Android 2.3 NFC简介

Android 2.3加入了NFC(近场通讯)的支持。官网developer.android.com的英文介绍如下：
Near Field Communications (NFC)
Android 2.3 includes an NFC stack and framework API that lets developers read NDEF tags that are discovered as a user touches an NFC-enabled device to tag elements embedded in stickers, smart posters, and even other devices.
The platform provides the underlying NFC services that work with the device hardware to discover tags when they come into range. On discovering a tag, the platform notifies applications by broadcasting an Intent, appending the tag's NDEF messages to the Intent as extras. Applications can create Intent filters to recognize and handle targeted tags and messages. For example, after receiving a tag by Intent, applications extract the NDEF messages, store them, alert the user, or handle them in other ways.


The NFC API is available in the android.nfc package. The key classes are:
  • NfcAdapter, which represents the NFC hardware on the device.
  • NdefMessage, which represents an NDEF data message, the standard format in which "records" carrying data are transmitted between devices and tags. Applications can receive these messages from ACTION_TAG_DISCOVERED Intents.
  • NdefRecord, delivered in an NdefMessage, which describes the type of data being shared and carries the data itself.
NFC communication relies on wireless technology in the device hardware, so support for the platform's NFC features on specific devices is determined by their manufacturers. To determine the NFC support on the current device, applications can call isEnabled() to query the NfcAdapter. The NFC API is always present, however, regardless of underlying hardware support.


To use the NFC API, applications must request permission from the user by declaring <uses-permission android:name="android.permission.NFC"> in their manifest files.
Additionally, developers can request filtering on Android Market, such that their applications are not discoverable to users whose devices do not support NFC. To request filtering, add <uses-feature android:name="android.hardware.nfc" android:required="true"> to the application's manifest.


NFC的应用场景有很多，但Android 2.3目前API只提供了电子标签阅读器的功能。相信随着Android的版本升级，后续会增加很多应用场景和 API。
功能是少一点，但从无到有的第一步总是不容易的。从架构上看，至少有以下几部分的改动：
  • Android Framework层，为应用开发增加了相关API（即英文中的framework API）。
  • 协议层，增加NFC的协议栈（即英文中的NFC Stack）。
  • 硬件适配层，为硬件生产商提供适配接口，此层应该与NFC硬件的 Linux驱动通讯（实现方式待验证）。
工作流程是当支持NFC功能的手机或其他电子终端（后面简称手机）在用户开启NFC功能的时候，如果手机内置的NFC扫描器（相当于类NfcAdapter的功能）扫描到电子标签后，就会向相关用户程序发送ACTION_TAG_DISCOVERED的Intent，Itent的 extras架构中会包含NDEF（NFC Data Exchange Format）消息。如何处理此NDEF消息，就是用户程序的事情了。
NFC 的API在android.nfc的包中提供，这个包主要提供三个大类，其中：NfcAdapter描述的就是手机中的NFC硬件，Android 2.3中可以暂时理解为电子标签扫描器。电子标签和扫描器中的消息通过NdefMessage来表示，这个类很简单，只是封装了NdefRecord。每 个NdefMessage中可以包含多个NdefRecord，通过类NdefMessage的方法getRecords() 可以查询到消息的所有NdefRecord。NdefRecord才是信息的真正载体，正确理解这个类是理解NFC技术的一个重点。NFC的技术规范 [url=link.php?url=http://www.nfc-forum.org%2Fspecs%2Fspec_list%2F]http: //www.nfc-forum.org/specs/spec_list/[/url]是理解这个类不可或缺的资料。
应用程序的编程思路是：
  • 通过android.nfc.NfcAdapter.getDefaultAdapter() 取得手机的objNfcAdapter
  • 通过objNfcAdapter.isEnabled()查询该手机是否支持NFC
  • 如果手机支持NFC，就申请接收ACTION_TAG_DISCOVERED 的Intent
  • 如果接收到ACTION_TAG_DISCOVERED，就提取NdefMessage，并在此基础上进而提取 NdefRecord，整个是一个消息解析过程