增加Listener的辅助代码

flowable-core/src/main/java/com/lanyuanxiaoyao/flowable/core/model/FlowableListener.java

@@ -16,4 +16,26 @@ public interface FlowableListener {
   void onAction(FlowableInstance instance, FlowableNode node, FlowableAction action);
 
   void onActionComplete(FlowableInstance instance, FlowableNode node, FlowableAction action);
+
+  abstract class AbstractFlowableListener implements FlowableListener {
+    @Override
+    public void onFlowStart(FlowableInstance instance, FlowableNode node) {
+    }
+
+    @Override
+    public void onFlowEnd(FlowableInstance instance, FlowableNode node, FlowableAction action) {
+    }
+
+    @Override
+    public void onActionStart(FlowableInstance instance, FlowableNode node, FlowableAction action) {
+    }
+
+    @Override
+    public void onAction(FlowableInstance instance, FlowableNode node, FlowableAction action) {
+    }
+
+    @Override
+    public void onActionComplete(FlowableInstance instance, FlowableNode node, FlowableAction action) {
+    }
+  }
 }