Implement event listeners of the FlowManager.

- This commit adds two types of event objects: FlowBatchStateChangedEvent and FlowStatesChangedEvent.
- Listeners for these objects are defined by FlowManagerListener interface.
- FlowBatchStateChangedEvent notifies state changes of the flow batch operation.
 -- The state is defined by FlowBatchState.
- FlowStatesChangedEvent notifies state changes of the set of Flow objects.
 -- The state is defined by FlowState.
- This task is a part of ONOS-1739.

Change-Id: Ie8b3bb1c58dbe6bc608936e4a269e1de12dee96c
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/FlowStateChange.java b/src/main/java/net/onrc/onos/api/flowmanager/FlowStateChange.java
new file mode 100644
index 0000000..9d7f0d6
--- /dev/null
+++ b/src/main/java/net/onrc/onos/api/flowmanager/FlowStateChange.java
@@ -0,0 +1,50 @@
+package net.onrc.onos.api.flowmanager;
+
+/**
+ * The class which expresses the state changes of Flow object.
+ */
+public class FlowStateChange {
+    private final FlowId flowId;
+    private final FlowState current;
+    private final FlowState previous;
+
+    /**
+     * Creates {@link FlowStateChange} instance.
+     *
+     * @param flowId the ID of the target flow
+     * @param current the current state of the flow
+     * @param previous the previous state of the flow
+     */
+    FlowStateChange(FlowId flowId, FlowState current, FlowState previous) {
+        this.flowId = flowId;
+        this.current = current;
+        this.previous = previous;
+    }
+
+    /**
+     * Gets the ID of the flow.
+     *
+     * @return the flow ID
+     */
+    public FlowId getFlowId() {
+        return flowId;
+    }
+
+    /**
+     * Gets the current state of the flow.
+     *
+     * @return the current state of the flow
+     */
+    public FlowState getCurrentState() {
+        return current;
+    }
+
+    /**
+     * Gets the previous state of the flow.
+     *
+     * @return the previous state of the flow
+     */
+    public FlowState getPreviousState() {
+        return previous;
+    }
+}