What is lifecycle observer and how to use it correctly?

You can use ProcessLifecycleOwner to get your Application's LifeCycle and to add a class as an observer of these events. You can implement LifecycleObserver in your Application Class:

public class MyApplication extends MultiDexApplication implements LifecycleObserver

@Override
public void onCreate() {
    super.onCreate();

    ProcessLifecycleOwner.get().getLifecycle().addObserver(this);

}

// Add these Lifecycle methods to observe when your app goes into the background or to the foreground:

@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void appInResumeState() {
    Toast.makeText(this,"In Foreground",Toast.LENGTH_LONG).show();
}

@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void appInPauseState() {
    Toast.makeText(this,"In Background",Toast.LENGTH_LONG).show();
}

// Add the following in your build.gradle file

implementation 'android.arch.lifecycle:extensions:1.1.1'

//Also In Activities or Fragments

You can also use them to reduce the complexity of code by creating different components which are implementing LifecycleObserver and then pass the lifecycle of activity to these components. This way you can split up the huge complexity to different components.

class MainActivity : AppCompatActivity(), LifecycleObserver {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        ReduceComplexComponent().registerLifecycle(lifecycle)

    }

}

class ReduceComplexComponent : LifecycleObserver{

    registerLifecycle(lifecycle : Lifecycle){
       lifecycle.addObserver(this)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun resume() {
       Log.d("OnResume","ON_RESUME")
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun pause() {
       Log.d("onPause","ON_PAUSE")
    }
}

This way you can listen to activity or fragment lifecycle events in separate components.

We can also manually fetch the current state of our lifecycle instance in Activity and that we can do by using its getCurrentState()

A State also has an isAtLeast() method that we can use to perform comparisons against the current lifecycle state

@OnLifecycleEvent is deprecated so many of the answers that used to be helpful here are no longer helpful. If you're not currently using Java 8 yet, you'll need to update your build.gradle first. We can now utilize DefaultLifecycleObserver or LifecycleEventObserver. See examples below.

To start, you can use ProcessLifecycleOwner to get your Application's lifecycle:

ProcessLifecycleOwner.get().getLifecycle()

DefaultLifecycleObserver:

lifecycle.addObserver(object: DefaultLifecycleObserver {
    override fun onResume(owner: LifecycleOwner) {
        super.onResume(owner)
        TODO()
    }
    override fun onPause(owner: LifecycleOwner) {
        TODO()
        super.onPause(owner)
    }
})

LifecycleEventObserver:

lifecycle.addObserver(object: LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
        when (event) {
            Lifecycle.Event.ON_RESUME -> TODO()
            Lifecycle.Event.ON_PAUSE -> TODO()
            else -> { }
        }
    }
})

How to update to Java 8 and explanation of why OnLifecycleEvent is deprecated: https://stackoverflow.com/a/70384221/3422470

May be a little late to the party, but another nice use-case of lifecycles (except for the obvious ViewModel stuff), is to let many components of the app un-register themselves when the relevant activity is getting destroyed, or simply out of screen.

For example, I have a static factory that creates dialogs, and using lifecycle I can dismiss the dialogs without cluttering the host activity with the old stuff like Dialog mDialog = ... and void onPause(){ ... if (mDialog !null && mDialog.isShowing()) mDialog.cancel() }

Some code:

DialogUtils.java:

public static void showConfirmDialog(Lifecycle lifecycle, String title, String msg, Runnable okRunnable) {
    AlertDialog dialog = AlertDialog.Builder(mAppContext)
    /* configuration stuff ... */
        .build();

    dialog.show();

    lifecycle.addObserver(new LifecycleObserver() {
          @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
          public void cancelDialog() {
            if (dialog.isShowing()) { // if not already dismissed by main-button tap 
              dialog.cancel();
            }
          }
    });
}

MyActivity.java:

public class MyActivity extends AppCompatActivity {

    /* stuff... onCreate()... other stuff... */

    private void confirmDeleteUser(User user){
        DialogUtils.showConfirmDialog(
            MyActivity.this.getLifecycle(), // all activities & fragment have lifecycles
            "Confirm Delete",
            "Action cannot be undone. Sure to continue?",
            new Runnable() { /* stuff... */ }
        );
        // Voilà! 
        // activity no needs to store reference to the dialog and cancel manually on pause
        // it's a fire-and-forget action
    }
}

lifecycle-extensions API have been deprecated.

lifecycle-extensions Artifact Deprecation: With the above deprecation of ViewModelProviders.of(), this release marks the deprecation of the last API in lifecycle-extensions and this artifact should now be considered deprecated in its entirety. We strongly recommend depending on the specific Lifecycle artifacts you need (such as lifecycle-service if you’re using LifecycleService and lifecycle-process if you’re using ProcessLifecycleOwner) rather than lifecycle-extensions as there will not be a future 2.3.0 release of lifecycle-extensions.

If you want to continue to use ProcessLifecycleOwner, it is recommended to add this dependency:

implementation "androidx.lifecycle:lifecycle-process:2.2.0"

Ref:
https://developer.android.com/jetpack/androidx/releases/lifecycle#version_220_3

https://androidx.tech/artifacts/lifecycle/lifecycle-extensions/

https://developer.android.com/reference/androidx/lifecycle/ProcessLifecycleOwner