Wi-Fi 點(diǎn)對(duì)點(diǎn)（P2P）API 允許應(yīng)用程序在無(wú)需連接到網(wǎng)絡(luò)和熱點(diǎn)的情況下連接到附近的設(shè)備。（Android Wi-Fi P2P 使用 Wi-Fi Direct? 驗(yàn)證程序進(jìn)行編譯）。Wi-Fi P2P 技術(shù)使得應(yīng)用程序可以快速發(fā)現(xiàn)附近的設(shè)備并與之交互。相比于藍(lán)牙技術(shù)，Wi-Fi P2P 的優(yōu)勢(shì)是具有較大的連接范圍。

本節(jié)主要內(nèi)容是使用 Wi-Fi P2P 技術(shù)發(fā)現(xiàn)并連接到附近的設(shè)備。

配置應(yīng)用權(quán)限

使用 Wi-Fi P2P 技術(shù)，需要添加 CHANGE_WIFI_STATE，ACCESS_WIFI_STATE 以及 INTERNET 三種權(quán)限到應(yīng)用的 manifest 文件。Wi-Fi P2P 技術(shù)雖然不需要訪問(wèn)互聯(lián)網(wǎng)，但是它會(huì)使用標(biāo)準(zhǔn)的 Java socket（需要 INTERNET 權(quán)限）。下面是使用 Wi-Fi P2P 技術(shù)需要申請(qǐng)的權(quán)限。

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.nsdchat"
    ...

    <uses-permission
        android:required="true"
        android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission
        android:required="true"
        android:name="android.permission.CHANGE_WIFI_STATE"/>
    <uses-permission
        android:required="true"
        android:name="android.permission.INTERNET"/>
    ...

設(shè)置廣播接收器（BroadCast Receiver）和 P2P 管理器

使用 Wi-Fi P2P 的時(shí)候，需要偵聽(tīng)當(dāng)某個(gè)事件出現(xiàn)時(shí)發(fā)出的broadcast intent。在應(yīng)用中，實(shí)例化一個(gè) IntentFilter，并將其設(shè)置為偵聽(tīng)下列事件：

WIFI_P2P_STATE_CHANGED_ACTION

　　指示　Wi-Fi P2P　是否開(kāi)啟

WIFI_P2P_PEERS_CHANGED_ACTION

　　代表對(duì)等節(jié)點(diǎn)（peer）列表發(fā)生了變化

WIFI_P2P_CONNECTION_CHANGED_ACTION

　　表明Wi-Fi P2P的連接狀態(tài)發(fā)生了改變

WIFI_P2P_THIS_DEVICE_CHANGED_ACTION

　　指示設(shè)備的詳細(xì)配置發(fā)生了變化

private final IntentFilter intentFilter = new IntentFilter();
...
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    //  Indicates a change in the Wi-Fi P2P status.
    intentFilter.addAction(WifiP2pManager.WIFI_P2P_STATE_CHANGED_ACTION);

    // Indicates a change in the list of available peers.
    intentFilter.addAction(WifiP2pManager.WIFI_P2P_PEERS_CHANGED_ACTION);

    // Indicates the state of Wi-Fi P2P connectivity has changed.
    intentFilter.addAction(WifiP2pManager.WIFI_P2P_CONNECTION_CHANGED_ACTION);

    // Indicates this device's details have changed.
    intentFilter.addAction(WifiP2pManager.WIFI_P2P_THIS_DEVICE_CHANGED_ACTION);

    ...
}

在　onCreate()　方法的最后，需要獲得　WifiP2pManager　的實(shí)例，并調(diào)用它的　initialize() 方法。該方法將返回 WifiP2pManager.Channel 對(duì)象。我們的應(yīng)用將在后面使用該對(duì)象連接 Wi-Fi P2P 框架。

@Override

Channel mChannel;

public void onCreate(Bundle savedInstanceState) {
    ....
    mManager = (WifiP2pManager) getSystemService(Context.WIFI_P2P_SERVICE);
    mChannel = mManager.initialize(this, getMainLooper(), null);
}

接下來(lái)，創(chuàng)建一個(gè)新的 BroadcastReceiver 類(lèi)偵聽(tīng)系統(tǒng)中 Wi-Fi P2P 狀態(tài)的變化。在 onReceive() 方法中，加入對(duì)上述四種不同 P2P 狀態(tài)變化的處理。

    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (WifiP2pManager.WIFI_P2P_STATE_CHANGED_ACTION.equals(action)) {
            // Determine if Wifi P2P mode is enabled or not, alert
            // the Activity.
            int state = intent.getIntExtra(WifiP2pManager.EXTRA_WIFI_STATE, -1);
            if (state == WifiP2pManager.WIFI_P2P_STATE_ENABLED) {
                activity.setIsWifiP2pEnabled(true);
            } else {
                activity.setIsWifiP2pEnabled(false);
            }
        } else if (WifiP2pManager.WIFI_P2P_PEERS_CHANGED_ACTION.equals(action)) {

            // The peer list has changed!  We should probably do something about
            // that.

        } else if (WifiP2pManager.WIFI_P2P_CONNECTION_CHANGED_ACTION.equals(action)) {

            // Connection state changed!  We should probably do something about
            // that.

        } else if (WifiP2pManager.WIFI_P2P_THIS_DEVICE_CHANGED_ACTION.equals(action)) {
            DeviceListFragment fragment = (DeviceListFragment) activity.getFragmentManager()
                    .findFragmentById(R.id.frag_list);
            fragment.updateThisDevice((WifiP2pDevice) intent.getParcelableExtra(
                    WifiP2pManager.EXTRA_WIFI_P2P_DEVICE));

        }
    }

最后，在主 activity 開(kāi)啟時(shí)，加入注冊(cè) intent filter 和 broadcast receiver 的代碼，并在 activity 暫?；蜿P(guān)閉時(shí)，注銷(xiāo)它們。上述做法最好放在 onResume() 和 onPause() 方法中。

    /** register the BroadcastReceiver with the intent values to be matched */
    @Override
    public void onResume() {
        super.onResume();
        receiver = new WiFiDirectBroadcastReceiver(mManager, mChannel, this);
        registerReceiver(receiver, intentFilter);
    }

    @Override
    public void onPause() {
        super.onPause();
        unregisterReceiver(receiver);
    }

初始化對(duì)等節(jié)點(diǎn)發(fā)現(xiàn)（Peer Discovery）

調(diào)用 discoverPeers() 開(kāi)始搜尋附近帶有 Wi-Fi P2P 的設(shè)備。該方法需要以下參數(shù)：

上節(jié)中調(diào)用 WifiP2pManager 的 initialize() 函數(shù)獲得的 WifiP2pManager.Channel 對(duì)象
一個(gè)對(duì) WifiP2pManager.ActionListener 接口的實(shí)現(xiàn)，包括了當(dāng)系統(tǒng)成功和失敗發(fā)現(xiàn)所調(diào)用的方法。

mManager.discoverPeers(mChannel, new WifiP2pManager.ActionListener() {

        @Override
        public void onSuccess() {
            // Code for when the discovery initiation is successful goes here.
            // No services have actually been discovered yet, so this method
            // can often be left blank.  Code for peer discovery goes in the
            // onReceive method, detailed below.
        }

        @Override
        public void onFailure(int reasonCode) {
            // Code for when the discovery initiation fails goes here.
            // Alert the user that something went wrong.
        }
});

需要注意的是，這僅僅表示對(duì)Peer發(fā)現(xiàn)（Peer Discovery）完成初始化。discoverPeers() 方法開(kāi)啟了發(fā)現(xiàn)過(guò)程并且立即返回。系統(tǒng)會(huì)通過(guò)調(diào)用 WifiP2pManager.ActionListener 中的方法通知應(yīng)用對(duì)等節(jié)點(diǎn)發(fā)現(xiàn)過(guò)程初始化是否正確。同時(shí)，對(duì)等節(jié)點(diǎn)發(fā)現(xiàn)過(guò)程本身仍然繼續(xù)運(yùn)行，直到一條連接或者一個(gè) P2P 小組建立。

獲取對(duì)等節(jié)點(diǎn)列表

在完成對(duì)等節(jié)點(diǎn)發(fā)現(xiàn)過(guò)程的初始化后，我們需要進(jìn)一步獲取附近的對(duì)等節(jié)點(diǎn)列表。第一步是實(shí)現(xiàn) WifiP2pManager.PeerListListener 接口。該接口提供了 Wi-Fi P2P 框架發(fā)現(xiàn)的對(duì)等節(jié)點(diǎn)信息。下列代碼實(shí)現(xiàn)了相應(yīng)功能：

 private List peers = new ArrayList();
    ...

    private PeerListListener peerListListener = new PeerListListener() {
        @Override
        public void onPeersAvailable(WifiP2pDeviceList peerList) {

            // Out with the old, in with the new.
            peers.clear();
            peers.addAll(peerList.getDeviceList());

            // If an AdapterView is backed by this data, notify it
            // of the change.  For instance, if you have a ListView of available
            // peers, trigger an update.
            ((WiFiPeerListAdapter) getListAdapter()).notifyDataSetChanged();
            if (peers.size() == 0) {
                Log.d(WiFiDirectActivity.TAG, "No devices found");
                return;
            }
        }
    }

接下來(lái)，完善 Broadcast Receiver 的 onReceiver() 方法。當(dāng)收到 WIFI_P2P_PEERS_CHANGED_ACTION 事件時(shí)，調(diào)用 requestPeer() 方法獲取對(duì)等節(jié)點(diǎn)列表。我們需要將 WifiP2pManager.PeerListListener 傳遞給 receiver。一種方法是在 broadcast receiver 的構(gòu)造函數(shù)中，將對(duì)象作為參數(shù)傳入。

public void onReceive(Context context, Intent intent) {
    ...
    else if (WifiP2pManager.WIFI_P2P_PEERS_CHANGED_ACTION.equals(action)) {

        // Request available peers from the wifi p2p manager. This is an
        // asynchronous call and the calling activity is notified with a
        // callback on PeerListListener.onPeersAvailable()
        if (mManager != null) {
            mManager.requestPeers(mChannel, peerListListener);
        }
        Log.d(WiFiDirectActivity.TAG, "P2P peers changed");
    }...
}

現(xiàn)在，一個(gè)帶有 WIFI_P2P_PEERS_CHANGED_ACTION action 的 intent 將觸發(fā)應(yīng)用對(duì) Peer 列表的更新。

連接一個(gè)對(duì)等節(jié)點(diǎn)

為了連接到一個(gè)對(duì)等節(jié)點(diǎn)，我們需要?jiǎng)?chuàng)建一個(gè)新的 WifiP2pConfig 對(duì)象，并將要連接的設(shè)備信息從表示我們想要連接設(shè)備的 WifiP2pDevice 拷貝到其中。然后調(diào)用 connect() 方法。

    @Override
    public void connect() {
        // Picking the first device found on the network.
        WifiP2pDevice device = peers.get(0);

        WifiP2pConfig config = new WifiP2pConfig();
        config.deviceAddress = device.deviceAddress;
        config.wps.setup = WpsInfo.PBC;

        mManager.connect(mChannel, config, new ActionListener() {

            @Override
            public void onSuccess() {
                // WiFiDirectBroadcastReceiver will notify us. Ignore for now.
            }

            @Override
            public void onFailure(int reason) {
                Toast.makeText(WiFiDirectActivity.this, "Connect failed. Retry.",
                        Toast.LENGTH_SHORT).show();
            }
        });
    }

在本段代碼中的 WifiP2pManager.ActionListener 實(shí)現(xiàn)僅能通知我們初始化的成功或失敗。想要監(jiān)聽(tīng)連接狀態(tài)的變化，需要實(shí)現(xiàn) WifiP2pManager.ConnectionInfoListener 接口。接口中的 onConnectionInfoAvailable() 回調(diào)函數(shù)會(huì)在連接狀態(tài)發(fā)生改變時(shí)通知應(yīng)用程序。當(dāng)有多個(gè)設(shè)備同時(shí)試圖連接到一臺(tái)設(shè)備時(shí)（例如多人游戲或者聊天群），這一臺(tái)設(shè)備將被指定為“群主”（group owner）。

    @Override
    public void onConnectionInfoAvailable(final WifiP2pInfo info) {

        // InetAddress from WifiP2pInfo struct.
        InetAddress groupOwnerAddress = info.groupOwnerAddress.getHostAddress());

        // After the group negotiation, we can determine the group owner.
        if (info.groupFormed && info.isGroupOwner) {
            // Do whatever tasks are specific to the group owner.
            // One common case is creating a server thread and accepting
            // incoming connections.
        } else if (info.groupFormed) {
            // The other device acts as the client. In this case,
            // you'll want to create a client thread that connects to the group
            // owner.
        }
    }

此時(shí)，回頭繼續(xù)完善 broadcast receiver 的 onReceive() 方法，并修改對(duì) [WIFI_P2P_CONNECTION_CHANGED_ACTION]() intent 的監(jiān)聽(tīng)部分的代碼。當(dāng)接收到該 intent 時(shí)，調(diào)用 requestConnectionInfo() 方法。此方法為異步，所以結(jié)果將會(huì)被我們提供的 WifiP2pManager.ConnectionInfoListener 所獲取。

        ...
        } else if (WifiP2pManager.WIFI_P2P_CONNECTION_CHANGED_ACTION.equals(action)) {

            if (mManager == null) {
                return;
            }

            NetworkInfo networkInfo = (NetworkInfo) intent
                    .getParcelableExtra(WifiP2pManager.EXTRA_NETWORK_INFO);

            if (networkInfo.isConnected()) {

                // We are connected with the other device, request connection
                // info to find group owner IP

                mManager.requestConnectionInfo(mChannel, connectionListener);
            }
            ...

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