wifi连接上后是怎么提供网络的?

干了六个月的网络协议栈，又回到了wifi老本行，所以我最近又开始研读 Android wifi fwk的源码了
之前还在干wifi的时候就思考过一个问题，wifi区别于蓝牙的一个很明显的点是，wifi可以提供 access to Internet
所以我想看看wifi连接成功后是怎么给上层app提供网络的

前言：研读过程中发现整个机制中有大量的类，且它们的命名非常符合自身的功能，这是我觉得oop语言很有趣也是很容易上手的原因
与之前解决 WNS 选网过程一样，解读的过程中我会尝试给出一个大概的通俗语言描述的模型

注册 NetworkOffer

WifiService(SystemService的子类)在完全开机后，会执行 handleBootCompleted() 方法，注册自己魔改的几个NetworkFactory
我们只关注 WifiNetworkFactory

//WifiServiceImpl.java
public void handleBootCompleted() {mWifiThreadRunner.post(() -> {Log.d(TAG, "Handle boot completed");//...mWifiInjector.getWifiNetworkFactory().register();mWifiInjector.getUntrustedWifiNetworkFactory().register();mWifiInjector.getOemWifiNetworkFactory().register();});
}

NetworkFactory register动作主要做的是

ConnectivityManager.registerNetworkProvider(new NetworkProvider)
handleOfferNetwork -> ConnectivityManager.offerNetwork

//NetworkFactoryImpl.java
private void register(final String logTag, final boolean listenToAllRequests) {mProvider = new NetworkProvider(mContext, NetworkFactoryImpl.this.getLooper(), logTag) {@Overridepublic void onNetworkRequested(@NonNull NetworkRequest request, int score,int servingProviderId) {handleAddRequest(request);}@Overridepublic void onNetworkRequestWithdrawn(@NonNull NetworkRequest request) {handleRemoveRequest(request);}};((ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE)).registerNetworkProvider(mProvider);if (listenToAllRequests) {sendMessage(obtainMessage(CMD_LISTEN_TO_ALL_REQUESTS));} else {sendMessage(obtainMessage(CMD_OFFER_NETWORK));}
}//handle CMD_OFFER_NETWORK msg 的逻辑
//此处 mScore == 0
private void handleOfferNetwork(@NonNull final NetworkScore score) {mProvider.registerNetworkOffer(score, mCapabilityFilter, mExecutor, mRequestCallback);
}

//NetworkProvider.java
public void registerNetworkOffer(@NonNull final NetworkScore score,@NonNull final NetworkCapabilities caps, @NonNull final Executor executor,@NonNull final NetworkOfferCallback callback) {mContext.getSystemService(ConnectivityManager.class).offerNetwork(providerId, score, caps, proxy);
}

简单总结：到这里，wifi fwk已经成功地向 CS 注册了一个 NetworkProvider，并且经由这个 NetworkProvider 给出了一个 NetworkOffer
这个 NetworkOffer 就是用来去 satisfy 上层app发起的 NetworkRequest 的，当 CS match 某NetworkOffer 和某NetworkRequest 的时候，就意味着这个app发起的网络请求将由匹配的网络来处理
不过，wifi fwk给出的这个 NetworkOffer尚处于不能 handle network request 的状态，因为它的 score 是 0

创建NetworkAgent

wifi fwk STA mode的状态机进入 L2ConnectedState 后(L2表示关联上了，之后会进行DHCP，拿到ip后进入L3ConnectedState)，会创建 NetworkAgent

//ClientModeImpl.java
//class L2ConnectedState的EA
mNetworkAgent = mWifiInjector.makeWifiNetworkAgent(nc, mLinkProperties, naConfig,mNetworkFactory.getProvider(), new WifiNetworkAgentCallback());
mWifiScoreReport.setNetworkAgent(mNetworkAgent);

如上代码所示，WifiScoreReport 持 WifiNetworkAgent ，它会根据wifi链路状态实时地更新wifi网络在 CS 中的评分（通过调用 sendNetworkScore 方法）

创建 NetworkAgent 触发的一系列动作如下
最主要的是创建NetworkMonitor

//WifiNetworkAgent.java
public WifiNetworkAgent(@NonNull Context context,@NonNull Looper looper,@NonNull NetworkCapabilities nc,@NonNull LinkProperties lp,@NonNull NetworkAgentConfig config,@Nullable NetworkProvider provider,@NonNull Callback wifiNetworkAgentCallback) {super(context, looper, TAG, nc, lp, ConnectedScore.WIFI_INITIAL_SCORE, config, provider);register();
}//NetworkAgent.java
public Network register() {synchronized (mRegisterLock) {final ConnectivityManager cm = (ConnectivityManager) mInitialConfiguration.context.getSystemService(Context.CONNECTIVITY_SERVICE);mNetwork = cm.registerNetworkAgent(new NetworkAgentBinder(mHandler),new NetworkInfo(mInitialConfiguration.info),mInitialConfiguration.properties, mInitialConfiguration.capabilities,mInitialConfiguration.score, mInitialConfiguration.config, providerId);}return mNetwork;
}//ConnectivityService.java
private Network registerNetworkAgentInternal(INetworkAgent na, NetworkInfo networkInfo,LinkProperties linkProperties, NetworkCapabilities networkCapabilities,NetworkScore currentScore, NetworkAgentConfig networkAgentConfig, int providerId,int uid) {LinkProperties lp = new LinkProperties(linkProperties);final NetworkCapabilities nc = new NetworkCapabilities(networkCapabilities);final NetworkAgentInfo nai = new NetworkAgentInfo(na,new Network(mNetIdManager.reserveNetId()), new NetworkInfo(networkInfo), lp, nc,currentScore, mContext, mTrackerHandler, new NetworkAgentConfig(networkAgentConfig),this, mNetd, mDnsResolver, providerId, uid, mLingerDelayMs,mQosCallbackTracker, mDeps);mDeps.getNetworkStack().makeNetworkMonitor(nai.network, name, new NetworkMonitorCallbacks(nai));return nai.network;
}

NetworkMonitor创建成功后，CS 会触发 onNetworkMonitorCreated 回调，处理逻辑是 mNetworkMonitor.start()
我们在 main log中经常可以看到 NetworkMonitor http/https/dns probe fail，这说明 NetworkMonitor 的功能是受控去 probe 指定url，根据返回的status code判断当前链接的状态

实时更新 Network

wifi fwk STA mode处于 L2ConnectedState，接收到 CMD_IP_CONFIGURATION_SUCCESSFUL msg(意味着DHCP完成，这个网络有了自己的ip)

//ClientModeImpl.java
case CMD_IP_CONFIGURATION_SUCCESSFUL: {if (getConnectedWifiConfigurationInternal() == null || mNetworkAgent == null) {mWifiNative.disconnect(mInterfaceName);} else {handleSuccessfulIpConfiguration();sendConnectedState();transitionTo(mL3ConnectedState);}break;
}private void sendConnectedState() {mNetworkAgent.markConnected();sendNetworkChangeBroadcast(DetailedState.CONNECTED);
}//NetworkAgent.java
public void markConnected() {mNetworkInfo.setDetailedState(NetworkInfo.DetailedState.CONNECTED, null /* reason */,mNetworkInfo.getExtraInfo());queueOrSendNetworkInfo(mNetworkInfo);
}

markConnected 之后，NetworkAgent 会把更新了状态的 NetworkInfo 同步给 CS
同步过程如下

//NetworkAgentInfo.java
public void sendNetworkInfo(@NonNull NetworkInfo info) {mHandler.obtainMessage(NetworkAgent.EVENT_NETWORK_INFO_CHANGED,new Pair<>(NetworkAgentInfo.this, info)).sendToTarget();
}//ConnectivityService.java
//NetworkStateTrackerHandler handle this msg
case NetworkAgent.EVENT_NETWORK_INFO_CHANGED: {NetworkInfo info = (NetworkInfo) arg.second;updateNetworkInfo(nai, info);break;
}private void updateNetworkInfo(NetworkAgentInfo networkAgent, NetworkInfo info) {if (!networkAgent.everConnected && state == NetworkInfo.State.CONNECTED) {rematchAllNetworksAndRequests();} else if (state == NetworkInfo.State.DISCONNECTED) {networkAgent.disconnect();}
}

rematchAllNetworksAndRequests 做的事情便是重新匹配 Network 和 NetworkRequest
我们在main log中经常可以见到 CS NetworkRequest reassign 的打印，这代表原来由某网络负责的NetworkRequest被分配给另一网络处理了
任何一个网络的状态稍有改变都会触发这个动作

WifiScoreReport sendNetworkScore 后触发的流程与上面类似，只不过 msg 换成了 EVENT_NETWORK_SCORE_CHANGED

//ConnectivityService.java
private void updateNetworkScore(@NonNull final NetworkAgentInfo nai, final NetworkScore score) {nai.setScore(score);rematchAllNetworksAndRequests();
}

总结

WifiService 在开机完成后就像 CS 注册了一个 NetworkProvider，并经由这个NetworkProvider注册了一个 NetworkOffer，只不过处于一个无法向外提供服务的状态。
当连接上某wifi（完成了数据链路层的连接）后，wifi 创建一个 NetworkAgent 用于管理这个网络，同时CS创建一个 NetworkMonitor 用于监管这个网络。
当DHCP完成，wifi网络理论上可以与外界通信后，wifi 的NetworkAgent实时地告知 CS 其NetworkInfo 的改动， CS 会重新调整 NetworkRequest 与 Network 的匹配关系。