//在 SystemServer 啟動的時候,啟動WifiService呼叫關係如下:
public static void main(String[] args) {
        new SystemServer().run();
    }
private void run() {
……
startOtherServices();
……
}
startOtherServices(){
......
               mSystemServiceManager.startService(WIFI_P2P_SERVICE_CLASS);
                mSystemServiceManager.startService(WIFI_SERVICE_CLASS);
                mSystemServiceManager.startService(
                            "com.android.server.wifi.WifiScanningService"
 
);

                mSystemServiceManager.startService("com.android.server.wifi.RttService");
……
}

當用戶按下 Wifi 按鈕後, 在設定裡呼叫 WifiEnabler 的 onSwitchChanged()方法, 在這個方法裡又 呼叫 WifiManager 的 setWifiEnabled 介面函式,通過 AIDL,實際呼叫的是 WifiService (即WifiServiceImpl)的setWifiEnabled 函式:

//setWifiEnabled 函式:
// WifiServiceImpl.java
 

public synchronized boolean setWifiEnabled(boolean enable) {
    ……
    mWifiController.obtainMessage(CMD_WIFI_TOGGLED, mWifiIpoOff ? 1 : 0)
    ……
}
// WifiController.java
//其建構函式如下:
WifiController(Context context, WifiServiceImpl service, Looper looper) {
        super(TAG, looper);
         ……
         addState(mDefaultState);
            addState(mApStaDisabledState, mDefaultState);
            addState(mStaEnabledState, mDefaultState);
                addState(mDeviceActiveState, mStaEnabledState);
……
if
 
 (isScanningAlwaysAvailable) {
            setInitialState(mStaDisabledWithScanState);
        } else {
            setInitialState(mApStaDisabledState);
        }
……
}

然後看類:class ApStaDisabledState extends State {
……

public boolean processMessage(Message msg) {
            SXlog.d(TAG, getName() + msg.toString() + "\n");
            switch (msg.what) {
                case CMD_WIFI_TOGGLED:
                case CMD_AIRPLANE_TOGGLED:
    ……
     if (mSettingsStore.isWifiToggleEnabled()) {

                        if (doDeferEnable(msg)) {
                            if (mHaveDeferredEnable) {
                                //  have 2 toggles now, inc serial number an ignore both
                                mDeferredEnableSerialNumber++;
                            }
                            mHaveDeferredEnable = !mHaveDeferredEnable;
                            break;
                        }
                        if (mDeviceIdle == false) {
                            transitionTo(mDeviceActiveState);
                        } else {
                            checkLocksAndTransitionWhenDeviceIdle();
                        }
                    ///M: check scan always avaliable only when ipo change from ipo on to off or airplane   //mode with no  ipo off
                    }
    ……
class StaEnabledState extends State {
        @Override
        public void enter() {

            if (DBG) log(getName() + "\n");
            mWifiStateMachine.setSupplicantRunning(true);
        }
       ……
}

class DeviceActiveState extends State {
        @Override
        public void enter() {
            if (DBG) log(getName() + "\n");

            mWifiStateMachine.setOperationalMode(WifiStateMachine.CONNECT_MODE);
            mWifiStateMachine.setDriverStart(true);
            mWifiStateMachine.setHighPerfModeEnabled(false);
        }
        ……
}

// WifiStateMachine.java
public void setSupplicantRunning(boolean enable) {
        if (enable) {
            sendMessage(CMD_START_SUPPLICANT);
        } else {
            sendMessage(CMD_STOP_SUPPLICANT);
        }
    }
public void setDriverStart(boolean enable) {
        if (enable) {
            sendMessage(CMD_START_DRIVER);
        } else {
            sendMessage(CMD_STOP_DRIVER);
        }
    }
這樣一來又要看WifiStateMachine裡的狀態
InitialState
public boolean processMessage(Message message) {
            logStateAndMessage(message, getClass().getSimpleName());
            switch (message.what) {
                case CMD_START_SUPPLICANT:
                    if (mWifiNative.loadDriver()) {
                        ……
                       /* Stop a running supplicant after a runtime restart
                        * Avoids issues with drivers that do not handle interface down
                        * on a running supplicant properly.
                        */
                        mWifiMonitor.killSupplicant(mP2pSupported);
                        if(mWifiNative.startSupplicant(mP2pSupported)) {
                            setWifiState(WIFI_STATE_ENABLING);
                            if (DBG) log("Supplicant start successful");
                            mWifiMonitor.startMonitoring();
                            transitionTo(mSupplicantStartingState);
                        } ……

}

首先裝載 WIFI 核心模組(該模組的位置硬編碼為“/system/lib/modules/wlan.ko” ), 然 後 啟 動 wpa_supplicant ( 配 置 文 件 硬 編 碼 為“/data/misc/wifi/wpa_supplicant.conf”) 啟動 WifiMonitor 中的監視執行緒。

當使能成功後,會廣播發送 WIFI_STATE_CHANGED_ACTION 這個 Intent 通知外界 WIFI已 經 成 功 使 能 了 。 WifiSettings 創 建 的 時 候 就 會 向 Android 注 冊 接 收WIFI_STATE_CHANGED_ACTION,因此它會收到該 Intent,從而開始掃描。          if (WifiManager.WIFI_STATE_CHANGED_ACTION.equals(action)) {
            updateWifiState(intent.getIntExtra(WifiManager.EXTRA_WIFI_STATE,
                    WifiManager.WIFI_STATE_UNKNOWN));
        } 

private void updateWifiState(int state) {
        Activity activity = getActivity();
        if (activity != null) {
            activity.invalidateOptionsMenu();
        }

        switch (state) {
            case WifiManager.WIFI_STATE_ENABLED:
                mScanner.resume();
……

void resume() {
            if (!hasMessages(0)) {
                sendEmptyMessage(0);
            }
        }
public void handleMessage(Message message) {
            if (mWifiSettings.mWifiManager.startScan()) {
                mRetry = 0;
            }
……
//最後就是往 wpa_supplicant 傳送 SCAN 命令.

當 wpa_supplicant 處理完 SCAN 命令後,它會向控制通道傳送事件通知掃描完成,從而wifi_wait_for_event 函式會接收到該事件,由此 WifiMonitor 中的 MonitorThread 會被執行,處理事件: void handleEvent(int event, String remainder) { case SCAN_RESULTS:
mStateMachine.sendMessage(SCAN_RESULTS_EVENT);
最後就是在WifiStateMachine中傳送廣播:SCAN_RESULTS_AVAILABLE_ACTION.
WifiSettings註冊了此廣播, 收到後會更新UI顯示列表.