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近场通讯(NFC)是一系列短距离无线技术，一般需要4cm或者更短去初始化连接。近场通讯(NFC)允许你在NFC tag和Android设备或者两个Android设备间共享小负载数据。
典型的应用为刷卡应用，如刷信用卡，公交车卡，吃饭的饭卡之类。腾讯2011年1月份文章“Android首款NFC近场通信应用推出”，基于Android的NFC应用目前已经有了，得益于日本在手机刷卡的应用氛围。据2011年7月网易文章“PayPal推出Android系统NFC移动支付服务”报道，PayPal已经做了尝试了。
下面我们从技术的层面来分析一下这个技术。
相关的类代码有：NfcAdapter,NdefMessage, NdefRecord,ACTION_TAG_DISCOVERED.
在功能层面上，涉及到了NFC的读写功能。下面我们分别来做总结。
使用的时候，需要在AndroidManifest.xml里面加一些权限以及属性。
<uses-permission android:name="android.permission.NFC" />
<uses-feature android:name="android.hardware.nfc" android:required="true" />
<uses-sdk android:minSdkVersion="10"/>这里注意，在Android Version 9的时候仅仅支持了ACTION_TAG_DISCOVERED，对于其他的需要10以上。
在上面的基础上，还需要增加intent-filter的支持。
<intent-filter>
<action android:name="android.nfc.action.NDEF_DISCOVERED"/>
<category android:name="android.intent.category.DEFAULT"/>
<data android:mimeType="text/plain" />
</intent-filter>获取NfcAdapter的代码示例：
public static String getStatusNfcDevice () {
NfcAdapter nfcAdapter = NfcAdapter.getDefaultAdapter();
if (nfcAdapter.isEnabled()) {
String status = "enabled";
return status;
}
else {
String status = "disabled";
return status;
}
}处理函数代码：
public void onResume() {
super.onResume();
if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(getIntent().getAction())) {
Parcelable[] rawMsgs = intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);
if (rawMsgs != null) {
msgs = new NdefMessage[rawMsgs.length];
for (int i = 0; i < rawMsgs.length; i++) {
msgs[i] = (NdefMessage) rawMsgs[i];
}
}
}
//process the msgs array
}完整的一个操作代码来自于Google Android NFC Guide代码（有注释）：
package com.example.android.beam;
import android.app.Activity;
import android.content.Intent;
import android.nfc.NdefMessage;
import android.nfc.NdefRecord;
import android.nfc.NfcAdapter;
import android.nfc.NfcAdapter.CreateNdefMessageCallback;
import android.nfc.NfcEvent;
import android.os.Bundle;
import android.os.Parcelable;
import android.widget.TextView;
import android.widget.Toast;
import java.nio.charset.Charset;
//继承并实现接口CreateNdefMessageCallback方法createNdefMessage
public class Beam extends Activity implements CreateNdefMessageCallback {
NfcAdapter mNfcAdapter;
TextView textView;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
TextView textView = (TextView) findViewById(R.id.textView);
// Check for available NFC Adapter
//检测是否有NFC适配器
mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
if (mNfcAdapter == null) {
Toast.makeText(this, "NFC is not available", Toast.LENGTH_LONG).show();
finish();
return;
}
// Register callback
//注册回调函数
mNfcAdapter.setNdefPushMessageCallback(this, this);
}
@Override
public NdefMessage createNdefMessage(NfcEvent event) {
String text = ("Beam me up, Android!\n\n" +
"Beam Time: " + System.currentTimeMillis());
//回调函数，构造NdefMessage格式
NdefMessage msg = new NdefMessage(
new NdefRecord[] { createMimeRecord(
"application/com.example.android.beam", text.getBytes())
/**
* The Android Application Record (AAR) is commented out. When a device
* receives a push with an AAR in it, the application specified in the AAR
* is guaranteed to run. The AAR overrides the tag dispatch system.
* You can add it back in to guarantee that this
* activity starts when receiving a beamed message. For now, this code
* uses the tag dispatch system.
*/
//,NdefRecord.createApplicationRecord("com.example.android.beam")
});
return msg;
}
@Override
public void onResume() {
super.onResume();
// Check to see that the Activity started due to an Android Beam
//得到是否检测到ACTION_NDEF_DISCOVERED触发
if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(getIntent().getAction())) {
processIntent(getIntent());
}
}
//重载Activity类方法处理当新Intent到来事件
@Override
public void onNewIntent(Intent intent) {
// onResume gets called after this to handle the intent
setIntent(intent);
}
/**
* Parses the NDEF Message from the intent and prints to the TextView
*/
//关键处理函数，处理扫描到的NdefMessage
void processIntent(Intent intent) {
textView = (TextView) findViewById(R.id.textView);
Parcelable[] rawMsgs = intent.getParcelableArrayExtra(
NfcAdapter.EXTRA_NDEF_MESSAGES);
// only one message sent during the beam
NdefMessage msg = (NdefMessage) rawMsgs[0];
// record 0 contains the MIME type, record 1 is the AAR, if present
textView.setText(new String(msg.getRecords()[0].getPayload()));
}
/**
* Creates a custom MIME type encapsulated in an NDEF record
*/
public NdefRecord createMimeRecord(String mimeType, byte[] payload) {
byte[] mimeBytes = mimeType.getBytes(Charset.forName("US-ASCII"));
NdefRecord mimeRecord = new NdefRecord(
NdefRecord.TNF_MIME_MEDIA, mimeBytes, new byte[0], payload);
return mimeRecord;
}
}以上代码还需要在AndroidManifest.xml文件里面添加：
<intent-filter>
<action android:name="android.nfc.action.NDEF_DISCOVERED"/>
<category android:name="android.intent.category.DEFAULT"/>
<data android:mimeType="application/com.example.android.beam"/>
</intent-filter>在对NFC设备进行写操作的时候，代码：
private void enableTagWriteMode() {
mWriteMode = true;
IntentFilter tagDetected = new IntentFilter(NfcAdapter.ACTION_TAG_DISCOVERED);
mWriteTagFilters = new IntentFilter[] { tagDetected };
mNfcAdapter.enableForegroundDispatch(this, mNfcPendingIntent, mWriteTagFilters, null);
}
@Override
protected void onNewIntent(Intent intent) {
// Tag writing mode
if (mWriteMode && NfcAdapter.ACTION_TAG_DISCOVERED.equals(intent.getAction())) {
Tag detectedTag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
if (NfcUtils.writeTag(NfcUtils.getPlaceidAsNdef(placeidToWrite), detectedTag)) {
Toast.makeText(this, "Success: Wrote placeid to nfc tag", Toast.LENGTH_LONG)
.show();
NfcUtils.soundNotify(this);
} else {
Toast.makeText(this, "Write failed", Toast.LENGTH_LONG).show();
}
}
}
/*
* Writes an NdefMessage to a NFC tag
*/
public static boolean writeTag(NdefMessage message, Tag tag) {
int size = message.toByteArray().length;
try {
Ndef ndef = Ndef.get(tag);
if (ndef != null) {
ndef.connect();
if (!ndef.isWritable()) {
return false;
}
if (ndef.getMaxSize() < size) {
return false;
}
ndef.writeNdefMessage(message);
return true;
} else {
NdefFormatable format = NdefFormatable.get(tag);
if (format != null) {
try {
format.connect();
format.format(message);
return true;
} catch (IOException e) {
return false;
}
} else {
return false;
}
}
} catch (Exception e) {
return false;
}
}相关的AndroidManifest.xml文件配置如下：
<activity android:name=".CheckInActivity">
<intent-filter>
<action android:name="android.nfc.action.NDEF_DISCOVERED"/>
<data android:mimeType="application/vnd.facebook.places"/>
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</activity>有不足之处敬请谅解或者留言指出，谢谢。