Add setUnknownTagVisitor() method in all decoders

artio-codecs/src/main/java/uk/co/real_logic/artio/builder/CommonDecoderImpl.java

@@ -31,6 +31,7 @@ public abstract class CommonDecoderImpl
     protected int invalidTagId = Decoder.NO_ERROR;
     protected int rejectReason = Decoder.NO_ERROR;
     protected AsciiBuffer buffer;
+    protected UnknownTagVisitor unknownTagVisitor;
 
     public int invalidTagId()
     {
@@ -42,6 +43,11 @@ public int rejectReason()
         return rejectReason;
     }
 
+    public void setUnknownTagVisitor(final UnknownTagVisitor unknownTagVisitor)
+    {
+        this.unknownTagVisitor = unknownTagVisitor;
+    }
+
     public int getInt(
         final AsciiBuffer buffer,
         final int startInclusive, final int endExclusive, final int tag, final boolean validation)

artio-codecs/src/main/java/uk/co/real_logic/artio/builder/Decoder.java

@@ -91,4 +91,11 @@ public interface Decoder extends CharAppender
      * @return the encoder passed as an argument.
      */
     Encoder toEncoder(Encoder encoder);
+
+    /**
+     * Sets a visitor to be called when an unknown tag is encountered while decoding a message.
+     *
+     * @param visitor the visitor to invoke
+     */
+    void setUnknownTagVisitor(UnknownTagVisitor visitor);
 }

artio-codecs/src/main/java/uk/co/real_logic/artio/builder/UnknownTagVisitor.java

@@ -0,0 +1,26 @@
+package uk.co.real_logic.artio.builder;
+
+import uk.co.real_logic.artio.util.AsciiBuffer;
+
+/**
+ * Interface for visiting unknown tags.
+ *
+ * @see CommonDecoderImpl#setUnknownTagVisitor(UnknownTagVisitor)
+ */
+public interface UnknownTagVisitor
+{
+    /**
+     * Called when an unknown tag is encountered while decoding a message.
+     *
+     * @param tag    The tag number of the unknown tag
+     * @param buffer The buffer containing the unknown tag
+     * @param offset The offset at which the unknown tag starts
+     * @param length The length of the unknown tag
+     */
+    void onUnknownTag(
+        int tag,
+        AsciiBuffer buffer,
+        int offset,
+        int length
+    );
+}

artio-codecs/src/main/java/uk/co/real_logic/artio/dictionary/generation/DecoderGenerator.java

@@ -1705,7 +1705,10 @@ private String decodeMethod(final List<Entry> entries, final Aggregate aggregate
             "                        unknownFields.add(tag);\n" +
             "                    }\n" +
             "                }\n") +
-
+            "                if (unknownTagVisitor != null)\n" +
+            "                {\n" +
+            "                    unknownTagVisitor.onUnknownTag(tag, buffer, valueOffset, valueLength);\n" +
+            "                }\n" +
             // Skip the thing if it's a completely unknown field and you aren't validating messages
             "                if (" + CODEC_REJECT_UNKNOWN_FIELD_ENABLED +
             " || " + unknownFieldPredicate(type) + ")\n" +

artio-codecs/src/test/java/uk/co/real_logic/artio/dictionary/ExampleDictionary.java

@@ -244,6 +244,10 @@ public final class ExampleDictionary
         "8=FIX.4.4\0019=53\00135=0\001115=abc\001116=2\001117=A\001127=19700101-00:00:00.001" +
         "\00110=043\001";
 
+    public static final String UNKNOWN_TAG_MESSAGE =
+        "8=FIX.4.4\0019=53\00135=0\001115=abc\00110100=FOO\001116=2\001117=A\00110101=BAR\001127=19700101-00:00:00.001" +
+        "\00110=043\001";
+
     public static final String OUT_OF_RANGE_FLOAT_VALUE_MESSAGE =
         "8=FIX.4.4\0019=53\00135=0\001115=abc\001116=2\001117=10000000000000000000000\001" +
         "127=19700101-00:00:00.001\00110=043\001";

artio-codecs/src/test/java/uk/co/real_logic/artio/dictionary/generation/DecoderGeneratorFlyweightTest.java

@@ -19,6 +19,10 @@
 import org.junit.BeforeClass;
 import org.junit.Test;
 import uk.co.real_logic.artio.builder.Decoder;
+import uk.co.real_logic.artio.builder.UnknownTagVisitor;
+import uk.co.real_logic.artio.util.AsciiBuffer;
+
+import java.util.HashMap;
 
 import static org.junit.Assert.assertEquals;
 import static uk.co.real_logic.artio.dictionary.ExampleDictionary.*;
@@ -65,4 +69,15 @@ public void shouldNotThrowWhenAccessingUnsetString() throws Exception
         assertTargetThrows(() -> getAsciiSequenceView(decoder, "testReqID"), IllegalArgumentException.class,
             "No value for optional field: TestReqID");
     }
+
+    @Test
+    public void shouldVisitUnknownTags() throws Exception
+    {
+        final Decoder decoder = newHeartbeat();
+        final HashMap<Integer, String> map = new HashMap<>();
+        decoder.setUnknownTagVisitor((tag, buffer, offset, length) -> map.put(tag, buffer.getAscii(offset, length)));
+        decode(UNKNOWN_TAG_MESSAGE, decoder);
+        assertEquals("FOO", map.get(10100));
+        assertEquals("BAR", map.get(10101));
+    }
 }