android - 接收到 NFC 后保持当前 Activity

Question

我有一个应用程序似乎工作正常并且可以很好地通过 NFC 传输数据。我有一个主要 Activity 、一个传输数据的 Activity 和一个接收数据的不同 Activity 。

发送方 Activity 运行良好，但当接收方收到 NFC Intent 时，它会将应用重新启动回到主 Activity。

我不太确定这是为什么。我希望它拒绝任何推送，除非用户已经在该 Activity 中，如果用户已经在该 Activity 中，则留在该 Activity 中并处理 NFC Intent 。

这是 list :

<activity android:name=".MainActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>
<activity android:name=".Timer" />
<activity android:name=".AddSlaves"
          android:label="Add Slave Devices"
          android:launchMode="singleTask">
    <intent-filter>
        <action android:name="android.nfc.action.NDEF_DISCOVERED" />
        <category android:name="android.intent.category.DEFAULT" />
        <data android:mimeType="text/plain" />
    </intent-filter>
</activity>
<activity android:name=".JoinSrv"
          android:launchMode="singleTask">
    <intent-filter>
        <action android:name="android.nfc.action.NDEF_DISCOVERED" />
        <category android:name="android.intent.category.DEFAULT"/>
        <data android:mimeType="text/plain" />
    </intent-filter>
</activity>

这是发件人类:

public class JoinSrv extends Activity implements NfcAdapter.OnNdefPushCompleteCallback, NfcAdapter.CreateNdefMessageCallback {
    //The array lists to hold our messages
    private ArrayList<String> messagesToSendArray = new ArrayList<>();
    private ArrayList<String> messagesReceivedArray = new ArrayList<>();

    //Text boxes to add and display our messages
    private NfcAdapter mNfcAdapter;

    //Save our Array Lists of Messages for if the user navigates away
    @Override
    public void onSaveInstanceState(@NonNull Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putStringArrayList("messagesToSend", messagesToSendArray);
        savedInstanceState.putStringArrayList("lastMessagesReceived", messagesReceivedArray);
    }

    //Load our Array Lists of Messages for when the user navigates back
    @Override
    public void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        messagesToSendArray = savedInstanceState.getStringArrayList("messagesToSend");
        messagesReceivedArray = savedInstanceState.getStringArrayList("lastMessagesReceived");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_join_srv);


        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if (mNfcAdapter != null) {
            //Handle some NFC initialization here
        } else {
            Toast.makeText(this, "NFC not available on this device",
                    Toast.LENGTH_SHORT).show();
        }

        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if (mNfcAdapter != null) {
            //This will refer back to createNdefMessage for what it will send
            mNfcAdapter.setNdefPushMessageCallback(this, this);

            //This will be called if the message is sent successfully
            mNfcAdapter.setOnNdefPushCompleteCallback(this, this);
        }
    }

    @Override
    public NdefMessage createNdefMessage(NfcEvent event) {
        //This will be called when another NFC capable device is detected.
        //We'll write the createRecords() method in just a moment
        NdefRecord[] recordsToAttach = createRecords();
        //When creating an NdefMessage we need to provide an NdefRecord[]
        return new NdefMessage(recordsToAttach);
    }

    @Override
    public void onNdefPushComplete(NfcEvent event) {
        //This is called when the system detects that our NdefMessage was
        //Successfully sent.
        messagesToSendArray.clear();
    }

    public NdefRecord[] createRecords() {
        NdefRecord[] records = new NdefRecord[1];
        //To Create Messages Manually if API is less than
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {

            byte[] payload = "192.168.1.100".
                    getBytes(Charset.forName("UTF-8"));
            NdefRecord record = new NdefRecord(
                    NdefRecord.TNF_WELL_KNOWN,      //Our 3-bit Type name format
                    NdefRecord.RTD_TEXT,            //Description of our payload
                    new byte[0],                    //The optional id for our Record
                    payload);                       //Our payload for the Record

            records[1] = record;

        }
        //Api is high enough that we can use createMime, which is preferred.
        else {

                byte[] payload = "192.168.1.100".
                        getBytes(Charset.forName("UTF-8"));

                NdefRecord record = NdefRecord.createMime("text/plain",payload);
                records[1] = record;

        }
        records[messagesToSendArray.size()] =
                NdefRecord.createApplicationRecord(getPackageName());
        return records;
    }

    private void handleNfcIntent(Intent NfcIntent) {
        if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(NfcIntent.getAction())) {
            Parcelable[] receivedArray =
                    NfcIntent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);

            if (receivedArray != null) {
                messagesReceivedArray.clear();
                NdefMessage receivedMessage = (NdefMessage) receivedArray[0];
                NdefRecord[] attachedRecords = receivedMessage.getRecords();

                for (NdefRecord record : attachedRecords) {
                    String string = new String(record.getPayload());
                    //Make sure we don't pass along our AAR (Android Application Record)
                    if (string.equals(getPackageName())) {
                        continue;
                    }
                    messagesReceivedArray.add(string);
                }
                Toast.makeText(this, "Received " + messagesReceivedArray.size() +
                        " Messages", Toast.LENGTH_LONG).show();
            } else {
                Toast.makeText(this, "Received Blank Parcel", Toast.LENGTH_LONG).show();
            }
        }
    }

    @Override
    public void onNewIntent(Intent intent) {
        handleNfcIntent(intent);
    }

    @Override
    public void onResume() {
        super.onResume();
        handleNfcIntent(getIntent());
    }
}

这是接收类:

public class AddSlaves extends Activity implements NfcAdapter.OnNdefPushCompleteCallback, NfcAdapter.CreateNdefMessageCallback{
    //The array lists to hold our messages
    private ArrayList<String> messagesToSendArray = new ArrayList<>();
    private ArrayList<String> messagesReceivedArray = new ArrayList<>();

    //Text boxes to add and display our messages
    private EditText txtBoxAddMessage;
    private TextView txtReceivedMessages;
    private TextView txtMessagesToSend;
    private NfcAdapter mNfcAdapter;

    private  void updateTextViews() {
        txtReceivedMessages.setText("Messages Received:\n");
        //Populate our list of messages we have received
        if (messagesReceivedArray.size() > 0) {
            for (int i = 0; i < messagesReceivedArray.size(); i++) {
                txtReceivedMessages.append(messagesReceivedArray.get(i));
                txtReceivedMessages.append("\n");
            }
        }
    }

    //Save our Array Lists of Messages for if the user navigates away
    @Override
    public void onSaveInstanceState(@NonNull Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        savedInstanceState.putStringArrayList("lastMessagesReceived",messagesReceivedArray);
    }

    //Load our Array Lists of Messages for when the user navigates back
    @Override
    public void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        messagesReceivedArray = savedInstanceState.getStringArrayList("lastMessagesReceived");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_slaves);

        txtReceivedMessages = (TextView) findViewById(R.id.txtMessagesReceived);
        Button btnAddMessage = (Button) findViewById(R.id.buttonAddMessage);


        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if(mNfcAdapter != null) {
            //Handle some NFC initialization here
        }
        else {
            Toast.makeText(this, "NFC not available on this device",
                    Toast.LENGTH_SHORT).show();
        }

        //Check if NFC is available on device
        mNfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if(mNfcAdapter != null) {
            //This will refer back to createNdefMessage for what it will send
            mNfcAdapter.setNdefPushMessageCallback(this, this);

            //This will be called if the message is sent successfully
            mNfcAdapter.setOnNdefPushCompleteCallback(this, this);
        }
    }

    @Override
    public NdefMessage createNdefMessage(NfcEvent event) {
        //This will be called when another NFC capable device is detected.
        return null;

    }

    @Override
    public void onNdefPushComplete(NfcEvent event) {
        //This is called when the system detects that our NdefMessage was
        //Successfully sent.
        messagesToSendArray.clear();
    }


    private void handleNfcIntent(Intent NfcIntent) {
        if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(NfcIntent.getAction())) {
            Parcelable[] receivedArray =
                    NfcIntent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);

            if(receivedArray != null) {
                messagesReceivedArray.clear();
                NdefMessage receivedMessage = (NdefMessage) receivedArray[0];
                NdefRecord[] attachedRecords = receivedMessage.getRecords();

                for (NdefRecord record:attachedRecords) {
                    String string = new String(record.getPayload());
                    //Make sure we don't pass along our AAR (Android Application Record)
                    if (string.equals(getPackageName())) { continue; }
                    messagesReceivedArray.add(string);
                }
                Toast.makeText(this, "Received " + messagesReceivedArray.size() +
                        " Messages", Toast.LENGTH_LONG).show();
                updateTextViews();
            }
            else {
                Toast.makeText(this, "Received Blank Parcel", Toast.LENGTH_LONG).show();
            }
        }
    }


    @Override
    public void onNewIntent(Intent intent) {
        handleNfcIntent(intent);
    }

    @Override
    public void onResume() {
        super.onResume();
        updateTextViews();
        handleNfcIntent(getIntent());
    }
}

Answer 1

您的发件人 Activity 代码中存在一些问题:

1. 您存储了 messagesToSendArray，但实际上您从未用数据填充此数组列表(即 messagesToSendArray.size() 始终为 0)。由于每当调用 createNdefMessage() 时您都会重新创建 NDEF 消息，因此无需保存和恢复 messagesToSendArray。

2. 您写道，您希望在一个 Activity 中发送 NDEF 消息，但您希望在另一个 Activity 中接收 NFC 事件。但是，您注册了您的发送者 Activity 以在 list 中接收 NDEF_DISCOVERED 事件。如果您不想接收和处理这些事件，则无需在 list 中使用 NDEF_DISCOVERED Intent 过滤器。

3. 此外，无需在您的发送者 Activity 中处理 NDEF_DISCOVERED Intent (即您可以安全地删除方法 onNewIntent() 和 handleNfcIntent() ).

4. 在 Jelly Bean 以下的 Android 版本中，您创建了一个结构无效的 NFC 论坛文本记录。文本 RTD 需要一个以表单编码的有效负载(另请参阅 this post )

   +----------+---------------+--------------------------------------+
   | Status   | Language Code | Text                                 |
   | (1 byte) | (n bytes)     | (m bytes)                            |
   +----------+---------------+--------------------------------------+
   

   where Status equals to the length n of the Language Code if the Text is UTF-8 encoded and Language Code is an IANA language code (e.g. "en" for English). Consequently, the proper way to encode that record would be:

   public static NdefRecord createTextRecord(String language, String text) {
       byte[] languageBytes;
       byte[] textBytes;
       try {
           languageBytes = language.getBytes("US-ASCII");
           textBytes = text.getBytes("UTF-8");
       } catch (UnsupportedEncodingException e) {
           throw new AssertionError(e);
       }
   
       byte[] recordPayload = new byte[1 + (languageBytes.length & 0x03F) + textBytes.length];
   
       recordPayload[0] = (byte)(languageBytes.length & 0x03F);
       System.arraycopy(languageBytes, 0, recordPayload, 1, languageBytes.length & 0x03F);
       System.arraycopy(textBytes, 0, recordPayload, 1 + (languageBytes.length & 0x03F), textBytes.length);
   
       return new NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT, null, recordPayload);
   }
   

5. 我不清楚为什么在 Jelly Bean 及以上版本的 Android 版本上创建 NFC 论坛文本记录，而在 Jelly Bean 及更高版本上创建 MIME 类型记录。您应该保持一致并在所有平台上创建相同的记录类型(请参阅 Method NdefRecord.createTextRecord("en" , "string") not working below API level 21 ):

   String text = "192.168.1.100";
   String language = "en";
   
   NdefRecord record;
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
       record = NdefRecord.createTextRecord(language, text);
   } else {
       record = createTextRecord(language, text);
   }
   

6. 最后，在 createRecords() 中创建数组 records as

   NdefRecord[] records = new NdefRecord[1];
   

   因此，该数组在索引 0 处有一个可访问元素。但是，您稍后尝试访问元素 1:

   records[1] = record;
   

   这会导致 IndexOutOfBounds 异常。由于 createRecords() 由 Android 通过 createNdefMessage() 回调调用，回调失败(由于运行时异常)并且 Android 不会使用您的 NDEF 消息。相反，Android 将为您的应用使用默认的 NDEF 消息。此默认 NDEF 消息包含一个 Android 应用程序记录，它将导致您的主要 Activity 被调用(因为您的其他任何 Activity 都没有注册以针对默认 NDEF 消息的特定内容启动)；见NFC tag detection is not calling onNewIntent and it's Launching From Main Activity .因此，您需要将存储新创建的 NDEF 记录的 records 中的偏移量更改为 0:

   records[0] = record;
   

   此外，您需要删除该行

   records[messagesToSendArray.size()] =
       NdefRecord.createApplicationRecord(getPackageName());
   

   因为这会用 Android 应用程序记录覆盖先前存储在索引 0 处的 NDEF 记录(messagesToSendArray.size() 为 0)。同样，这将导致您的主要 Activity 开始，因为您没有在 list 中注册该特定记录类型。

最后，如果你想拒绝推送，除非用户在接收 Activity 中，你应该考虑使用前台调度系统。在这种情况下，您将从 list 中完全删除所有 NDEF_DISCOVERED Intent 过滤器，而是将每个 Activity (接收者、发送者、和 main)注册到前台调度系统。在接收器中，您将通过 onNewIntent() 接收 NDEF 消息。在发件人和主要 Activity 中，您只需忽略并丢弃任何收到的 NDEF 消息。参见 Android app enable NFC only for one Activity举个例子。