Android WIFI 分析

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初始化
在 SystemServer 启动的时候,会生成一个 ConnectivityService 的实例,
 try {
         Log.i(TAG, “Starting Connectivity Service.”);
         ServiceManager.addService(Context.CONNECTIVITY_SERVICE, new ConnectivityService(context));
  } catch (Throwable e) {
         Log.e(TAG, “Failure starting Connectivity Service”, e);
   }
ConnectivityService 的构造函数会创建 WifiService,
if (DBG) Log.v(TAG, “Starting Wifi Service.”);
mWifiStateTracker = new WifiStateTracker(context, handler);
WifiService wifiService = new WifiService(context, mWifiStateTracker);
ServiceManager.addService(Context.WIFI_SERVICE, wifiService);
WifiStateTracker 会创建 WifiMonitor 接收来自底层的事件,WifiService 和 WifiMonitor 是整个模块的核心。WifiService 负责启动关闭 wpa_supplicant、启动关闭 WifiMonitor 监视线程和把命令下发给 wpa_supplicant,而 WifiMonitor 则负责从 wpa_supplicant 接收事件通知。
连接 AP
1. 使能 WIFI
WirelessSettings 在初始化的时候配置了由 WifiEnabler 来处理 Wifi 按钮,
private void initToggles() {
          mWifiEnabler = new WifiEnabler(
                       this,
                       (WifiManager) getSystemService(WIFI_SERVICE),
                       (CheckBoxPreference) findPreference(KEY_TOGGLE_WIFI));
当用户按下 Wifi 按钮后,  Android 会调用 WifiEnabler 的 onPreferenceChange, 再由 WifiEnabler调用 WifiManager 的 setWifiEnabled 接口函数,通过 AIDL,实际调用的是 WifiService 的setWifiEnabled 函数, WifiService 接着向自身发送一条 MESSAGE_ENABLE_WIFI 消息,在处理该消息的代码中做真正的使能工作:首先装载 WIFI 内核模块(该模块的位置硬编码为”/system/lib/modules/wlan.ko” ), 然后启动 wpa_supplicant ( 配置文件硬编码为”/data/misc/wifi/wpa_supplicant.conf”) 再通过 WifiStateTracker 来启动 WifiMonitor 中的监视线程。
private boolean setWifiEnabledBlocking(boolean enable) {
          final  int  eventualWifiState   =   enable  ?  WIFI_STATE_ENABLED  : WIFI_STATE_DISABLED;
          updateWifiState(enable ? WIFI_STATE_ENABLING : WIFI_STATE_DISABLING);
          if (enable) {
                if (!WifiNative.loadDriver()) {
                       Log.e(TAG, “Failed to load Wi-Fi driver.”);
                       updateWifiState(WIFI_STATE_UNKNOWN);
                       return false;
                }
                if (!WifiNative.startSupplicant()) {
                       WifiNative.unloadDriver();
                       Log.e(TAG, “Failed to start supplicant daemon.”);
                       updateWifiState(WIFI_STATE_UNKNOWN);
                       return false;
                }
                mWifiStateTracker.startEventLoop();
          }
             // Success!
             persistWifiEnabled(enable);
             updateWifiState(eventualWifiState);
             return true;
}
当使能成功后,会广播发送 WIFI_STATE_CHANGED_ACTION 这个 Intent 通知外界 WIFI已经成功使能了。WifiEnabler 创建的时候就会向Android 注册接收WIFI_STATE_CHANGED_ACTION，因此它会收到该 Intent,从而开始扫描。
private void handleWifiStateChanged(int wifiState) {
       if (wifiState == WIFI_STATE_ENABLED) {
            loadConfiguredAccessPoints();
            attemptScan();
       }
}
2. 查找 AP
扫描的入口函数是 WifiService 的 startScan,它其实也就是往 wpa_supplicant 发送 SCAN 命令。
static jboolean android_net_wifi_scanCommand(JNIEnv* env, jobject clazz)
{
      jboolean result;
      // Ignore any error from setting the scan mode.
      // The scan will still work.
      (void)doBooleanCommand(“DRIVER SCAN-ACTIVE”, “OK”);
      result = doBooleanCommand(“SCAN”, “OK”);
      (void)doBooleanCommand(“DRIVER SCAN-PASSIVE”, “OK”);
      return result;
}
当 wpa_supplicant 处理完 SCAN 命令后,它会向控制通道发送事件通知扫描完成,从而wifi_wait_for_event 函数会接收到该事件,由此 WifiMonitor 中的 MonitorThread 会被执行来出来这个事件,
void handleEvent(int event, String remainder) {
                        case SCAN_RESULTS:
                        mWifiStateTracker.notifyScanResultsAvailable();
                        break;
WifiStateTracker 则接着广播发送 SCAN_RESULTS_AVAILABLE_ACTION 这个 Intent
case EVENT_SCAN_RESULTS_AVAILABLE:
         mContext.sendBroadcast(new Intent(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
WifiLayer 注册了接收 SCAN_RESULTS_AVAILABLE_ACTION 这个 Intent,所以它的相关处理函数 handleScanResultsAvailable 会被调用,在该函数中,先会去拿到 SCAN 的结果(最终是往 wpa_supplicant 发送 SCAN_RESULT 命令并读取返回值来实现的)，
List list = mWifiManager.getScanResults();
对每一个扫描返回的 AP,WifiLayer 会调用 WifiSettings 的 onAccessPointSetChanged 函数,
从而最终把该 AP 加到 GUI 显示列表中。
public void onAccessPointSetChanged(AccessPointState ap, boolean added) {
          AccessPointPreference pref = mAps.get(ap);
          if (added) {
                if (pref == null) {
                      pref = new AccessPointPreference(this, ap);
                      mAps.put(ap, pref);
                } else {
                      pref.setEnabled(true);
                }
                mApCategory.addPreference(pref);
          }
}
3. 配置 AP 参数
当用户在 WifiSettings 界面上选择了一个 AP 后,会显示配置 AP 参数的一个对话框,
public boolean onPreferenceTreeClick(PreferenceScreen preferenceScreen, Preference
preference) {
          if (preference instanceof AccessPointPreference) {
                AccessPointState state =  ((AccessPointPreference)preference).getAccessPointState();
                showAccessPointDialog(state, AccessPointDialog.MODE_INFO);
          }
 }
4. 连接
当用户在 AcessPointDialog 中选择好加密方式和输入密钥之后,再点击连接按钮,Android就会去连接这个 AP。
private void handleConnect() {
          String password = getEnteredPassword();
          if (!TextUtils.isEmpty(password)) {
                mState.setPassword(password);
          }
          mWifiLayer.connectToNetwork(mState);
}
WifiLayer 会先检测这个 AP 是不是之前被配置过,这个是通过向 wpa_supplicant 发送LIST_NETWORK 命令并且比较返回值来实现的,
// Need WifiConfiguration for the AP
WifiConfiguration config = findConfiguredNetwork(state);
如果 wpa_supplicant 没有这个 AP 的配置信息, 则会向 wpa_supplicant 发送 ADD_NETWORK命令来添加该 AP,
 if (config == null) {
         // Connecting for the first time, need to create it
          config =  addConfiguration(state, ADD_CONFIGURATION_ENABLE|ADD_CONFIGURATION_SAVE);
}
ADD_NETWORK 命令会返回一个ID , WifiLayer再用这个返回的ID作为参数向wpa_supplicant 发送 ENABLE_NETWORK 命令,从而让 wpa_supplicant 去连接该 AP。
// Make sure that network is enabled, and disable others
mReenableApsOnNetworkStateChange = true;
if (!mWifiManager.enableNetwork(state.networkId, true)) {
         Log.e(TAG, “Could not enable network ID ” + state.networkId);
         error(R.string.error_connecting);
         return false;
}
5. 配置 IP 地址
当 wpa_supplicant 成功连接上 AP 之后,它会向控制通道发送事件通知连接上 AP 了,从而wifi_wait_for_event 函数会接收到该事件,由此 WifiMonitor 中的 MonitorThread 会被执行来出来这个事件,
void handleEvent(int event, String remainder) {
        case CONNECTED:
         handleNetworkStateChange(NetworkInfo.DetailedState.CONNECTED,remainder);
         break;
WifiMonitor 再调用 WifiStateTracker 的 notifyStateChange,WifiStateTracker 则接着会往自身发送 EVENT_DHCP_START 消息来启动 DHCP 去获取 IP 地址,
private void handleConnectedState() {
          setPollTimer();
          mLastSignalLevel = -1;
          if (!mHaveIPAddress && !mObtainingIPAddress) {
                mObtainingIPAddress = true;
                mDhcpTarget.obtainMessage(EVENT_DHCP_START).sendToTarget();
          }
}
然后再广播发送 NETWORK_STATE_CHANGED_ACTION 这个 Intent
case EVENT_NETWORK_STATE_CHANGED:
         if (result.state != DetailedState.DISCONNECTED || !mDisconnectPending) {
                 intent  =  new Intent(WifiManager.NETWORK_STATE_CHANGED_ACTION);
                 intent.putExtra(WifiManager.EXTRA_NETWORK_INFO,mNetworkInfo);
                 if (result.BSSID != null)
                       intent.putExtra(WifiManager.EXTRA_BSSID, result.BSSID);
                  mContext.sendStickyBroadcast(intent);
         }
}
break;
WifiLayer 注册了接收 NETWORK_STATE_CHANGED_ACTION 这个 Intent,所以它的相关处理函数 handleNetworkStateChanged 会被调用,
当 DHCP 拿到 IP 地址之后,会再发送 EVENT_DHCP_SUCCEEDED 消息,
private class DhcpHandler extends Handler {
          public void handleMessage(Message msg) {
                switch (msg.what) {
                     case EVENT_DHCP_START:
                          if (NetworkUtils.runDhcp(mInterfaceName, mDhcpInfo)) {
                                event = EVENT_DHCP_SUCCEEDED;
                          }
WifiLayer 处 理 EVENT_DHCP_SUCCEEDED 消息, 会再次广播发送NETWORK_STATE_CHANGED_ACTION 这个Intent,这次带上完整的IP 地址信息。
case EVENT_DHCP_SUCCEEDED:
         mWifiInfo.setIpAddress(mDhcpInfo.ipAddress);
         setDetailedState(DetailedState.CONNECTED);
         intent  =   new Intent(WifiManager.NETWORK_STATE_CHANGED_ACTION);
         intent.putExtra(WifiManager.EXTRA_NETWORK_INFO, mNetworkInfo);
         mContext.sendStickyBroadcast(intent);
break;
至此为止,整个连接过程完成。