Fix NDEF documentation to indicate the message may be null.

The current NFC stack formats tags to the INITIALIZED state
as defined by NFC forum; in that state the tag has the
NDEF Capability Container, but does not contain any message
yet.

Tags in that state (correctly) return the NDEF technology,
but the documentation does not specify that the message
may be null.

Also, get rid of buggy getLastErrorCode and use
(cached) presence check value to determine if tag was
lost during read.

Change-Id: If4293428093024ba9cda5dd7c9979b8b06353234

Author: Martijn Coenen

core/java/android/nfc/INfcTag.aidl

@@ -34,8 +34,6 @@ interface INfcTag
     boolean isPresent(int nativeHandle);
     TransceiveResult transceive(int nativeHandle, in byte[] data, boolean raw);
 
-    int getLastError(int nativeHandle);
-
     NdefMessage ndefRead(int nativeHandle);
     int ndefWrite(int nativeHandle, in NdefMessage msg);
     int ndefMakeReadOnly(int nativeHandle);

core/java/android/nfc/tech/Ndef.java

@@ -176,8 +176,11 @@ public Ndef(Tag tag) throws RemoteException {
      * <p>If the NDEF Message is modified by an I/O operation then it
      * will not be updated here, this function only returns what was discovered
      * when the tag entered the field.
+     * <p>Note that this method may return null if the tag was in the
+     * INITIALIZED state as defined by NFC Forum, as in this state the
+     * tag is formatted to support NDEF but does not contain a message yet.
      * <p>Does not cause any RF activity and does not block.
-     * @return NDEF Message read from the tag at discovery time
+     * @return NDEF Message read from the tag at discovery time, can be null
      */
     public NdefMessage getCachedNdefMessage() {
         return mNdefMsg;
@@ -245,11 +248,15 @@ public boolean isWritable() {
      *
      * <p>This always reads the current NDEF Message stored on the tag.
      *
+     * <p>Note that this method may return null if the tag was in the
+     * INITIALIZED state as defined by NFC Forum, as in that state the
+     * tag is formatted to support NDEF but does not contain a message yet.
+     *
      * <p>This is an I/O operation and will block until complete. It must
      * not be called from the main application thread. A blocked call will be canceled with
      * {@link IOException} if {@link #close} is called from another thread.
      *
-     * @return the NDEF Message, never null
+     * @return the NDEF Message, can be null
      * @throws TagLostException if the tag leaves the field
      * @throws IOException if there is an I/O failure, or the operation is canceled
      * @throws FormatException if the NDEF Message on the tag is malformed
@@ -265,17 +272,8 @@ public NdefMessage getNdefMessage() throws IOException, FormatException {
             int serviceHandle = mTag.getServiceHandle();
             if (tagService.isNdef(serviceHandle)) {
                 NdefMessage msg = tagService.ndefRead(serviceHandle);
-                if (msg == null) {
-                    int errorCode = tagService.getLastError(serviceHandle);
-                    switch (errorCode) {
-                        case ErrorCodes.ERROR_IO:
-                            throw new IOException();
-                        case ErrorCodes.ERROR_INVALID_PARAM:
-                            throw new FormatException();
-                        default:
-                            // Should not happen
-                            throw new IOException();
-                    }
+                if (msg == null && !tagService.isPresent(serviceHandle)) {
+                    throw new TagLostException();
                 }
                 return msg;
             } else {