这是@d60402回答的修改版本:https://stackoverflow.com/a/15573121/4747587

按照上面提到的去做。但是，与其有一个Base Activity，并将其作为每个活动的父活动，并重写onResume()和onPause，不如执行以下操作:

在你的应用程序类中，添加这样一行:

registerActivityLifecycleCallbacks(应用程序。ActivityLifecycleCallbacks回调);

这个回调有所有的活动生命周期方法，你现在可以覆盖onactivityresume()和onActivityPaused()。

看看这个Gist: https://gist.github.com/thsaravana/1fa576b6af9fc8fff20acfb2ac79fa1b

我喜欢ProcessLifecycleOwner方法，但实际上可以跳过所有这些，因为在Activity的onCreate()方法中，可以很容易地确定它是第一次运行还是后续运行:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (savedInstanceState == null) {
        /* savedInstanceState is always null on first run */
    } else {
        /* it's a subsequent run */
    }
}

我所做的是确保所有应用程序内的活动启动startActivityForResult，然后检查onActivityResult是否在onResume之前被调用。如果不是，这意味着我们刚刚从应用程序之外的某个地方返回。

boolean onActivityResultCalledBeforeOnResume;

@Override
public void startActivity(Intent intent) {
    startActivityForResult(intent, 0);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent intent) {
    super.onActivityResult(requestCode, resultCode, intent);
    onActivityResultCalledBeforeOnResume = true;
}

@Override
protected void onResume() {
    super.onResume();
    if (!onActivityResultCalledBeforeOnResume) {
        // here, app was brought to foreground
    }
    onActivityResultCalledBeforeOnResume = false;
}

我知道有点晚了，但我认为所有这些答案都有一些问题，而我这样做了，这是完美的。

创建一个活动生命周期回调，如下所示:

 class ActivityLifeCycle implements ActivityLifecycleCallbacks{

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {

    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    Activity lastActivity;
    @Override
    public void onActivityResumed(Activity activity) {
        //if (null == lastActivity || (activity != null && activity == lastActivity)) //use this condition instead if you want to be informed also when  app has been killed or started for the first time
        if (activity != null && activity == lastActivity) 
        {
            Toast.makeText(MyApp.this, "NOW!", Toast.LENGTH_LONG).show();
        }

        lastActivity = activity;
    }

    @Override
    public void onActivityPaused(Activity activity) {

    }

    @Override
    public void onActivityStopped(Activity activity) {

    }

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

    }

    @Override
    public void onActivityDestroyed(Activity activity) {

    }
}

然后像下面这样在你的应用类上注册它:

public class MyApp extends Application {

@Override
public void onCreate() {
    super.onCreate();
    registerActivityLifecycleCallbacks(new ActivityLifeCycle());
}

这些答案似乎不正确。当另一个活动开始和结束时，也会调用这些方法。你能做的是保留一个全局标志(是的，全局不好:)，并在每次启动一个新活动时将其设置为true。在每个活动的onCreate中将其设置为false。然后，在onPause中检查这个标志。如果为假，你的应用就会进入后台，或者被杀死。

我们可以使用LiveData扩展这个解决方案:

class AppForegroundStateLiveData : LiveData<AppForegroundStateLiveData.State>() {

    private var lifecycleListener: LifecycleObserver? = null

    override fun onActive() {
        super.onActive()
        lifecycleListener = AppLifecycleListener().also {
            ProcessLifecycleOwner.get().lifecycle.addObserver(it)
        }
    }

    override fun onInactive() {
        super.onInactive()
        lifecycleListener?.let {
            this.lifecycleListener = null
            ProcessLifecycleOwner.get().lifecycle.removeObserver(it)
        }
    }

    internal inner class AppLifecycleListener : LifecycleObserver {

        @OnLifecycleEvent(Lifecycle.Event.ON_START)
        fun onMoveToForeground() {
            value = State.FOREGROUND
        }

        @OnLifecycleEvent(Lifecycle.Event.ON_STOP)
        fun onMoveToBackground() {
            value = State.BACKGROUND
        }
    }

    enum class State {
        FOREGROUND, BACKGROUND
    }
}

现在我们可以订阅这个LiveData并捕获所需的事件。例如:

appForegroundStateLiveData.observeForever { state ->
    when(state) {
        AppForegroundStateLiveData.State.FOREGROUND -> { /* app move to foreground */ }
        AppForegroundStateLiveData.State.BACKGROUND -> { /* app move to background */ }
    }
}