二，Wifi模塊解析
1）框架分析
首先，用戶(hù)程序使用WifiManager類(lèi)來(lái)管理Wifi模塊，它能夠獲得Wifi模塊的狀態(tài)，配置和控制Wifi模塊，而所有這些操作都要依賴(lài)Wifiservice類(lèi)來(lái)實(shí)現。
WifiService和WifiMonitor類(lèi)是Wifi框架的核心，如圖所示。下面先來(lái)看看WifiService是什么時(shí)候，怎么被創(chuàng )建和初始化的。
在systemServer啟動(dòng)之后，它會(huì )創(chuàng )建一個(gè)ConnectivityServer對象，這個(gè)對象的構造函數會(huì )創(chuàng )建一個(gè)WifiService的實(shí)例，代碼如下所示：
framework/base/services/java/com/android/server/ConnectivityService.java
{
……
case ConnectivityManager.TYPE_WIFI:
if (DBG) Slog.v(TAG, "Starting Wifi Service.");
WifiStateTracker wst = new WifiStateTracker(context, mHandler); //創(chuàng )建WifiStateTracker實(shí)例
WifiService wifiService = new WifiService(context, wst);//創(chuàng )建WifiService實(shí)例
ServiceManager.addService(Context.WIFI_SERVICE, wifiService); //向服務(wù)管理系統添加Wifi服務(wù)
wifiService.startWifi(); //啟動(dòng)Wifi
mNetTrackers[ConnectivityManager.TYPE_WIFI] = wst;
wst.startMonitoring(); //啟動(dòng)WifiMonitor中的WifiThread線(xiàn)程
……
}
WifiService的主要工作：WifiMonitor和Wpa_supplicant的啟動(dòng)和關(guān)閉，向Wpa_supplicant發(fā)送命令。
WifiMonitor的主要工作：阻塞監聽(tīng)并接收來(lái)自Wpa_supplicant的消息，然后發(fā)送給WifiStateTracker。
上面兩個(gè)線(xiàn)程通過(guò)AF_UNIX套接字和Wpa_supplicant通信，在通信過(guò)程中有兩種連接方式：控制連接和監聽(tīng)連接。它們創(chuàng )建代碼如下：
ctrl_conn = wpa_ctrl_open(ifname);
.. .. ..
monitor_conn = wpa_ctrl_open(ifname);
2）Wifi啟動(dòng)流程
（1）使能Wifi
要想使用Wifi模塊，必須首先使能Wifi，當你第一次按下Wifi使能按鈕時(shí)，WirelessSettings會(huì )實(shí)例化一個(gè)WifiEnabler對象，實(shí)例化代碼如下：
packages/apps/settings/src/com/android/settings/WirelessSettings.java
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
……
CheckBoxPreference wifi = (CheckBoxPreference) findPreference(KEY_TOGGLE_WIFI);
mWifiEnabler = new WifiEnabler(this, wifi);
……
}
WifiEnabler類(lèi)的定義大致如下，它實(shí)現了一個(gè)監聽(tīng)接口，當WifiEnabler對象被初始化后，它監聽(tīng)到你按鍵的動(dòng)作，會(huì )調用響應函數onPreferenceChange（），這個(gè)函數會(huì )調用WifiManager的setWifiEnabled（）函數。
public class WifiEnabler implements Preference.OnPreferenceChangeListener {
……
public boolean onPreferenceChange(Preference preference, Object value) {
boolean enable = (Boolean) value;
……
if (mWifiManager.setWifiEnabled(enable)) {
mCheckBox.setEnabled(false);
……
}
……
}
我們都知道Wifimanager只是個(gè)服務(wù)代理，所以它會(huì )調用WifiService的setWifiEnabled（）函數，而這個(gè)函數會(huì )調用sendEnableMessage（）函數，了解android 消息處理機制的都知道，這個(gè)函數最終會(huì )給自己發(fā)送一個(gè)MESSAGE_ENABLE_WIFI的消息，被WifiService里面定義的handlermessage()函數處理，會(huì )調用setWifiEnabledBlocking（）函數。下面是調用流程：
mWifiEnabler.onpreferencechange()=>mWifiManage.setWifienabled()=>mWifiService.setWifiEnabled()=>mWifiService.sendEnableMessage()=>mWifiService.handleMessage()=>mWifiService.setWifiEnabledBlocking().
在setWifiEnabledBlocking()函數中主要做如下工作：加載Wifi驅動(dòng)，啟動(dòng)wpa_supplicant，注冊廣播接收器，啟動(dòng)WifiThread監聽(tīng)線(xiàn)程。代碼如下：
……
if (enable) {
if (!mWifiStateTracker.loadDriver()) {
Slog.e(TAG, "Failed to load Wi-Fi driver.");
setWifiEnabledState(WIFI_STATE_UNKNOWN, uid);
return false;
}
if (!mWifiStateTracker.startSupplicant()) {
mWifiStateTracker.unloadDriver();
Slog.e(TAG, "Failed to start supplicant daemon.");
setWifiEnabledState(WIFI_STATE_UNKNOWN, uid);
return false;
}
registerForBroadcasts();
mWifiStateTracker.startEventLoop();
……
至此，Wifi使能結束，自動(dòng)進(jìn)入掃描階段。
(2) 掃描AP
當驅動(dòng)加載成功后，如果配置文件的AP_SCAN = 1，掃描會(huì )自動(dòng)開(kāi)始，WifiMonitor將會(huì )從supplicant收到一個(gè)消息EVENT_DRIVER_STATE_CHANGED，調用handleDriverEvent（），然后調用mWifiStateTracker.notifyDriverStarted()，該函數向消息隊列添加EVENT_DRIVER_STATE_CHANGED，handlermessage()函數處理消息時(shí)調用scan()函數，并通過(guò)WifiNative將掃描命令發(fā)送到wpa_supplicant。
Frameworks/base/wifi/java/android/net/wifi/WifiMonitor.java
private void handleDriverEvent(String state) {
if (state == null) {
return;
}
if (state.equals("STOPPED")) {
mWifiStateTracker.notifyDriverStopped();
} else if (state.equals("STARTED")) {
mWifiStateTracker.notifyDriverStarted();
} else if (state.equals("HANGED")) {
mWifiStateTracker.notifyDriverHung();
}
}
Frameworks/base/wifi/java/android/net/wifi/WifiStateTracker.java
case EVENT_DRIVER_STATE_CHANGED:
switch (msg.arg1) {
case DRIVER_STARTED:
setNumAllowedChannels();
synchronized (this) {
if (mRunState == RUN_STATE_STARTING) {
mRunState = RUN_STATE_RUNNING;
if (!mIsScanOnly) {
reconnectCommand();
} else {
// In some situations, supplicant needs to be kickstarted to
// start the background scanning
scan(true);
}
}
}
break;
上面是啟動(dòng)Wifi時(shí)，自動(dòng)進(jìn)行的AP的掃描，用戶(hù)當然也可以手動(dòng)掃描AP，這部分實(shí)現在WifiService里面，WifiService通過(guò)startScan()接口函數發(fā)送掃描命令到supplicant。
Frameworks/base/wifi/java/android/net/wifi/WifiStateTracker.java
public boolean startScan(boolean forceActive) {
enforceChangePermission();
switch (mWifiStateTracker.getSupplicantState()) {
case DISCONNECTED:
case INACTIVE:
case SCANNING:
case DORMANT:
break;
default:
mWifiStateTracker.setScanResultHandling(
WifiStateTracker.SUPPL_SCAN_HANDLING_LIST_ONLY);
break;
}
return mWifiStateTracker.scan(forceActive);
}
然后下面的流程同上面的自動(dòng)掃描，我們來(lái)分析一下手動(dòng)掃描從哪里開(kāi)始的。我們應該知道手動(dòng)掃描是通過(guò)菜單鍵的掃描鍵來(lái)響應的，而響應該動(dòng)作的應該是WifiSettings類(lèi)中Scanner類(lèi)的handlerMessage()函數，它調用WifiManager的startScanActive()，這才調用WifiService的startScan()。
packages/apps/Settings/src/com/android/settings/wifi wifisettings.java
public boolean onCreateOptionsMenu(Menu menu) {
menu.add(Menu.NONE, MENU_ID_SCAN, 0, R.string.wifi_menu_scan)
.setIcon(R.drawable.ic_menu_scan_network);
menu.add(Menu.NONE, MENU_ID_ADVANCED, 0, R.string.wifi_menu_advanced)
.setIcon(android.R.drawable.ic_menu_manage);
return super.onCreateOptionsMenu(menu);
}
當按下菜單鍵時(shí)，WifiSettings就會(huì )調用這個(gè)函數繪制菜單。如果選擇掃描按鈕，WifiSettings會(huì )調用onOptionsItemSelected()。
packages/apps/Settings/src/com/android/settings/wifi wifisettings.java
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case MENU_ID_SCAN:
if (mWifiManager.isWifiEnabled()) {
mScanner.resume();
}
return true;
case MENU_ID_ADVANCED:
startActivity(new Intent(this, AdvancedSettings.class));
return true;
}
return super.onOptionsItemSelected(item);
}
private class Scanner extends Handler {
private int mRetry = 0;
void resume() {
if (!hasMessages(0)) {
sendEmptyMessage(0);
}
}
void pause() {
mRetry = 0;
mAccessPoints.setProgress(false);
removeMessages(0);
}
@Override
public void handleMessage(Message message) {
if (mWifiManager.startScanActive()) {
mRetry = 0;
} else if (++mRetry >= 3) {
mRetry = 0;
Toast.makeText(WifiSettings.this, R.string.wifi_fail_to_scan,
Toast.LENGTH_LONG).show();
return;
}
mAccessPoints.setProgress(mRetry != 0);
sendEmptyMessageDelayed(0, 6000);
}
}
這里的mWifiManager.startScanActive()就會(huì )調用WifiService里的startScan（）函數，下面的流程和上面的一樣，這里不贅述。
當supplicant完成了這個(gè)掃描命令后，它會(huì )發(fā)送一個(gè)消息給上層，提醒他們掃描已經(jīng)完成，WifiMonitor會(huì )接收到這消息，然后再發(fā)送給WifiStateTracker。
Frameworks/base/wifi/java/android/net/wifi/WifiMonitor.java
void handleEvent(int event, String remainder) {
switch (event) {
case DISCONNECTED:
handleNetworkStateChange(NetworkInfo.DetailedState.DISCONNECTED, remainder);
break;
case CONNECTED:
handleNetworkStateChange(NetworkInfo.DetailedState.CONNECTED, remainder);
break;
case SCAN_RESULTS:
mWifiStateTracker.notifyScanResultsAvailable();
break;
case UNKNOWN:
break;
}
}
WifiStateTracker將會(huì )廣播SCAN_RESULTS_AVAILABLE_ACTION消息：
Frameworks/base/wifi/java/android/net/wifi/WifiStateTracker.java
public void handleMessage(Message msg) {
Intent intent;
……
case EVENT_SCAN_RESULTS_AVAILABLE:
if (ActivityManagerNative.isSystemReady()) {
mContext.sendBroadcast(new Intent(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
}
sendScanResultsAvailable();
setScanMode(false);
break;
……
｝
由于WifiSettings類(lèi)注冊了intent，能夠處理SCAN_RESULTS_AVAILABLE_ACTION消息，它會(huì )調用handleEvent()，調用流程如下所示。
WifiSettings.handleEvent() => WifiSettings.updateAccessPoints() => mWifiManager.getScanResults() => mService.getScanResults() => mWifiStateTracker.scanResults() => WifiNative.scanResultsCommand()……
將獲取AP列表的命令發(fā)送到supplicant，然后supplicant通過(guò)Socket發(fā)送掃描結果，由上層接收并顯示。這和前面的消息獲取流程基本相同。
(3)配置，連接AP
當用戶(hù)選擇一個(gè)活躍的AP時(shí)，WifiSettings響應打開(kāi)一個(gè)對話(huà)框來(lái)配置AP，比如加密方法和連接AP的驗證模式。配置好AP后，WifiService添加或更新網(wǎng)絡(luò )連接到特定的AP。
packages/apps/settings/src/com/android/settings/wifi/WifiSetttings.java
public boolean onPreferenceTreeClick(PreferenceScreen screen, Preference preference) {
if (preference instanceof AccessPoint) {
mSelected = (AccessPoint) preference;
showDialog(mSelected, false);
} else if (preference == mAddNetwork) {
mSelected = null;
showDialog(null, true);
} else if (preference == mNotifyOpenNetworks) {
Secure.putInt(getContentResolver(),
Secure.WIFI_NETWORKS_AVAILABLE_NOTIFICATION_ON,
mNotifyOpenNetworks.isChecked() ? 1 : 0);
} else {
return super.onPreferenceTreeClick(screen, preference);
}
return true;
}
配置好以后，當按下“Connect Press”時(shí)，WifiSettings通過(guò)發(fā)送LIST_NETWORK命令到supplicant來(lái)檢查該網(wǎng)絡(luò )是否配置。如果沒(méi)有該網(wǎng)絡(luò )或沒(méi)有配置它，WifiService調用addorUpdateNetwork（）函數來(lái)添加或更新網(wǎng)絡(luò )，然后發(fā)送命令給supplicant，連接到這個(gè)網(wǎng)絡(luò )。下面是從響應連接按鈕到WifiService發(fā)送連接命令的代碼：
packages/apps/settings/src/com/android/settings/wifi/WifiSetttings.java
public void onClick(DialogInterface dialogInterface, int button) {
if (button == WifiDialog.BUTTON_FORGET && mSelected != null) {
forget(mSelected.networkId);
} else if (button == WifiDialog.BUTTON_SUBMIT && mDialog != null) {
WifiConfiguration config = mDialog.getConfig();
if (config == null) {
if (mSelected != null && !requireKeyStore(mSelected.getConfig())) {
connect(mSelected.networkId);
}
} else if (config.networkId != -1) {
if (mSelected != null) {
mWifiManager.updateNetwork(config);
saveNetworks();
}
} else {
int networkId = mWifiManager.addNetwork(config);
if (networkId != -1) {
mWifiManager.enableNetwork(networkId, false);
config.networkId = networkId;
if (mDialog.edit || requireKeyStore(config)) {
saveNetworks();
} else {
connect(networkId);
}
}
}
}
}
Frameworks\base\wifi\java\android\net\wifi\WifiManager.java
public int updateNetwork(WifiConfiguration config) {
if (config == null || config.networkId < 0) {
return -1;
}
return addOrUpdateNetwork(config);
}
private int addOrUpdateNetwork(WifiConfiguration config) {
try {
return mService.addOrUpdateNetwork(config);
} catch (RemoteException e) {
return -1;
}
}
WifiService.addOrUpdateNetwork()通過(guò)調用mWifiStateTracker.setNetworkVariable()將連接命令發(fā)送到Wpa_supplicant。
（4）獲取IP地址
當連接到supplicant后，WifiMonitor就會(huì )通知WifiStateTracker。
Frameworks/base/wifi/java/android/net/wifi/WifiMonitor.java
Public void Run(){
if (connectToSupplicant()) {
// Send a message indicating that it is now possible to send commands
// to the supplicant
mWifiStateTracker.notifySupplicantConnection();
} else {
mWifiStateTracker.notifySupplicantLost();
return;
}
……
}
WifiStateTracker發(fā)送EVENT_SUPPLICANT_CONNECTION消息到消息隊列，這個(gè)消息有自己的handlermessage()函數處理，它會(huì )啟動(dòng)一個(gè)DHCP線(xiàn)程，而這個(gè)線(xiàn)程會(huì )一直等待一個(gè)消息事件，來(lái)啟動(dòng)DHCP協(xié)議分配IP地址。
frameworks/base/wifi/java/android/net/wifi/WifiStateTracker.java
void notifySupplicantConnection() {
sendEmptyMessage(EVENT_SUPPLICANT_CONNECTION);
}
public void handleMessage(Message msg) {
Intent intent;
switch (msg.what) {
case EVENT_SUPPLICANT_CONNECTION:
……
HandlerThread dhcpThread = new HandlerThread("DHCP Handler Thread");
dhcpThread.start();
mDhcpTarget = new DhcpHandler(dhcpThread.getLooper(), this);
……
……
}
當Wpa_supplicant連接到AP后，它會(huì )發(fā)送一個(gè)消息給上層來(lái)通知連接成功，WifiMonitor會(huì )接受到這個(gè)消息并上報給WifiStateTracker。
Frameworks/base/wifi/java/android/net/wifi/WifiMonitor.java
void handleEvent(int event, String remainder) {
switch (event) {
case DISCONNECTED:
handleNetworkStateChange(NetworkInfo.DetailedState.DISCONNECTED, remainder);
break;
case CONNECTED:
handleNetworkStateChange(NetworkInfo.DetailedState.CONNECTED, remainder);
break;
……
}
private void handleNetworkStateChange(NetworkInfo.DetailedState newState, String data) {
String BSSID = null;
int networkId = -1;
if (newState == NetworkInfo.DetailedState.CONNECTED) {
Matcher match = mConnectedEventPattern.matcher(data);
if (!match.find()) {
if (Config.LOGD) Log.d(TAG, "Could not find BSSID in CONNECTED event string");
} else {
BSSID = match.group(1);
try {
networkId = Integer.parseInt(match.group(2));
} catch (NumberFormatException e) {
networkId = -1;
}
}
}
mWifiStateTracker.notifyStateChange(newState, BSSID, networkId);
}
void notifyStateChange(DetailedState newState, String BSSID, int networkId) {
Message msg = Message.obtain(
this, EVENT_NETWORK_STATE_CHANGED,
new NetworkStateChangeResult(newState, BSSID, networkId));
msg.sendToTarget();
}
case EVENT_NETWORK_STATE_CHANGED:
……
configureInterface();
……
private void configureInterface() {
checkPollTimer();
mLastSignalLevel = -1;
if (!mUseStaticIp) { //使用DHCP線(xiàn)程動(dòng)態(tài)IP
if (!mHaveIpAddress && !mObtainingIpAddress) {
mObtainingIpAddress = true;
//發(fā)送啟動(dòng)DHCP線(xiàn)程獲取IP
mDhcpTarget.sendEmptyMessage(EVENT_DHCP_START);
}
} else { //使用靜態(tài)IP，IP信息從mDhcpInfo中獲取
int event;
if (NetworkUtils.configureInterface(mInterfaceName, mDhcpInfo)) {
mHaveIpAddress = true;
event = EVENT_INTERFACE_CONFIGURATION_SUCCEEDED;
if (LOCAL_LOGD) Log.v(TAG, "Static IP configuration succeeded");
} else {
mHaveIpAddress = false;
event = EVENT_INTERFACE_CONFIGURATION_FAILED;
if (LOCAL_LOGD) Log.v(TAG, "Static IP configuration failed");
}
sendEmptyMessage(event); //發(fā)送IP獲得成功消息事件
}
}
DhcpThread獲取EVENT_DHCP_START消息事件后，調用handleMessage（）函數，啟動(dòng)DHCP獲取IP地址的服務(wù)。
public void handleMessage(Message msg) {
int event;
switch (msg.what) {
case EVENT_DHCP_START:
……
Log.d(TAG, "DhcpHandler: DHCP request started");
//啟動(dòng)一個(gè)DHCP client的精靈進(jìn)程，為mInterfaceName請求分配一個(gè)IP地//址
if (NetworkUtils.runDhcp(mInterfaceName, mDhcpInfo)) {
event = EVENT_INTERFACE_CONFIGURATION_SUCCEEDED;
if (LOCAL_LOGD) Log.v(TAG, "DhcpHandler: DHCP request succeeded");
} else {
event = EVENT_INTERFACE_CONFIGURATION_FAILED;
Log.i(TAG, "DhcpHandler: DHCP request failed: " +
NetworkUtils.getDhcpError());
}
……
}
這里調用了一個(gè)NetworkUtils.runDhcp（）函數，NetworkUtils類(lèi)是一個(gè)網(wǎng)絡(luò )服務(wù)的輔助類(lèi)，它主要定義了一些本地接口，這些接口會(huì )通過(guò)他們的JNI層android_net_NetUtils.cpp文件和DHCP client通信，并獲取IP地址。
至此，IP地址獲取完畢，Wifi啟動(dòng)流程結束。
