Custom Application which can detect application state of whether it enter background or enter foreground.

StatusApplication.java

/**
 * Custom Application which can detect application state of whether it enter
 * background or enter foreground.
 *
 * @author shuang
 * @reference http://www.vardhan-justlikethat.blogspot.sg/2014/02/android-solution-to-detect-when-android.html
 */
public abstract class StatusApplication extends Application implements ActivityLifecycleCallbacks {

    public static final int STATE_UNKNOWN = 0x00;
    public static final int STATE_CREATED = 0x01;
    public static final int STATE_STARTED = 0x02;
    public static final int STATE_RESUMED = 0x03;
    public static final int STATE_PAUSED = 0x04;
    public static final int STATE_STOPPED = 0x05;
    public static final int STATE_DESTROYED = 0x06;

    private static final int FLAG_STATE_FOREGROUND = -1;
    private static final int FLAG_STATE_BACKGROUND = -2;

    private int mCurrentState = STATE_UNKNOWN;
    private int mStateFlag = FLAG_STATE_BACKGROUND;

    @Override
    public void onCreate() {
        super.onCreate();
        mCurrentState = STATE_UNKNOWN;
        registerActivityLifecycleCallbacks(this);
    }

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        // mCurrentState = STATE_CREATED;
    }

    @Override
    public void onActivityStarted(Activity activity) {
        if (mCurrentState == STATE_UNKNOWN || mCurrentState == STATE_STOPPED) {
            if (mStateFlag == FLAG_STATE_BACKGROUND) {
                applicationWillEnterForeground();
                mStateFlag = FLAG_STATE_FOREGROUND;
            }
        }
        mCurrentState = STATE_STARTED;

    }

    @Override
    public void onActivityResumed(Activity activity) {
        mCurrentState = STATE_RESUMED;

    }

    @Override
    public void onActivityPaused(Activity activity) {
        mCurrentState = STATE_PAUSED;

    }

    @Override
    public void onActivityStopped(Activity activity) {
        mCurrentState = STATE_STOPPED;

    }

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

    }

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

    @Override
    public void onTrimMemory(int level) {
        super.onTrimMemory(level);
        if (mCurrentState == STATE_STOPPED && level >= TRIM_MEMORY_UI_HIDDEN) {
            if (mStateFlag == FLAG_STATE_FOREGROUND) {
                applicationDidEnterBackground();
                mStateFlag = FLAG_STATE_BACKGROUND;
            }
        }else if (mCurrentState == STATE_DESTROYED && level >= TRIM_MEMORY_UI_HIDDEN) {
            if (mStateFlag == FLAG_STATE_FOREGROUND) {
                applicationDidDestroyed();
                mStateFlag = FLAG_STATE_BACKGROUND;
            }
        }
    }

    /**
     * The method be called when the application been destroyed. But when the
     * device screen off,this method will not invoked.
     */
    protected abstract void applicationDidDestroyed();

    /**
     * The method be called when the application enter background. But when the
     * device screen off,this method will not invoked.
     */
    protected abstract void applicationDidEnterBackground();

    /**
     * The method be called when the application enter foreground.
     */
    protected abstract void applicationWillEnterForeground();

}