WIFI连接原理

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, jobjectclazz)
{
     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<ScanResult> 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, booleanadded) {
         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(PreferenceScreenpreferenceScreen, 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。