Added piston retract event listener (#32)

* Check all blocks moved by piston instead of only one.

* Added retract event listener

src/main/java/com/derongan/minecraft/deeperworld/synchronization/SectionSyncListener.kt

@@ -57,15 +57,21 @@ object SectionSyncListener : Listener {
         updateCorrespondingBlock(location, updateBlockData)
     }
 
-
-    /** Disables pistons if they are in the overlap of two sections */
+    /** Disables pistons extending if they are in the overlap of two sections */
     @EventHandler
-    fun onPistonEvent(event: BlockPistonExtendEvent) {
+    fun onPistonExtendEvent(event: BlockPistonExtendEvent) {
         //TODO handle pistons properly instead of just cancelling the event
         if (event.blocks.any { it.location.correspondingSection != null })
             event.isCancelled = true
     }
 
+    /** Disables pistons retracting if they are in the overlap of two sections */
+    @EventHandler
+    fun onPistonRetractEvent(event: BlockPistonRetractEvent) {
+        if (event.blocks.any { it.location.correspondingSection != null })
+            event.isCancelled = true
+    }
+
     @EventHandler
     fun onWaterEmptyEvent(event: PlayerBucketEmptyEvent) =
             updateCorrespondingBlock(event.block.location) { orig, corr ->