1.api14之前靠定时器手动监测

if (Build.VERSION.SDK_INT < Build.VERSION_CODES.ICE_CREAM_SANDWICH){
            // 4.0 之前的系统,每 2s 在后台检测一次当前是否在前台
            mSubscriptionCheckOldSys = Observable.interval(2, TimeUnit.SECONDS)
                    .subscribeOn(Schedulers.io())
                    .observeOn(Schedulers.computation())
                    .subscribe(new Action1<Long>() {
                        @Override
                        public void call(Long aLong) {
                            if (AppUtils.isRunningForeground2(IMEnvironment.getInstance().getContext())) {
                                checkServiceAlive();
                            } else {
                                stopLink();
                            }
                        }
                    });
        }
/*
     * 判断程序是否在前台运行
     * 存在问题，如果程序有一个前台运行的service则会始终返回true
     */
    public static boolean isRunningForeground2(Context context) {
        ActivityManager activityManager = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        List<ActivityManager.RunningAppProcessInfo> appProcesses = activityManager.getRunningAppProcesses();
        for (ActivityManager.RunningAppProcessInfo appProcess : appProcesses) {
            if (appProcess.processName.equals(context.getPackageName())) {
                if (appProcess.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_BACKGROUND) {
                    Log.d("background", appProcess.processName);
                    return false;
                } else {
                    Log.i("foreground", appProcess.processName);
                    return true;
                }
            }
        }
        return false;
    }

2.在api14之后用ActivityLifecycleCallbacks

@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
public class BJApplicationStateCallbacks implements ActivityLifecycleCallbacks {

    public static interface BJApplicationStateListener {
        public void applicationDidEnterForeground();

        public void applicationDidEnterBackground();
    }

    private int mForegroundActivities;
    private boolean hasSeenFirstActivity;
//    private boolean isChangingConfiguration;
    private BJApplicationStateListener mListener = null;

    public BJApplicationStateCallbacks(BJApplicationStateListener listener) {
        mListener = listener;

    }

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onActivityStarted(Activity activity) {

        mForegroundActivities++;
        if (hasSeenFirstActivity && mForegroundActivities == 1/* && !isChangingConfiguration*/) {
            if (mListener != null) {
                mListener.applicationDidEnterForeground();
            }
        }
        hasSeenFirstActivity = true;
//        isChangingConfiguration = false;
    }

    @Override
    public void onActivityResumed(Activity activity) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onActivityPaused(Activity activity) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onActivityStopped(Activity activity) {
        mForegroundActivities--;
        if (mForegroundActivities == 0) {
            if (mListener != null) {
                mListener.applicationDidEnterBackground();
            }
        }
//        isChangingConfiguration = activity.isChangingConfigurations();
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        // TODO Auto-generated method stub

    }

}
//demo
 /**
     * 监听 APP 进入前后台的状态。只在 4.0+ 有效
     */
    @TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
    private void registerActivityLifeCycleCallback() {
        ((Application) IMEnvironment.getInstance().getContext())
                .registerActivityLifecycleCallbacks(new BJApplicationStateCallbacks(
                        new BJApplicationStateCallbacks.BJApplicationStateListener() {

                            @Override
                            public void applicationDidEnterForeground() {
                                mIMServiceV2.applicationDidEnterForeground();
                            }

                            @Override
                            public void applicationDidEnterBackground() {
                                mIMServiceV2.applicationDidEnterBackground();
                            }
                        }));
    }