Android Wifi的扫描机制

Android Wifi 的扫描场景分为下面四种情况：

1.亮屏情况下，在Wifi settings界面，固定扫描，扫描时间为10s。

2.亮屏情况下，在非Wifi settings界面，二进制指数退避扫描，退避：interval*(2^n), 最小间隔min=20s, 最大间隔max=160s.

3.灭屏情况下，有保存网络时，若已连接，不扫描，否则，PNO扫描，即只扫描已保存的网络。最小间隔min=20s，最大间隔max=20s*3=60s。

4.无保存网络情况下，固定扫描，间隔为5分钟，用于通知用户周围存在可用开放网络。

1.亮屏情况下，在Wifi settings界面，固定扫描，扫描时间为10s。

从 wifi扫描流程梳理 可以看到wifi扫描流程会走到WifiTracker的handleMessage函数中，我们看sendEmptyMessageDelayed，会发送一条扫描的信息，其中包含了WIFI_RESCAN_INTERVAL_MS，WIFI_RESCAN_INTERVAL_MS就是默认的扫描周期，可以看到系统设置的默认周期是10 * 1000，也就是10秒。

WifiTracker.java

public void handleMessage(Message message) {if (message.what != MSG_SCAN) return;if (mWifiManager.startScan()) {mRetry = 0;} else if (++mRetry >= 3) {mRetry = 0;if (mContext != null) {Toast.makeText(mContext, R.string.wifi_fail_to_scan, Toast.LENGTH_LONG).show();}return;}sendEmptyMessageDelayed(MSG_SCAN, WIFI_RESCAN_INTERVAL_MS);}

2.亮屏情况下，在非Wifi settings界面，二进制指数退避扫描，退避：interval*(2^n), 最小间隔min=20s, 最大间隔max=160s.

WifiConnectivityManager.java

startConnectivityScan --> startPeriodicScan --> startPeriodicSingleScan

private void startConnectivityScan(boolean scanImmediately) {// Always stop outstanding connecivity scan if there is anystopConnectivityScan();if (mWifiState != WIFI_STATE_CONNECTED && mWifiState != WIFI_STATE_DISCONNECTED) {return;}if (mScreenOn) {startPeriodicScan(scanImmediately);} else {if (mWifiState == WIFI_STATE_DISCONNECTED && !mPnoScanStarted) {startDisconnectedPnoScan();}}}

private void startPeriodicScan(boolean scanImmediately) {mPnoScanListener.resetLowRssiNetworkRetryDelay();if (scanImmediately) {resetLastPeriodicSingleScanTimeStamp();}mPeriodicSingleScanInterval = PERIODIC_SCAN_INTERVAL_MS;startPeriodicSingleScan();}

// Start a single scan and set up the interval for next single scan.private void startPeriodicSingleScan() {long currentTimeStamp = mClock.getElapsedSinceBootMillis();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) {mLastPeriodicSingleScanTimeStamp = currentTimeStamp;startSingleScan(isFullBandScan, WIFI_WORK_SOURCE);schedulePeriodicScanTimer(mPeriodicSingleScanInterval);// Set up the next scan interval in an exponential backoff fashion.mPeriodicSingleScanInterval *= 2;if (mPeriodicSingleScanInterval >  MAX_PERIODIC_SCAN_INTERVAL_MS) {mPeriodicSingleScanInterval = MAX_PERIODIC_SCAN_INTERVAL_MS;}} else {// Since we already skipped this scan, keep the same scan interval for next scan.schedulePeriodicScanTimer(mPeriodicSingleScanInterval);}}

对于扫描间隔的设置

    // Periodic scan interval in milli-seconds. This is the scan// performed when screen is on.public static final int PERIODIC_SCAN_INTERVAL_MS = 20 * 1000; // 20 seconds// When screen is on and WiFi traffic is heavy, exponential backoff// connectivity scans are scheduled. This constant defines the maximum// scan interval in this scenario.@VisibleForTestingpublic static final int MAX_PERIODIC_SCAN_INTERVAL_MS = 160 * 1000; // 160 seconds

3.灭屏情况下，有保存网络时，若已连接，不扫描，否则，PNO扫描，即只扫描已保存的网络。最小间隔min=20s，最大间隔max=20s*3=60s。

待续

4.无保存网络情况下，固定扫描，间隔为5分钟，用于通知用户周围存在可用开放网络。

（注：第四种情况属于Android O的逻辑，因为在P的代码里还没有找到关于这个扫描的代码，也可能取消了）

WifiStateMachine.java

class DisconnectedState extends State {@Overridepublic void enter() {Log.i(TAG, "disconnectedstate enter");// We dont scan frequently if this is a temporary disconnect// due to p2pif (mTemporarilyDisconnectWifi) {p2pSendMessage(WifiP2pServiceImpl.DISCONNECT_WIFI_RESPONSE);return;}/** clear the roaming state, if we were roaming, we failed */mIsAutoRoaming = false;mWifiConnectivityManager.handleConnectionStateChanged(WifiConnectivityManager.WIFI_STATE_DISCONNECTED);/*** If we have no networks saved, the supplicant stops doing the periodic scan.* The scans are useful to notify the user of the presence of an open network.* Note that these are not wake up scans.*/if (mNoNetworksPeriodicScan != 0 && !mP2pConnected.get()&& mWifiConfigManager.getSavedNetworks().size() == 0) {sendMessageDelayed(obtainMessage(CMD_NO_NETWORKS_PERIODIC_SCAN,++mPeriodicScanToken, 0), mNoNetworksPeriodicScan);}mDisconnectedTimeStamp = mClock.getWallClockMillis();mWifiStateTracker.updateState(WifiStateTracker.DISCONNECTED);}

mNoNetworksPeriodicScan = mContext.getResources().getInteger(R.integer.config_wifi_no_network_periodic_scan_interval);

/frameworks/base/core/res/res/values/config.xml
mNoNetworksPeriodicScan在config.xml中注册，周期为5分钟。

<!-- Integer indicating the framework no networks periodic scan interval in milliseconds. -->
<integer translatable="false" name="config_wifi_no_network_periodic_scan_interval">300000</integer>