原文地址：http://android.xsoftlab.net/training/connect-devices-wirelessly/wifi-direct.html#permissions
Wi-Fi peer-to-peer (P2P) APIs可以使程序与附近的设备进行直接通讯，Android的Wi-Fi P2P框架由Wi-Fi Direct™提供技术支持。WI-FI P2P技术可以使程序快速的检索附近的设备并与之建立连接。其覆盖范围超过蓝牙的覆盖范围。
这节课会学习如何通过WI-FI P2P技术搜索附近的设备并与之建立连接。
设置应用权限
如果要使用WI-FI P2P技术，需要在程序的清单文件中添加CHANGE\_WIFI\_STATE, ACCESS\_WIFI\_STATE, INTERNET三项权限。Wi-Fi P2P并不需要互联网连接，但是它需要使用标准的Java Socket通讯技术，所以需要使用INTERNET权限:
<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"/>
...设置广播接收器及P2P管理员
使用WI-FI P2P技术，需要监听广播意图，广播意图会通知程序某些事件的发生。所以在程序中需要添加IntentFilter，并设置其监听以下行为：
WIFI\_P2P\_STATE\_CHANGED\_ACTION
监听Wi-Fi P2P是否可用
WIFI\_P2P\_PEERS\_CHANGED\_ACTION
监听WI-FI P2P列表的变化
WIFI\_P2P\_CONNECTION\_CHANGED\_ACTION
监听Wi-Fi P2P的连接状态
WIFI\_P2P\_THIS\_DEVICE\_CHANGED\_ACTION
监听设备的配置变化
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()方法的末尾，需要获取WifiP2pManager的实例，然后调用它的initialize()方法。这个方法会返回一个WifiP2pManager.Channel的对象，它用于使程序应用层与Wi-Fi P2P框架建立连接。
@Override
Channel mChannel;
public void onCreate(Bundle savedInstanceState) {
....
mManager = (WifiP2pManager) getSystemService(Context.WIFI_P2P_SERVICE);
mChannel = mManager.initialize(this, getMainLooper(), null);
}接下来创建一个新的BroadcastReceiver类，它用于监听系统的Wi-Fi P2P的状态变化，在onReceive()方法中，需要添加一些基本的判断条件来处理每种P2P的状态并处理：
@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，将广播接收器与意图过滤器添加到上下文中，并需要在Activity暂停的时候注销这个广播接收器。放置这些代码的最佳位置就是onResume()方法与onPause()方法。
/** 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);
}初始化端点搜索
如果开始要使用Wi-Fi P2P来搜索附近的设备，需要调用discoverPeers()方法。这个方法要求传入以下参数：
- 在初始化P2P管理员时获得的WifiP2pManager.Channel对象。
- WifiP2pManager.ActionListener的实现，它用于监听搜索的成功与否。
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.
}
});要记住，这里只是初始化了端点搜索。discoverPeers()方法启动搜索进程后会立即返回。如果端点搜索进程成功初始化，那么系统会自动调用初始化时设置的回调方法。另外，端点搜索功能会一直保持在活动状态，直到连接初始化完成或者P2P组建立连接。
获取端点列表
接下来需要获得并处理端点列表。首先需要实现WifiP2pManager.PeerListListener接口，它提供了WI-FI P2P所搜索到的端点信息。下面的代码演示了这个过程：
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;
}
}
}
现在需要修改广播接收器的onReceive()方法，在收到WIFI\_P2P\_STATE\_CHANGED\_ACTION行为时调用requestPeers()方法。在这之前需要将监听器的实例传入到广播接收器中，常规的方式是在广播接收器的构造方法中将这个监听器传进来。、
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");
}...
}现在，WIFI\_P2P\_STATE\_CHANGED\_ACTION的行为将会触发端点列表的更新。
连接到端点
为了可以连接到端点，需要创建一个新的WifiP2pConfig对象，然后将WifiP2pDevice中的数据拷贝进这个对象中。WifiP2pDevice代表的将要连接的设备。然后调用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();
}
});
}上述代码中的WifiP2pManager.ActionListener接口只有在初始化成功或者失败的情况下才会调用。如果要监听连接状态的变化，需要实现WifiP2pManager.ConnectionInfoListener接口，它的方法onConnectionInfoAvailable()会在连接状态发生变化的时候回调。在多台设备连接一台设备的情况下(比如多人互动的游戏或者聊天类的APP)，其中一台设备会被指定为”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.
}
}现在回到广播接收器的onReceive()方法，修改监听WIFI\_P2P\_CONNECTION\_CHANGED\_ACTION的部分，当这个意图接收到时，调用requestConnectionInfo()方法。这是一个异步方法，所以结果会通过参数:连接信息监听器回调回来。
...
} 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);
}
...
