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Class for detecting and eventing whether an Android app is currently foreground or background (requires API level 14+)
package com.sjl.util;
import android.app.Activity;
import android.app.Application;
import android.content.Context;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
/**
* Usage:
*
* 1. Get the Foreground Singleton, passing a Context or Application object unless you
* are sure that the Singleton has definitely already been initialised elsewhere.
*
* 2.a) Perform a direct, synchronous check: Foreground.isForeground() / .isBackground()
*
* or
*
* 2.b) Register to be notified (useful in Service or other non-UI components):
*
* Foreground.Listener myListener = new Foreground.Listener(){
* public void onBecameForeground(){
* // ... whatever you want to do
* }
* public void onBecameBackground(){
* // ... whatever you want to do
* }
* }
*
* public void onCreate(){
* super.onCreate();
* Foreground.get(this).addListener(listener);
* }
*
* public void onDestroy(){
* super.onCreate();
* Foreground.get(this).removeListener(listener);
* }
*/
public class Foreground implements Application.ActivityLifecycleCallbacks {
public static final long CHECK_DELAY = 500;
public static final String TAG = Foreground.class.getName();
public interface Listener {
public void onBecameForeground();
public void onBecameBackground();
}
private static Foreground instance;
private boolean foreground = false, paused = true;
private Handler handler = new Handler();
private List<Listener> listeners = new CopyOnWriteArrayList<Listener>();
private Runnable check;
/**
* Its not strictly necessary to use this method - _usually_ invoking
* get with a Context gives us a path to retrieve the Application and
* initialise, but sometimes (e.g. in test harness) the ApplicationContext
* is != the Application, and the docs make no guarantees.
*
* @param application
* @return an initialised Foreground instance
*/
public static Foreground init(Application application){
if (instance == null) {
instance = new Foreground();
application.registerActivityLifecycleCallbacks(instance);
}
return instance;
}
public static Foreground get(Application application){
if (instance == null) {
init(application);
}
return instance;
}
public static Foreground get(Context ctx){
if (instance == null) {
Context appCtx = ctx.getApplicationContext();
if (appCtx instanceof Application) {
init((Application)appCtx);
}
throw new IllegalStateException(
"Foreground is not initialised and " +
"cannot obtain the Application object");
}
return instance;
}
public static Foreground get(){
if (instance == null) {
throw new IllegalStateException(
"Foreground is not initialised - invoke " +
"at least once with parameterised init/get");
}
return instance;
}
public boolean isForeground(){
return foreground;
}
public boolean isBackground(){
return !foreground;
}
public void addListener(Listener listener){
listeners.add(listener);
}
public void removeListener(Listener listener){
listeners.remove(listener);
}
@Override
public void onActivityResumed(Activity activity) {
paused = false;
boolean wasBackground = !foreground;
foreground = true;
if (check != null)
handler.removeCallbacks(check);
if (wasBackground){
Log.i(TAG, "went foreground");
for (Listener l : listeners) {
try {
l.onBecameForeground();
} catch (Exception exc) {
Log.e(TAG, "Listener threw exception!", exc);
}
}
} else {
Log.i(TAG, "still foreground");
}
}
@Override
public void onActivityPaused(Activity activity) {
paused = true;
if (check != null)
handler.removeCallbacks(check);
handler.postDelayed(check = new Runnable(){
@Override
public void run() {
if (foreground && paused) {
foreground = false;
Log.i(TAG, "went background");
for (Listener l : listeners) {
try {
l.onBecameBackground();
} catch (Exception exc) {
Log.e(TAG, "Listener threw exception!", exc);
}
}
} else {
Log.i(TAG, "still foreground");
}
}
}, CHECK_DELAY);
}
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {}
@Override
public void onActivityStarted(Activity activity) {}
@Override
public void onActivityStopped(Activity activity) {}
@Override
public void onActivitySaveInstanceState(Activity activity, Bundle outState) {}
@Override
public void onActivityDestroyed(Activity activity) {}
}
@xadh00m
xadh00m commented Feb 17, 2015

Thanks for this great way to figure out the foreground state of an application!
We have adapted this class (to a BackgroundManager) which has ended up to be even a bit simpler:
https://gist.github.com/xadh00m/1584a58ddb3d724cdd24

@YkmLo
YkmLo commented Aug 31, 2015

Hi, I got NoClassDefFoundError in line 156. Any idea? It seems the error occurs whenever implementing new Runnable(). My stackoverflow question is http://stackoverflow.com/questions/32303800/noclassdeffounderror-on-implementing-runnable-class-in-android-studio

@j796160836

Hi Steve (@steveliles), I found a bug that I use the Foreground class.
https://gist.github.com/steveliles/11116937#file-foreground-java-L92

public static Foreground get(Context ctx){
    if (instance == null) {
        Context appCtx = ctx.getApplicationContext();
        if (appCtx instanceof Application) {
            init((Application)appCtx);
        }
        throw new IllegalStateException(
            "Foreground is not initialised and " +
            "cannot obtain the Application object");
    }
    return instance;
}

I received the exception when get instances with no init in Application object.
Their will be always throw exception when instances is null.

I think you are lack a return here.

return init((Application)appCtx);
@j796160836

@steveliles I leave this commit here because GitHub gist cannot send pull request.
Please check it out, thanks.

@Ankita1405

How to implement it in my app

@azorrozua

Really useful. Thank you!

@susemi99

@Ankita1405

  1. download that file and add your project.
  2. create Application class see here
  3. insert Foreground.init(this); to onCreate() of Application class
  4. call Foreground.get().isForeground() where you want.
@prgomet
prgomet commented Jun 7, 2016 edited

You have to change private boolean foreground = false, paused = true; to private boolean foreground = true, paused = false;,
otherwise this wont work first time after you open app and put it to background.

@vladiulianbogdan

Good job, mate! Thank you!

@saintjab
saintjab commented Nov 7, 2016

Good job. Thanks!

@af-fess
af-fess commented Nov 8, 2016

Doesn't work for hybrid applications such: Cordova, Unity, Nativescript, react Native

@Iamgiam
Iamgiam commented Feb 1, 2017

Thank you for you hard work. I had a nightmare trying to flag when I went Background to Foreground with crashes on resume with fragments.
This helps massively.

@llew2011

good , thanks

@ecramer
ecramer commented Mar 16, 2017

🙌

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