoundService.class);
bindService(intent, connection, BIND_AUTO_CREATE);
backgroundSoundService.start();
}
@Override
public void onDestroy() {
super.onDestroy();
backgroundSoundService.destroy();
}
}
我使用活动来播放,暂停和恢复背景音乐。我将在这里省略这个问题的不必要的方法/行:
public class BackgroundSoundService extends Service {
private static final String TAG = null;
public IBinder binder = new LocalBinder();
public IBinder onBind( Intent arg0 ) {
return binder;
}
public IBinder onUnBind( Intent arg0 ) {
return null;
}
public class LocalBinder extends Binder {
public BackgroundSoundService getServiceInstance() {
return BackgroundSoundService.this;
}
}
}
但是,当我运行应用程序时,我在NullPointerException
类中获得了MainActivity
(在onCreate
方法中,我在代码中对其进行了评论)。
该变量似乎尚未初始化,但我确实需要在用户打开应用程序时启动音乐。
我也尝试从backgroundSoundService.start();
方法中删除onCreate
,所以音乐会在调用onStart
时开始。但是,当我这样做时,我得到了同样的错误。
那么,如何在用于调用其方法之前初始化backgroundSoundService
?
首先从onCreate中删除这个backgroundSoundService.start()
并将其添加到onServiceConnected()
方法中
你需要在做任何与backgroundSoundService相关的事情之前检查null
@Override
public void onPause() {
super.onPause();
if(backgroundSoundService != null){
backgroundSoundService.pause();
}
}
在backgroundSoundService
的所有外观中添加这种空检查