使用WiFi建立P2P连接
最后更新于：2022-04-01 01:39:56
> 编写:[naizhengtan](https://github.com/naizhengtan) - 原文:[http://developer.android.com/training/connect-devices-wirelessly/wifi-direct.html](http://developer.android.com/training/connect-devices-wirelessly/wifi-direct.html)
Android提供的Wi-Fi点对点（P2P）APIs允许应用程序无需连接到网络和热点的情况下连接到附近的设备。（Android Wi-Fi P2P框架遵循[Wi-Fi Direct™](http://www.wi-fi.org/discover-and-learn/wi-fi-direct) 验证程序）Wi-Fi P2P技术使得应用程序可以快速发现附近的设备并与之交互。相比于蓝牙技术，Wi-Fi P2P的优势是具有较大的连接范围。
本节主要内容是使用Wi-Fi P2P技术发现并连接到附近的设备。
### 配置应用权限
使用Wi-Fi P2P技术，需要添加[CHANGE_WIFI_STATE](http://developer.android.com/reference/android/Manifest.permission.html#CHANGE_WIFI_STATE),[ACCESS_WIFI_STATE](http://developer.android.com/reference/android/Manifest.permission.html#ACCESS_WIFI_STATE)以及[INTERNET](http://developer.android.com/reference/android/Manifest.permission.html#INTERNET)三种权限到应用的manifest文件。Wi-Fi P2P技术虽然不需要访问互联网，但是它会使用Java中的标准socket。而使用socket需要具有INTERNET权限，这也是Wi-Fi P2P技术需要申请该权限的原因。
~~~
<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"/>
...
~~~
### 广播接收器(BroadCast Receiver)和点对点管理器(Peer-to-peer Manager)
使用Wi-Fi P2P的时候需要侦听相关的广播事件（broadcast intent）。所以在应用中需要实例化一个[IntentFilter](http://developer.android.com/reference/android/content/IntentFilter.html)，并将其设置为侦听下列事件：
-
[WIFI_P2P_STATE_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_STATE_CHANGED_ACTION)
指示Wi-Fi P2P是否开启
-
[WIFI_P2P_PEERS_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_PEERS_CHANGED_ACTION)
代表对等节点（peer）列表发生了变化
-
[WIFI_P2P_CONNECTION_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_CONNECTION_CHANGED_ACTION)
表明Wi-Fi P2P的连接状态发生了改变
-
[WIFI_P2P_THIS_DEVICE_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_THIS_DEVICE_CHANGED_ACTION)
指示设备的详细配置发生了变化
~~~
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()](http://developer.android.com/reference/android/app/Activity.html#onCreate(android.os.Bundle))方法的最后，需要获得[WifiPpManager](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html)的实例，并调用它的[initialize()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#initialize(android.content.Context, android.os.Looper, android.net.wifi.p2p.WifiP2pManager.ChannelListener))方法。该方法将返回[WifiP2pManager.Channel](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.Channel.html)对象。你的应用将使用该对象与Wi-Fi P2P框架进行交互。
~~~
@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](http://developer.android.com/reference/android/content/BroadcastReceiver.html)类侦听系统中Wi-Fi P2P的状态变化。在[onReceive()](http://developer.android.com/reference/android/content/BroadcastReceiver.html#onReceive(android.content.Context, android.content.Intent))方法中，加入对上述四种不同P2P状态变化的处理。
~~~
@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，在主界面开启时，加入注册intent filter和broadcast receiver的代码，并在暂停或关闭时，注销它们。最好的位置是在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);
}
~~~
### 初始化对等节点发现（Peer Discovery）过程
在Wi-Fi P2P中，应用通过调用[discoverPeers()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#discoverPeers(android.net.wifi.p2p.WifiP2pManager.Channel, android.net.wifi.p2p.WifiP2pManager.ActionListener))搜寻附近的设备。该方法需要以下参数：
- 上节中调用WifiP2pManager的initialize()函数获得的[WifiP2pManager.Channel](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.Channel.html)对象
- 一个对[WifiP2pManager.ActionListener](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.ActionListener.html)接口的实现，包括了当系统成功/失败发现所调用的方法
~~~
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，在此的成功仅仅表示对Peer发现（Peer Discovery）的过程完成初始化。方法discoverPeers()开启了发现过程并且立即返回。系统会通过调用WifiP2pManager.ActionListener中的方法通知应用对等节点发现过程初始化是否正确。同时，对等节点发现过程本身仍然继续运行，直到一条连接或者一个P2P小组建立。
### 获取对等节点列表
在完成对等节点发现过程的初始化后，我们需要进一步获取附近的对等节点列表。第一步是实现[WifiP2pManager.PeerListListener](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.PeerListListener.html)接口。该接口提供了Wi-Fi P2P框架发现的对等节点信息。下列代码实现了相应功能：
~~~
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;
}
}
}
~~~
接下来，完善上文广播接收者（Broadcast Receiver）的onReceiver()方法。当收到[WIFI_P2P_PEERS_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_PEERS_CHANGED_ACTION)事件时，调用[requestPeer()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#requestPeers(android.net.wifi.p2p.WifiP2pManager.Channel, android.net.wifi.p2p.WifiP2pManager.PeerListListener))方法获取对等节点列表。在此，需要将WifiP2pManager.PeerListListener对象传递给该方法。一种方法是在广播接收者构造时，就将对象作为参数传入。
~~~
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");
}...
}
~~~
现在，一个[WIFI_P2P_PEERS_CHANGED_ACTION](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_PEERS_CHANGED_ACTION)事件将触发应用对Peer列表的更新了。
### 连接一个对等节点
为了连接到一个对等节点，你需要创一个新的[WifiP2pConfig](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pConfig.html)对象，并将要连接的设备信息从[WifiP2pDevice](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pDevice.html)拷贝到其中，最后调用[connect()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#connect(android.net.wifi.p2p.WifiP2pManager.Channel, android.net.wifi.p2p.WifiP2pConfig, android.net.wifi.p2p.WifiP2pManager.ActionListener))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();
}
});
}
~~~
在本段代码中的[WifiP2pManager.ActionListener](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.ActionListener.html)仅能通知你初始化的成功或失败。想要侦听连接状态的变化，需要实现[WifiP2pManager.ConnectionInfoListener](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.ConnectionInfoListener.html)接口。接口中的[onConnectionInfoAvailable()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.ConnectionInfoListener.html#onConnectionInfoAvailable(android.net.wifi.p2p.WifiP2pInfo))回调函数会在连接状态发生改变时通知应用程序。当有多个设备同时试图连接到一台设备时（例如多人游戏或者聊天群），这一台设备将被指定为“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.
}
}
~~~
此时，回头继续完善广播接收者的onReceive()方法，并修改对[WIFI_P2P_CONNECTION_CHANGED_ACTION]()intent的监听部分的代码。当该intent接收到后，调用[requestConnectionInfo()](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#WIFI_P2P_CONNECTION_CHANGED_ACTION)方法。此方法为异步，所以结果将会被你提供的[WifiP2pManager.ConnectionInfoListener](http://developer.android.com/reference/android/net/wifi/p2p/WifiP2pManager.html#requestConnectionInfo(android.net.wifi.p2p.WifiP2pManager.Channel, android.net.wifi.p2p.WifiP2pManager.ConnectionInfoListener))所获取。
~~~
...
} 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);
}
...
~~~