app在前台还是后台的判断有很多种，如：
1.在BaseActivity中对Activity的进行保存管理，可以通过根据当前的所有Activity的状态来判断应用是否是在前台显示。
2.在application中注册registerActivityLifecycleCallbacks（）的回调，通过回调来判断是否在前台

public class MyApp extends Application {
    private static final String TAG = "TEST_APP";
    private static int mActivityCount;

    @Override
    public void onCreate() {
        super.onCreate();
        registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            }

            @Override
            public void onActivityStarted(Activity activity) {
                mActivityCount++;
                if(mActivityCount == 1) {
                    Log.e(TAG, "后台->前台");
                }
            }

            @Override
            public void onActivityResumed(Activity activity) {
            }

            @Override
            public void onActivityPaused(Activity activity) {
            }

            @Override
            public void onActivityStopped(Activity activity) {
                mActivityCount--;
                if(mActivityCount == 0) {
                    Log.e(TAG, "前台->后台");
                }
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
            }
        });
    }
}

3.通过获取系统ActivityManager来判断。
4.其他方法

新增一个方法

使用LifecycleObserver来获取对应生命周期的回调，从而判断是前台还是后台

public class ProcessLifecycleObserver implements LifecycleObserver {
    private static final String TAG = LogHelper.makeLogTag(ProcessLifecycleObserver.class);
    public ProcessLifecycleObserver() {
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    public void onProcessCreate() {
        LogHelper.d(TAG, "onProcessCreate: ");
    }

    /**
     * 进入前台
     */
    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    public void onEnterForeground() {
        Log.e(TAG, "onEnterForeground: ");
    }

    /**
     * 进入后台
     */
    @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
    public void onEnterBackground() {
        Log.e(TAG, "onEnterBackground: ");
    }
}

在项目的application中注册

public class ComicApps extends Application {
    @Override
    public void onCreate() {
        ProcessLifecycleOwner.get().getLifecycle().addObserver(new ProcessLifecycleObserver(）);
    }
}