#933: Add an interface LwM2mClientObserver2 to handle unexpected error

This interface extends `LwM2mClientObserver` interface.
And make `LwM2mClientObserverAdapter` and
`LwM2mClientObserverDispatcher`
implement that new interface.
This doesn't break the backward compatibility because
`LwM2mClientObserver2` is compatible with `LwM2mClientObserver` and each
implementing class conceals the difference between `LwM2mClientObserver`
and `LwM2mClientObserver2`.

Signed-off-by: moznion <moznion@gmail.com>

leshan-client-core/src/main/java/org/eclipse/leshan/client/observer/LwM2mClientObserver2.java

@@ -0,0 +1,14 @@
+package org.eclipse.leshan.client.observer;
+
+/**
+ * An extended interface of {@link LwM2mClientObserver} with handler for unexpected error.
+ * <p>
+ * Future plan: Since version 2.0, this interface is going to merge into {@link LwM2mClientObserver}.
+ *
+ * @since 1.3
+ */
+public interface LwM2mClientObserver2 extends LwM2mClientObserver {
+	// ============== Unexpected Error Handling =================
+
+	void onUnexpectedError(Throwable unexpectedError);
+}

leshan-client-core/src/main/java/org/eclipse/leshan/client/observer/LwM2mClientObserverAdapter.java

@@ -26,7 +26,7 @@
  * An abstract adapter class for observing registration life cycle. The methods in this class are empty. This class
  * exists as convenience for creating client observer objects.
  */
-public class LwM2mClientObserverAdapter implements LwM2mClientObserver {
+public class LwM2mClientObserverAdapter implements LwM2mClientObserver2 {
 
     @Override
     public void onBootstrapStarted(ServerIdentity bsserver, BootstrapRequest request) {
@@ -95,4 +95,8 @@ public void onDeregistrationFailure(ServerIdentity server, DeregisterRequest req
     @Override
     public void onDeregistrationTimeout(ServerIdentity server, DeregisterRequest request) {
     }
+
+    @Override
+    public void onUnexpectedError(Throwable unexpectedError) {
+    }
 }

leshan-client-core/src/main/java/org/eclipse/leshan/client/observer/LwM2mClientObserverDispatcher.java

@@ -24,7 +24,7 @@
  * A dispatcher for LwM2mClientObserver. It allow several observers on a LwM2mClient.
  *
  */
-public class LwM2mClientObserverDispatcher implements LwM2mClientObserver {
+public class LwM2mClientObserverDispatcher implements LwM2mClientObserver2 {
     private CopyOnWriteArrayList<LwM2mClientObserver> observers = new CopyOnWriteArrayList<>();
 
     public void addObserver(LwM2mClientObserver observer) {
@@ -151,4 +151,12 @@ public void onDeregistrationTimeout(ServerIdentity server, DeregisterRequest req
         }
     }
 
+    @Override
+    public void onUnexpectedError(Throwable unexpectedError) {
+        for (LwM2mClientObserver observer : observers) {
+            if (observer instanceof LwM2mClientObserver2) {
+                ((LwM2mClientObserver2) observer).onUnexpectedError(unexpectedError);
+            }
+        }
+    }
 }