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

您应该使用共享首选项来存储属性，并使用来自活动的服务绑定对其进行操作。如果你只使用绑定(也就是从不使用startService)，那么你的服务将只在你绑定它时运行，(绑定onResume和取消绑定onPause)，这将使它只在前台运行，如果你确实想在后台工作，你可以使用常规的启动停止服务。

Idolon的答案是容易出错的，更复杂的，尽管在这里重复检查android应用程序是在前台或不是?从后台任务或服务中确定当前的前台应用程序

有一个更简单的方法:

在所有活动扩展的BaseActivity上:

protected static boolean isVisible = false;

 @Override
 public void onResume()
 {
     super.onResume();
     setVisible(true);
 }


 @Override
 public void onPause()
 {
     super.onPause();
     setVisible(false);
 }

无论何时你需要检查你的应用程序活动是否在前台，只需检查isVisible();

要理解这种方法，请检查side-by-side活动生命周期的答案:activity side-by-side生命周期

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

这段代码将在任何情况下检查前台和后台:

Java代码:

private static boolean isApplicationForeground(Context context) {
    KeyguardManager keyguardManager =
            (KeyguardManager) context.getSystemService(Context.KEYGUARD_SERVICE);

    if (keyguardManager.isKeyguardLocked()) {
        return false;
    }
    int myPid = Process.myPid();

    ActivityManager activityManager =
            (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);

    List<ActivityManager.RunningAppProcessInfo> list;

    if ((list = activityManager.getRunningAppProcesses()) != null) {
        for (ActivityManager.RunningAppProcessInfo aList : list) {
            ActivityManager.RunningAppProcessInfo info;
            if ((info = aList).pid == myPid) {
                return info.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND;
            }
        }
    }
    return false;
}

芬兰湾的科特林代码:

private fun isApplicationForeground(context: Context): Boolean {
        val keyguardManager = context.getSystemService(Context.KEYGUARD_SERVICE) as KeyguardManager
        if (keyguardManager.isKeyguardLocked) {
            return false
        }
        val myPid = Process.myPid()
        val activityManager = context.getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
        var list: List<ActivityManager.RunningAppProcessInfo>
        if (activityManager.runningAppProcesses.also { list = it } != null) {
            for (aList in list) {
                var info: ActivityManager.RunningAppProcessInfo
                if (aList.also { info = it }.pid == myPid) {
                    return info.importance == ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND
                }
            }
        }
        return false
    }

如果你想知道一个特定的活动是否在前台，如果你是一个没有直接访问应用程序的SDK，那么没有一个答案非常适合特定的情况。对我来说，我在后台线程刚刚收到一个新的聊天消息的推送通知，只想在聊天屏幕不在前台时显示系统通知。

使用ActivityLifecycleCallbacks，正如在其他答案中推荐的那样，我已经创建了一个小的util类，它包含MyActivity是否在前台的逻辑。

class MyActivityMonitor(context: Context) : Application.ActivityLifecycleCallbacks {

private var isMyActivityInForeground = false

init {
    (context.applicationContext as Application).registerActivityLifecycleCallbacks(this)
}

fun isMyActivityForeground() = isMyActivityInForeground

override fun onActivityPaused(activity: Activity?) {
    if (activity is MyActivity) {
        isMyActivityInForeground = false
    }
}

override fun onActivityResumed(activity: Activity?) {
    if (activity is MyActivity) {
        isMyActivityInForeground = true
    }
}

}