add tile load and unload events

packages/engine/Source/Scene/Cesium3DTileset.js

@@ -3022,6 +3022,7 @@ function updateTiles(tileset, frameState, passOptions) {
     statistics.incrementSelectionCounts(tile.content);
     ++statistics.selected;
   }
+  //debugger;
   const emptyTiles = tileset._emptyTiles;
   for (let i = 0; i < emptyTiles.length; ++i) {
     const tile = emptyTiles[i];
@@ -3287,6 +3288,7 @@ function update(tileset, frameState, passStatistics, passOptions) {
 
   const statistics = tileset._statistics;
   statistics.clear();
+  //debugger;
 
   // Resets the visibility check for each pass
   ++tileset._updatedVisibilityFrame;

packages/engine/Source/Scene/VoxelPrimitive.js

@@ -450,6 +450,49 @@ function VoxelPrimitive(options) {
     }
   }
 
+  /**
+   * The event fired to indicate that a tile's content was loaded.
+   * <p>
+   * The loaded tile is passed to the event listener.
+   * </p>
+   * <p>
+   * This event is fired during the tileset traversal while the frame is being rendered
+   * so that updates to the tile take effect in the same frame.  Do not create or modify
+   * Cesium entities or primitives during the event listener.
+   * </p>
+   *
+   * @type {Event}
+   * @default new Event()
+   *
+   * @example
+   * voxelPrimitive.tileLoad.addEventListener(function(tile) {
+   *     console.log('A tile was loaded.');
+   * });
+   */
+  this.tileLoad = new Event();
+
+  /**
+   * The event fired to indicate that a tile's content was unloaded.
+   * <p>
+   * The unloaded {@link Cesium3DTile} is passed to the event listener.
+   * </p>
+   * <p>
+   * This event is fired immediately before the tile's content is unloaded while the frame is being
+   * rendered so that the event listener has access to the tile's content.  Do not create
+   * or modify Cesium entities or primitives during the event listener.
+   * </p>
+   *
+   * @type {Event}
+   * @default new Event()
+   *
+   * @example
+   * primitive.tileUnload.addEventListener(function(tile) {
+   *     console.log('A tile was unloaded from the cache.');
+   * });
+   *
+   */
+  this.tileUnload = new Event();
+
   // If the provider fails to initialize the primitive will fail too.
   const provider = this._provider;
   initialize(this, provider);

packages/engine/Source/Scene/VoxelTraversal.js

@@ -422,6 +422,7 @@ function requestData(that, keyframeNode) {
   function postRequestSuccess(result) {
     that._simultaneousRequestCount--;
     const length = provider.types.length;
+    that._primitive.tileLoad.raiseEvent();
 
     if (!defined(result)) {
       keyframeNode.state = KeyframeNode.LoadState.UNAVAILABLE;
@@ -645,6 +646,7 @@ function loadAndUnload(that, frameState) {
         destroyedCount++;
 
         const discardNode = keyframeNodesInMegatexture[addNodeIndex];
+        that._primitive.tileUnload.raiseEvent();
         discardNode.spatialNode.destroyKeyframeNode(
           discardNode,
           that.megatextures,