[webnfc] Implement watch method for Android nfc mojo service.

device/nfc/android/java/src/org/chromium/device/nfc/NfcTagHandler.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.device.nfc;
 
 import android.nfc.FormatException;
 import android.nfc.NdefMessage;
 import android.nfc.Tag;
 import android.nfc.TagLostException;
 import android.nfc.tech.Ndef;
 import android.nfc.tech.NdefFormatable;
 import android.nfc.tech.TagTechnology;
 
 import java.io.IOException;
 
 /**
  * Utility class that provides I/O operations for NFC tags.
  */
-public final class NfcTagHandler {
+public class NfcTagHandler {
     private final TagTechnology mTech;
     private final TagTechnologyHandler mTechHandler;
     private boolean mWasConnected;
 
     /**
      * Factory method that creates NfcTagHandler for a given NFC Tag.
      *
      * @param tag @see android.nfc.Tag
      * @return NfcTagHandler or null when unsupported Tag is provided.
      */
@@ skipping 11 matching lines @@
         return null;
     }
 
     /**
      * NdefFormatable and Ndef interfaces have different signatures for operating with NFC tags.
      * This interface provides generic methods.
      */
     private interface TagTechnologyHandler {
         public void write(NdefMessage message)
                 throws IOException, TagLostException, FormatException;
-        // TODO(crbug.com/625589): add read method for nfc.watch.
+        public NdefMessage read() throws IOException, TagLostException, FormatException;
     }
 
     /**
      * Implementation of TagTechnologyHandler that uses Ndef tag technology.
      * @see android.nfc.tech.Ndef
      */
     private static class NdefHandler implements TagTechnologyHandler {
         private final Ndef mNdef;
 
         NdefHandler(Ndef ndef) {
             mNdef = ndef;
         }
 
         public void write(NdefMessage message)
                 throws IOException, TagLostException, FormatException {
             mNdef.writeNdefMessage(message);
         }
+
+        public NdefMessage read() throws IOException, TagLostException, FormatException {
+            return mNdef.getNdefMessage();
+        }
     }
 
     /**
      * Implementation of TagTechnologyHandler that uses NdefFormatable tag technology.
      * @see android.nfc.tech.NdefFormatable
      */
     private static class NdefFormattableHandler implements TagTechnologyHandler {
         private final NdefFormatable mNdefFormattable;
 
         NdefFormattableHandler(NdefFormatable ndefFormattable) {
             mNdefFormattable = ndefFormattable;
         }
 
         public void write(NdefMessage message)
                 throws IOException, TagLostException, FormatException {
             mNdefFormattable.format(message);
         }
+
+        public NdefMessage read() throws IOException, TagLostException, FormatException {
+            return NfcTypeConverter.emptyNdefMessage();
+        }
     }
 
-    private NfcTagHandler(TagTechnology tech, TagTechnologyHandler handler) {
+    protected NfcTagHandler(TagTechnology tech, TagTechnologyHandler handler) {
         mTech = tech;
         mTechHandler = handler;
     }
 
     /**
      * Connects to NFC tag.
      */
     public void connect() throws IOException, TagLostException {
         if (!mTech.isConnected()) {
             mTech.connect();
             mWasConnected = true;
         }
     }
 
     /**
+     * Checks if NFC tag is connected.
+     */
+    public boolean isConnected() {
+        return mTech.isConnected();
+    }
+
+    /**
      * Closes connection.
      */
     public void close() throws IOException {
         mTech.close();
     }
 
     /**
      * Writes NdefMessage to NFC tag.
      */
     public void write(NdefMessage message) throws IOException, TagLostException, FormatException {
         mTechHandler.write(message);
     }
 
+    public NdefMessage read() throws IOException, TagLostException, FormatException {
+        return mTechHandler.read();
+    }
+
     /**
      * If tag was previously connected and subsequent connection to the same tag fails, consider
      * tag to be out of range.
      */
     public boolean isTagOutOfRange() {
         try {
             connect();
         } catch (IOException e) {
             return mWasConnected;
         }
         return false;
     }
 }