import android.app.Activity;
import android.app.Application;
import android.content.Context;
import android.os.Bundle;
import android.os.Handler;

import java.util.ArrayList;
import java.util.List;

/**
 * Detects when the current application enters background or foreground and notifies its listeners.
 */
public class BackgroundTransitionMonitor {
    public interface Listener {
        void onEnterBackground();
        void onEnterForeground();
    }

    private static final String TAG = "BackgroundTransitionMonitor";
    private static final long BACKGROUND_TRANSITION_DELAY = 500;
    private static BackgroundTransitionMonitor instance;
    private final Application application;
    private final Handler backgroundCheckHandler = new Handler();
    private final List<Listener> listeners = new ArrayList<>();
    private boolean isBackground = false;

    public static synchronized BackgroundTransitionMonitor getInstance(Context context) {
        if (instance == null) {
            instance = new BackgroundTransitionMonitor((Application) context.getApplicationContext());
        }
        return instance;
    }

    private BackgroundTransitionMonitor(Application application) {
        this.application = application;
    }

    public void addListener(Listener listener) {
        if (listeners.isEmpty()) {
            application.registerActivityLifecycleCallbacks(lifecycleCallbacks);
        }
        listeners.add(listener);
    }

    public void removeListener(Listener listener) {
        listeners.remove(listener);
        if (listeners.isEmpty()) {
            application.unregisterActivityLifecycleCallbacks(lifecycleCallbacks);
        }
    }

    private final Application.ActivityLifecycleCallbacks lifecycleCallbacks = new Application.ActivityLifecycleCallbacks() {
        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        }

        @Override
        public void onActivityStarted(Activity activity) {
        }

        @Override
        public void onActivityResumed(Activity activity) {
            backgroundCheckHandler.removeCallbacks(backgroundTransitionRunnable);
            if (isBackground) {
                isBackground = false;
                for (Listener listener : listeners) {
                    listener.onEnterForeground();
                }
            }
        }

        @Override
        public void onActivityPaused(Activity activity) {
            if (!isBackground) {
                backgroundCheckHandler.removeCallbacks(backgroundTransitionRunnable);
                backgroundCheckHandler.postDelayed(backgroundTransitionRunnable, BACKGROUND_TRANSITION_DELAY);
            }
        }

        @Override
        public void onActivityStopped(Activity activity) {
        }

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

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

    private final Runnable backgroundTransitionRunnable = new Runnable() {
        @Override
        public void run() {
            isBackground = true;
            for (Listener listener : listeners) {
                listener.onEnterBackground();
            }
        }
    };
}