我尝试了推荐的使用Application的解决方案。ActivityLifecycleCallbacks和许多其他的，但是它们没有像预期的那样工作。感谢Sarge，我想出了一个非常简单和直接的解决方案，我将在下面描述。

解决方案的关键是理解这样一个事实:如果我们有ActivityA和ActivityB，并且我们从ActivityA调用ActivityB(而不是调用ActivityA.finish)，那么ActivityB的onStart()将在ActivityA onStop()之前被调用。

这也是onStop()和onPause()之间的主要区别，在我读过的文章中没有人提到。

So based on this Activity's Lifecycle behavior, you can simply count how many times did onStart() and onPause() got called in your program. Note that for each Activity of your program, you must override onStart() and onStop(), in order to increment/decrement the static variable used for counting. Below is the code implementing this logic. Note that I am using a class that extends Application, so dont forget to declare on Manifest.xml inside Application tag: android:name=".Utilities", although it can be implemented using a simple custom class too.

public class Utilities extends Application
{
    private static int stateCounter;

    public void onCreate()
    {
        super.onCreate();
        stateCounter = 0;
    }

    /**
     * @return true if application is on background
     * */
    public static boolean isApplicationOnBackground()
    {
        return stateCounter == 0;
    }

    //to be called on each Activity onStart()
    public static void activityStarted()
    {
        stateCounter++;
    }

    //to be called on each Activity onStop()
    public static void activityStopped()
    {
        stateCounter--;
    }
}

现在，在我们程序的每个活动上，我们应该重写onStart()和onStop()和增量/减量，如下所示:

@Override
public void onStart()
{
    super.onStart();
    Utilities.activityStarted();
}

@Override
public void onStop()
{
    Utilities.activityStopped();
    if(Utilities.isApplicationOnBackground())
    {
        //you should want to check here if your application is on background
    }
    super.onStop();
}

根据这种逻辑，有两种可能的情况:

stateCounter = 0:停止活动的数量与启动活动的数量相等，这意味着应用程序正在后台运行。 stateCounter > 0:启动的次数大于停止的次数，即应用程序在前台运行。

注意:stateCounter < 0意味着有更多停止的活动而不是启动的活动，这是不可能的。如果您遇到这种情况，那么这意味着您没有像应该的那样增加/减少计数器。

你已经准备好了。你应该在onStop()中检查你的应用程序是否在后台。

我建议你仔细阅读这个页面:http://developer.android.com/reference/android/app/Activity.html

简而言之，你的活动在onStop()被调用后不再可见。

简单直接的回答:

override fun onPause() {
    Log.i("APP LIFECYCLE", "App Enter BACKground")
    isForeground = false
    super.onPause()
}

override fun onResume() {
    Log.i("APP LIFECYCLE", "App Enter FOREground")
    isForeground = true
    super.onResume()
}

然后只需使用您的活动的isForeground属性来检查状态。

参见onActivityDestroyed函数中的注释。

适用于SDK目标版本14>:

import android.app.Activity;
import android.app.Application;
import android.os.Bundle;
import android.util.Log;

public class AppLifecycleHandler implements Application.ActivityLifecycleCallbacks {

    public static int active = 0;

    @Override
    public void onActivityStopped(Activity activity) {
        Log.i("Tracking Activity Stopped", activity.getLocalClassName());
        active--;
    }

    @Override
    public void onActivityStarted(Activity activity) {
        Log.i("Tracking Activity Started", activity.getLocalClassName());
        active++;
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        Log.i("Tracking Activity SaveInstanceState", activity.getLocalClassName());
    }

    @Override
    public void onActivityResumed(Activity activity) {
        Log.i("Tracking Activity Resumed", activity.getLocalClassName());
        active++;
    }

    @Override
    public void onActivityPaused(Activity activity) {
        Log.i("Tracking Activity Paused", activity.getLocalClassName());
        active--;
    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        Log.i("Tracking Activity Destroyed", activity.getLocalClassName());
        active--;

        // if active var here ever becomes zero, the app is closed or in background
        if(active == 0){
            ...
        }

    }

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        Log.i("Tracking Activity Created", activity.getLocalClassName());
        active++;
    }
}

我尝试了推荐的使用Application的解决方案。ActivityLifecycleCallbacks和许多其他的，但是它们没有像预期的那样工作。感谢Sarge，我想出了一个非常简单和直接的解决方案，我将在下面描述。

解决方案的关键是理解这样一个事实:如果我们有ActivityA和ActivityB，并且我们从ActivityA调用ActivityB(而不是调用ActivityA.finish)，那么ActivityB的onStart()将在ActivityA onStop()之前被调用。

这也是onStop()和onPause()之间的主要区别，在我读过的文章中没有人提到。

So based on this Activity's Lifecycle behavior, you can simply count how many times did onStart() and onPause() got called in your program. Note that for each Activity of your program, you must override onStart() and onStop(), in order to increment/decrement the static variable used for counting. Below is the code implementing this logic. Note that I am using a class that extends Application, so dont forget to declare on Manifest.xml inside Application tag: android:name=".Utilities", although it can be implemented using a simple custom class too.

public class Utilities extends Application
{
    private static int stateCounter;

    public void onCreate()
    {
        super.onCreate();
        stateCounter = 0;
    }

    /**
     * @return true if application is on background
     * */
    public static boolean isApplicationOnBackground()
    {
        return stateCounter == 0;
    }

    //to be called on each Activity onStart()
    public static void activityStarted()
    {
        stateCounter++;
    }

    //to be called on each Activity onStop()
    public static void activityStopped()
    {
        stateCounter--;
    }
}

现在，在我们程序的每个活动上，我们应该重写onStart()和onStop()和增量/减量，如下所示:

@Override
public void onStart()
{
    super.onStart();
    Utilities.activityStarted();
}

@Override
public void onStop()
{
    Utilities.activityStopped();
    if(Utilities.isApplicationOnBackground())
    {
        //you should want to check here if your application is on background
    }
    super.onStop();
}

根据这种逻辑，有两种可能的情况:

stateCounter = 0:停止活动的数量与启动活动的数量相等，这意味着应用程序正在后台运行。 stateCounter > 0:启动的次数大于停止的次数，即应用程序在前台运行。

注意:stateCounter < 0意味着有更多停止的活动而不是启动的活动，这是不可能的。如果您遇到这种情况，那么这意味着您没有像应该的那样增加/减少计数器。

你已经准备好了。你应该在onStop()中检查你的应用程序是否在后台。

另一种没有额外依赖的方法是:

只需将此方法添加到应用程序类中，并在onCreate()中调用它即可。

var isInBackground = true

private fun setupActivityLifecycleCallbacks() {
    registerActivityLifecycleCallbacks(object : ActivityLifecycleCallbacks {
        override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {}
        override fun onActivityStarted(activity: Activity) {}
        override fun onActivityResumed(activity: Activity) {
            isInBackground = false
        }
        override fun onActivityPaused(activity: Activity) {
            isInBackground = true
        }
        override fun onActivityStopped(activity: Activity) {}
        override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) {}
        override fun onActivityDestroyed(activity: Activity) {}
    })
}

AFAIK，你甚至可以使isInBackground静态，这样你就可以在没有上下文的情况下使用伴生对象访问它