这是你想做的。a类：

Intent intent = new Intent(this,b.class);
startActivity(intent);

protected void onPause(){
   super.onPause();
   showNotification();
}
public void showNotification()
{
    NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this,"channelID")
            .setSmallIcon(R.drawable.icon)
            .setContentTitle("Title")
            .setContentText("Description")
            .setAutoCancel(true);

    NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    int notificationId = 1;
    createChannel(notificationManager);
    notificationManager.notify(notificationId, notificationBuilder.build());
}

public void createChannel(NotificationManager notificationManager){
    if (Build.VERSION.SDK_INT < 26) {
        return;
    }
    NotificationChannel channel = new NotificationChannel("channelID","title", NotificationManager.IMPORTANCE_DEFAULT);
    channel.setDescription("Description");
    notificationManager.createNotificationChannel(channel);
}

所以，如果通知关闭/从通知栏中删除，那么它将返回类a
然后，检查通知是否在类b中删除。