Android 9.0 WiFi BG Scan机制

        欢迎大家一起学习探讨通信之WLAN。本节我们讨论关于Android设备的WiFi Background 扫描机制。通过使用WiFi设备的经验可知。WiFi设备连接上WiFi网络，必要条件是需要扫描到目标网络。我们在使用Android设备时，设备不处于WiFi设置界面时，移动到已保存的WiFi网络覆盖范围内，设备会自动连接上网络。细心的使用者，可能已发现，有时设备很短时间内就连接上已保存网络，有时设备需要过几分钟才能连接上已保存网络。当不处于WiFi设置界面时，WiFi自动连接功能主要依赖BG Scan机制。这里我们主要探讨下Android 9.0 WiFi BG Scan机制背后代码实现的故事。

       好。我们先看下打开WiFi相关状态初始化流程。Android 框架中维护了一个WiFi状态机(WiFiStateMachine)，当用户点击打开WiFi按钮后，状态机最终要停留到DisconnectedState状态。在状态转变期间，状态机需经过ConnectModeState节点，在该状态的enter()中，调用setupClientMode()，启动一些client mode相关服务和获取其相关状态。在setupClientMode()方法中初始化了WifiConnectivityManager对象，为WiFi BG Scan奠定了基础。

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiStateMachine.java
class ConnectModeState extends State {
	public void enter() {
		setupClientMode();  //设置client Mode，即WiFi Station
	}
}
private void setupClientMode() {
    if (mWifiConnectivityManager == null) {
        synchronized (mWifiReqCountLock) {
        //创建注册WifiConnectivityManager对象
            mWifiConnectivityManager =mWifiInjector.makeWifiConnectivityManager(mWifiInfo,hasConnectionRequests());
            mWifiConnectivityManager.setUntrustedConnectionAllowed(mUntrustedReqCount > 0);
            mWifiConnectivityManager.handleScreenStateChanged(mScreenOn);
         }
    }
}

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiInjector.java
public WifiConnectivityManager makeWifiConnectivityManager(WifiInfo wifiInfo,
                                                               boolean hasConnectionRequests) {
    return new WifiConnectivityManager(mContext, getScoringParams(),
                mWifiStateMachine, getWifiScanner(),
                mWifiConfigManager, wifiInfo, mWifiNetworkSelector, mWifiConnectivityHelper,
                mWifiLastResortWatchdog, mOpenNetworkNotifier, mCarrierNetworkNotifier,
                mCarrierNetworkConfig, mWifiMetrics, mWifiStateMachineHandlerThread.getLooper(),
                mClock, mConnectivityLocalLog, hasConnectionRequests, mFrameworkFacade,
                mSavedNetworkEvaluator, mScoredNetworkEvaluator, mPasspointNetworkEvaluator);
}

       经过状态机多次转移，终于来到了DisconnectedState状态。Android状态机有个特性：状态机要进入某个状态，必须先进入其enter()；要退出某个状态，必须执行状态的exit()。在进入DisconnectedState状态时，触发了WiFi BG Scan 的流程。进入到WifiConnectivityManager这个类中，将BG Scan周期扫描逻辑的真正实现。

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiStateMachine.java
class DisconnectedState extends State {
    @Override
   //DisconnectedState 状态中调用mWifiConnectivityManager通知WiFi状态变化。
    public void enter() {
            mWifiConnectivityManager.handleConnectionStateChanged(
                    WifiConnectivityManager.WIFI_STATE_DISCONNECTED);
	}
}
framework/opt/net/wifi/service/java/com/android/server/wifi/WifiConnectivityManager.java
public void handleConnectionStateChanged(int state) {
        if (mWifiState == WIFI_STATE_DISCONNECTED) {
            mLastConnectionAttemptBssid = null;
            scheduleWatchdogTimer();
            startConnectivityScan(SCAN_IMMEDIATELY);//开始WiFi扫描
	}
}
private void startConnectivityScan(boolean scanImmediately) {
    if (mScreenOn) {
        startPeriodicScan(scanImmediately);//亮屏下，执行正常WiFi扫描
    } else {
        if (mWifiState == WIFI_STATE_DISCONNECTED && !mPnoScanStarted) {
            startDisconnectedPnoScan();//熄屏下，执行正常PNO wifi扫描
        }
    }
}
private void startPeriodicScan(boolean scanImmediately) {
    mPeriodicSingleScanInterval = PERIODIC_SCAN_INTERVAL_MS;
    startPeriodicSingleScan(); //开始周期扫描，并赋值mPeriodicSingleScanInterval=20S。
}

       到这里，WiFi BG Scan已经进入其机制具体实现中，详细学习下这块代码逻辑的实现，调用逻辑相对还是比较清晰简单。

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiConnectivityManager.java
正常扫描，在亮屏状态下进行，从代码中可看到对mScreen的判断。
public static final int PERIODIC_SCAN_INTERVAL_MS = 20 * 1000; // 最小扫描间隔20s
public static final int MAX_PERIODIC_SCAN_INTERVAL_MS = 160 * 1000; //最大扫描间隔160s

这里解释下，PNO扫描只扫描已保存的网络，在熄屏状态下执行的。
private static final int DISCONNECTED_PNO_SCAN_INTERVAL_MS = 20 * 1000; // 最小PNO扫描间隔20s
private static final int CONNECTED_PNO_SCAN_INTERVAL_MS = 160 * 1000; // 最大PNO扫描间隔160s

private int mPeriodicSingleScanInterval = PERIODIC_SCAN_INTERVAL_MS;

创建Alarm，时间到期执行periodicScanTimerHandler()方法。
private final AlarmManager.OnAlarmListener mPeriodicScanTimerListener =
            new AlarmManager.OnAlarmListener() {
                public void onAlarm() {
                    periodicScanTimerHandler();
                }
};

// 设置周期扫描Timer。并将周期扫描标志变量设置mPeriodicScanTimerSet=true。
private void schedulePeriodicScanTimer(int intervalMs) {
        mAlarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP,
                            mClock.getElapsedSinceBootMillis() + intervalMs,
                            PERIODIC_SCAN_TIMER_TAG,
                            mPeriodicScanTimerListener, mEventHandler);
        mPeriodicScanTimerSet = true;
 }

private void periodicScanTimerHandler() {
        localLog("periodicScanTimerHandler");

        //安排下一个定时器，如亮屏，则开始扫描。
        if (mScreenOn) {
            startPeriodicSingleScan();
        }
    }

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiConnectivityManager.java
    // 启动扫描并设置下一次扫描的间隔。
    private void startPeriodicSingleScan() {
        long currentTimeStamp = mClock.getElapsedSinceBootMillis();

        如果距上次扫描时间小于20s，在重新设置到时扫描Timer.
        if (mLastPeriodicSingleScanTimeStamp != RESET_TIME_STAMP) {
            long msSinceLastScan = currentTimeStamp - mLastPeriodicSingleScanTimeStamp;
            if (msSinceLastScan < PERIODIC_SCAN_INTERVAL_MS) {
                localLog("Last periodic single scan started " + msSinceLastScan
                        + "ms ago, defer this new scan request.");
                schedulePeriodicScanTimer(PERIODIC_SCAN_INTERVAL_MS - (int) msSinceLastScan);
                return;
            }
        }

        if (isScanNeeded) {
            //判断是否需要执行扫描，
            //如果为true。在执行扫描，并重新设置周期扫描Timer。
            //如果为false。则跳过本次扫描，重新设置周期扫描Timer同上次时长。
            mLastPeriodicSingleScanTimeStamp = currentTimeStamp;
            startSingleScan(isFullBandScan, WIFI_WORK_SOURCE);
            schedulePeriodicScanTimer(mPeriodicSingleScanInterval);

            //以指数回退方式设置下一个扫描间隔。
            //mPeriodicSingleScanInterval = mPeriodicSingleScanInterval  * 2
            //单次执行Timer时间为：20S  40S  80S 160S。后续一直保存为160S
            mPeriodicSingleScanInterval *= 2; 
            if (mPeriodicSingleScanInterval >  MAX_PERIODIC_SCAN_INTERVAL_MS) {
                mPeriodicSingleScanInterval = MAX_PERIODIC_SCAN_INTERVAL_MS;
            }
        } else {
            schedulePeriodicScanTimer(mPeriodicSingleScanInterval);
        }
    }

具体执行扫描实现如下。调用startSingleScan方法，将扫描动作传递到Scanner中。
framework/opt/net/wifi/service/java/com/android/server/wifi/WifiConnectivityManager.java
// Start a single scan
private void startSingleScan(boolean isFullBandScan, WorkSource workSource) {
	mScanner.startScan(settings, singleScanListener, workSource);
}
./base/wifi/java/android/net/wifi/WifiScanner.java
public void startScan(ScanSettings settings, ScanListener listener, WorkSource workSource) {
    //通过AsyncChannel发送消息CMD_START_SINGLE_SCAN，给到扫描服务。
    mAsyncChannel.sendMessage(CMD_START_SINGLE_SCAN, 0, key, scanParams);
}

//扫描服务处理WifiScanner发送的消息
framework/opt/net/wifi/service/java/com/android/server/wifi/scanner/WifiScanningServiceImpl.java
class DriverStartedState extends State {
	public boolean processMessage(Message msg) {
		case WifiScanner.CMD_START_SINGLE_SCAN:
			tryToStartNewScan();
	}
}
void tryToStartNewScan() {
	if (mScannerImpl.startSingleScan(settings, this)) {
	}
}

framework/opt/net/wifi/service/java/com/android/server/wifi/scanner/WifiScannerImpl.java
public abstract boolean startSingleScan(WifiNative.ScanSettings settings,
            WifiNative.ScanEventHandler eventHandler);

framework/opt/net/wifi/service/java/com/android/server/wifi/scanner/WificondScannerImpl.java
public boolean startSingleScan(WifiNative.ScanSettings settings,
        WifiNative.ScanEventHandler eventHandler) {
		success = mWifiNative.scan(
            mIfaceName, settings.scanType, freqs, hiddenNetworkSSIDSet);
}

      WiFiNative下发扫描命令给到WiFiCond，在WiFiCond进程中将消息发送至kernel nl802.11中，最后发送至driver执行WiFi扫描动作。

framework/opt/net/wifi/service/java/com/android/server/wifi/WifiNative.java

public boolean scan(@NonNull String ifaceName, int scanType, Set<Integer> freqs,

Set<String> hiddenNetworkSSIDs) {

return mWificondControl.scan(ifaceName, scanType, freqs, hiddenNetworkSSIDs);

}

       到此，我们已经分析完了WiFi BG scan的代码实现流程。但WiFiNative->kernel->驱动执行还有比较复杂的调用关系，后续再进行分析梳理。

注：

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