How to detect when an Android app goes to the background and come back to the foreground Ask Question

2018: Android supports this natively through lifecycle components.

March 2018 UPDATE: There is now a better solution. See ProcessLifecycleOwner. You will need to use the new architecture components 1.1.0 (latest at this time) but it’s specifically designed to do this.

There’s a simple sample provided in this answer but I wrote a sample app and a blog post about it.

Ever since I wrote this back in 2014, different solutions arose. Some worked, some were thought to be working, but had flaws (including mine!) and we, as a community (Android) learned to live with the consequences and wrote workarounds for the special cases.

Never assume a single snippet of code is the solution you’re looking for, it’s unlikely the case; better yet, try to understand what it does and why it does it.

The MemoryBoss class was never actually used by me as written here, it was just a piece of pseudo code that happened to work.

Unless there’s valid reason for you not to use the new architecture components (and there are some, especially if you target super old apis), then go ahead and use them. They are far from perfect, but neither were ComponentCallbacks2.

UPDATE / NOTES (November 2015): People has been making two comments, first is that >= should be used instead of == because the documentation states that you shouldn't check for exact values. This is fine for most cases, but bear in mind that if you only care about doing something when the app went to the background, you will have to use == and also combine it with another solution (like Activity Lifecycle callbacks), or you may not get your desired effect. The example (and this happened to me) is that if you want to lock your app with a password screen when it goes to the background (like 1Password if you're familiar with it), you may accidentally lock your app if you run low on memory and are suddenly testing for >= TRIM_MEMORY, because Android will trigger a LOW MEMORY call and that's higher than yours. So be careful how/what you test.

Additionally, some people have asked about how to detect when you get back.

The simplest way I can think of is explained below, but since some people are unfamiliar with it, I'm adding some pseudo code right here. Assuming you have YourApplication and the MemoryBoss classes, in your class BaseActivity extends Activity (you will need to create one if you don't have one).

@Override
protected void onStart() {
    super.onStart();

    if (mApplication.wasInBackground()) {
        // HERE YOU CALL THE CODE YOU WANT TO HAPPEN ONLY ONCE WHEN YOUR APP WAS RESUMED FROM BACKGROUND
        mApplication.setWasInBackground(false);
    }
}

public abstract void onTrimMemory (int level)

public static final int TRIM_MEMORY_UI_HIDDEN

public class MemoryBoss implements ComponentCallbacks2 {
    @Override
    public void onConfigurationChanged(final Configuration newConfig) {
    }

    @Override
    public void onLowMemory() {
    }

    @Override
    public void onTrimMemory(final int level) {
        if (level == ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN) {
            // We're in the Background
        }
        // you might as well implement some memory cleanup here and be a nice Android dev.
    }
}

MemoryBoss mMemoryBoss;
@Override
public void onCreate() {
   super.onCreate();
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
      mMemoryBoss = new MemoryBoss();
      registerComponentCallbacks(mMemoryBoss);
   } 
}

unregisterComponentCallbacks(mMemoryBoss);