LifecycleObserver就像名字上说的，它是用于观察宿主的生命周期，这里的宿主指的是Activity或Fragment。可以通过实现LifecycleObserver接口，在方法上添加@OnLifecycleEvent(Lifecycle.Event.ON_XXX)注释，当生命周期回调到相应的生命周期的时候，就会调用上面的方法。这是管理内容泄露的一个很好的方法，同时这也是生命周期的一个切面。

LifeCycle events

class ConnectivityChecker(private val connectivityManager: ConnectivityManager) : LifecycleObserver {
    

    private val _connectedStatus = MutableLiveData<Boolean>()

    val connectedStatus: LiveData<Boolean>
        get() = _connectedStatus


    private val connectivityCallback = object : ConnectivityManager.NetworkCallback() {
        override fun onAvailable(network: Network) {
            _connectedStatus.postValue(true)
        }

        override fun onLost(network: Network) {
            _connectedStatus.postValue(false)
        }
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun stopMonitoringConnectivity() {
        connectivityManager.unregisterNetworkCallback(connectivityCallback)
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun startMonitoringConnectivity() {
        val activeNetworkInfo: NetworkInfo? = connectivityManager.activeNetworkInfo
        val connected = activeNetworkInfo != null && activeNetworkInfo.isConnected
        _connectedStatus.postValue(connected)
        connectivityManager.registerNetworkCallback(
            NetworkRequest.Builder()
                .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET).build(),
            connectivityCallback
        )

    }
}

ConnectivityChecker代码很简单，在ON_RESUME时，注册监听器，在ON_PAUSE的时候取消注册监听器。然后通过connectedStatus暴露连接状态。下面是调用的代码。

   private fun listeningNetChange() {
        val connectivityManager = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        connectivityManager.apply {
            ConnectivityChecker(this).apply {
                lifecycle.addObserver(this)
                connectedStatus.observe(this@MainActivity, Observer {
                    if (it) {
                      //网络已经连接上
                    } else {
                      //网络已断开
                    }
                })
            }
        }
    }