Android中NFC功能流程图解析及代码演示『轉』

在Android4.0推出的时候，一个非常引人注目的功能就是NFC(Near Field Communication).

Near Field Communication (NFC) is a set of short-range wireless technologies, typically requiring a distance of 4cm or less to initiate a connection. NFC allows you to share small payloads of data between an NFC tag and an Android-powered device, or between two Android-powered devices.

翻译：
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”，说明了基于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，相信这股风很快要刮到中国。

下面我们从技术的层面来分析一下这个技术。

官方的图片示例为：

这是NFC的开发流程，参考文章 “【NFC在android中的应用API】”。

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。
在代码层上面：

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的代码为：

public static String getStatusNfcDevice () {
    NfcAdapter nfcAdapter = NfcAdapter.getDefaultAdapter();
    if (nfcAdapter.isEnabled()) {
         String status = "enabled" ;
         return status;
    }
    else {
        String status = "disabled" ;
        return status;
    }
}

处理函数为

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摘自Google Android NFC Guide代码（略加注释）：

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;
    }
}

上面代码还需要在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，相关代码：

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>

参考文章：

http://developer.android.com/guide/topics/nfc/nfc.html

http://stackoverflow.com/questions/5078649/android-nfc-sample-demo-reads-only-fake-information-from-the-tag

http://developer.android.com/guide/topics/nfc/index.html

http://money.163.com/11/0714/09/78TPVJOL00253CVK.html

http://www.cnblogs.com/jfttcjl/archive/2011/11/09/2242045.html

http://www.jessechen.net/blog/how-to-nfc-on-the-android-platform/

另外有几个专门推动NFC技术的联盟：

http://open-nfc.org/wp/nfchal/

http://www.classic.nxp.com/theme/nfc/

http://www.nfc-forum.org/home/

官方关于NFC的翻译文档见：

http://blog.csdn.net/nicebooks/article/details/6223956

参考库：

http://code.google.com/p/guava-libraries/