我们先来看一下WifiDisplay的代码，先从UI看起。 WifiDisplay是在Wifi P2P的基础上发展而来的，他的功能实现也离不开P2P。所以在WifiDisplaySettings的onCreate中，就先初始化了WifiP2P。 /packages/apps/Settings/src/com/android/settings/wfd/WifiDisplaySettings.java

    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);

        final Context context = getActivity();
        mRouter = (MediaRouter) context.getSystemService(Context.MEDIA_ROUTER_SERVICE);
        mDisplayManager = (DisplayManager) context.getSystemService(Context.DISPLAY_SERVICE);
        mWifiP2pManager = (WifiP2pManager) context.getSystemService(Context.WIFI_P2P_SERVICE);
        mWifiP2pChannel = mWifiP2pManager.initialize(context, Looper.getMainLooper(), null);

        addPreferencesFromResource(R.xml.wifi_display_settings);
        setHasOptionsMenu(true);
    }

然后找到开启投屏的开关组件，check以后，修改数据库相关参数WIFI_DISPLAY_ON，从false置为true。

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case MENU_ID_ENABLE_WIFI_DISPLAY:
                mWifiDisplayOnSetting = !item.isChecked();
                item.setChecked(mWifiDisplayOnSetting);
                Settings.Global.putInt(getContentResolver(),
                        Settings.Global.WIFI_DISPLAY_ON, mWifiDisplayOnSetting ? 1 : 0);
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

WifiDisplayController中从数据库查询WIFI_DISPLAY_ON参数，如果为true，表示开启WifiDisplay，然后开始一系列操作。 /frameworks/base/services/core/java/com/android/server/display/WifiDisplayController.java

    private void updateSettings() {
        final ContentResolver resolver = mContext.getContentResolver();
        mWifiDisplayOnSetting = Settings.Global.getInt(resolver,
                Settings.Global.WIFI_DISPLAY_ON, 0) != 0;

首先是修改状态，然后修改wfdinfo的相关配置信息。 WifiP2PManager也要进行相关配置，主要是setWFDInfo。

    private void updateWfdEnableState() {
        if (mWifiDisplayOnSetting && mWifiP2pEnabled) {
            // WFD should be enabled.
            if (!mWfdEnabled && !mWfdEnabling) {
                mWfdEnabling = true;

                WifiP2pWfdInfo wfdInfo = new WifiP2pWfdInfo();
                wfdInfo.setWfdEnabled(true);
                wfdInfo.setDeviceType(WifiP2pWfdInfo.WFD_SOURCE);
                wfdInfo.setSessionAvailable(true);
                wfdInfo.setControlPort(DEFAULT_CONTROL_PORT);
                wfdInfo.setMaxThroughput(MAX_THROUGHPUT);
                mWifiP2pManager.setWFDInfo(mWifiP2pChannel, wfdInfo, new ActionListener() {
                    @Override
                    public void onSuccess() {
                        if (DEBUG) {
                            Slog.d(TAG, "Successfully set WFD info.");
                        }
                        if (mWfdEnabling) {
                            mWfdEnabling = false;
                            mWfdEnabled = true;
                            reportFeatureState();
                            updateScanState();
                        }
                    }

我们看setWFDInfo函数，主要是调用WifiP2pServiceImpl的checkConfigureWifiDisplayPermission方法。

    public void setWFDInfo(
            Channel c, WifiP2pWfdInfo wfdInfo,
            ActionListener listener) {
        checkChannel(c);
        try {
            mService.checkConfigureWifiDisplayPermission();
        } catch (RemoteException e) {
            e.rethrowFromSystemServer();
        }
        c.mAsyncChannel.sendMessage(SET_WFD_INFO, 0, c.putListener(listener), wfdInfo);
    }

这里是检测wfd有没有相关的权限。 /frameworks/opt/net/wifi/service/java/com/android/server/wifi/p2p/WifiP2pServiceImpl.java

    @Override
    public void checkConfigureWifiDisplayPermission() {
        if (!getWfdPermission(Binder.getCallingUid())) {
            throw new SecurityException("Wifi Display Permission denied for uid = "
                    + Binder.getCallingUid());
        }
    }

然后执行完以后就执行到updateScanState，就到扫描阶段了 /frameworks/base/services/core/java/com/android/server/display/WifiDisplayController.java

    private void updateScanState() {
        if (mScanRequested && mWfdEnabled && mDesiredDevice == null) {
            if (!mDiscoverPeersInProgress) {
                Slog.i(TAG, "Starting Wifi display scan.");
                mDiscoverPeersInProgress = true;
                handleScanStarted();
                tryDiscoverPeers();
            }
        } else {
            if (mDiscoverPeersInProgress) {
                // Cancel automatic retry right away.
                mHandler.removeCallbacks(mDiscoverPeers);

                // Defer actually stopping discovery if we have a connection attempt in progress.
                // The wifi display connection attempt often fails if we are not in discovery
                // mode.  So we allow discovery to continue until we give up trying to connect.
                if (mDesiredDevice == null || mDesiredDevice == mConnectedDevice) {
                    Slog.i(TAG, "Stopping Wifi display scan.");
                    mDiscoverPeersInProgress = false;
                    stopPeerDiscovery();
                    handleScanFinished();
                }
            }
        }
    }