以前的方案

通过lifecycle监控，每次打开一个Activity则加1，每次关闭一个Activity则减1

根据统计的数值是否为0，判断当前App是否处于前台/后台

更简洁的方案

  ProcessLifecycleOwner.get().lifecycle.addObserver(object : LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
      when (event) {
        Lifecycle.Event.ON_START -> _isForeground = true // 处于前台
        Lifecycle.Event.ON_STOP -> _isForeground = false // 处于后台
        else -> Unit
      }
    }
  })

来自：androidx.lifecycle:lifecycle-process:xxx

参考资料

• https://blog.csdn.net/awy1988/article/details/99431616
• 遇到的实例