Merge pull request #413 from jmlee2k/AnimationStateListener-optional

Made AnimationStateListener functions optional

packages/runtime-4.0/src/core/AnimationState.ts

@@ -1179,24 +1179,24 @@ export enum EventType {
  * */
 export interface AnimationStateListener extends IAnimationStateListener {
     /** Invoked when this entry has been set as the current entry. */
-    start (entry: TrackEntry): void;
+    start? (entry: TrackEntry): void;
 
     /** Invoked when another entry has replaced this entry as the current entry. This entry may continue being applied for
      * mixing. */
-    interrupt (entry: TrackEntry): void;
+    interrupt? (entry: TrackEntry): void;
 
     /** Invoked when this entry is no longer the current entry and will never be applied again. */
-    end (entry: TrackEntry): void;
+    end? (entry: TrackEntry): void;
 
     /** Invoked when this entry will be disposed. This may occur without the entry ever being set as the current entry.
      * References to the entry should not be kept after dispose is called, as it may be destroyed or reused. */
-    dispose (entry: TrackEntry): void;
+    dispose? (entry: TrackEntry): void;
 
     /** Invoked every time this entry's animation completes a loop. */
-    complete (entry: TrackEntry): void;
+    complete? (entry: TrackEntry): void;
 
     /** Invoked when this entry's animation triggers an event. */
-    event (entry: TrackEntry, event: Event): void;
+    event? (entry: TrackEntry, event: Event): void;
 }
 
 /**