New Tile unload approach.

Cesium3DTilesSelection/include/Cesium3DTilesSelection/Tile.h

@@ -531,6 +531,9 @@ class CESIUM3DTILESSELECTION_API Tile final {
    */
   void decrementDoNotUnloadSubtreeCountOnParent(const char* reason) noexcept;
 
+  bool addViewGroupReference() const noexcept;
+  bool releaseViewGroupReference() const noexcept;
+
 private:
   void incrementDoNotUnloadSubtreeCount(const std::string& reason) noexcept;
 
@@ -587,6 +590,7 @@ class CESIUM3DTILESSELECTION_API Tile final {
 
   // tile content
   CesiumUtility::DoublyLinkedListPointers<Tile> _loadedTilesLinks;
+  CesiumUtility::DoublyLinkedListPointers<Tile> _unusedTilesLinks;
   TileContent _content;
   TilesetContentLoader* _pLoader;
   TileLoadState _loadState;
@@ -599,6 +603,11 @@ class CESIUM3DTILESSELECTION_API Tile final {
   // external tileset (if any) from being unloaded from the tree.
   int32_t _doNotUnloadSubtreeCount = 0;
 
+  // The number of ViewGroups that currently reference this tile. While a tile
+  // is referenced by a view group, its content may not be unloaded, nor can the
+  // external tileset that contains this tile be unloaded.
+  mutable int32_t _viewGroupReferences = 0;
+
   friend class TilesetContentManager;
   friend class MockTilesetContentManagerTestFixture;
 
@@ -608,6 +617,9 @@ class CESIUM3DTILESSELECTION_API Tile final {
    */
   typedef CesiumUtility::DoublyLinkedList<Tile, &Tile::_loadedTilesLinks>
       LoadedLinkedList;
+
+  typedef CesiumUtility::DoublyLinkedList<Tile, &Tile::_unusedTilesLinks>
+      UnusedLinkedList;
 };
 
 } // namespace Cesium3DTilesSelection

Cesium3DTilesSelection/include/Cesium3DTilesSelection/TileSelectionState.h

@@ -99,7 +99,7 @@ class TileSelectionState final {
    * @param frameNumber The previous frame number.
    * @return The {@link TileSelectionState::Result}
    */
-  constexpr Result getResult(int32_t frameNumber) const noexcept {
+  constexpr Result getResult(int32_t /* frameNumber */) const noexcept {
     // if (this->_frameNumber != frameNumber) {
     //   return Result::None;
     // }

Cesium3DTilesSelection/include/Cesium3DTilesSelection/Tileset.h

@@ -8,7 +8,7 @@
 #include <Cesium3DTilesSelection/TilesetExternals.h>
 #include <Cesium3DTilesSelection/TilesetLoadFailureDetails.h>
 #include <Cesium3DTilesSelection/TilesetOptions.h>
-#include <Cesium3DTilesSelection/ViewGroup.h>
+#include <Cesium3DTilesSelection/TilesetViewGroup.h>
 #include <Cesium3DTilesSelection/ViewState.h>
 #include <Cesium3DTilesSelection/ViewUpdateResult.h>
 #include <CesiumAsync/AsyncSystem.h>
@@ -29,6 +29,7 @@ class TilesetMetadata;
 class TilesetHeightQuery;
 struct TilesetHeightRequest;
 class TilesetSharedAssetSystem;
+class TilesetViewGroup;
 
 /**
  * @brief A <a
@@ -218,7 +219,7 @@ class CESIUM3DTILESSELECTION_API Tileset final {
   updateView(const std::vector<ViewState>& frustums, float deltaTime = 0.0f);
 
   const ViewUpdateResult& updateView(
-      ViewGroup& viewGroup,
+      TilesetViewGroup& viewGroup,
       const std::vector<ViewState>& frustums,
       float deltaTime = 0.0f);
 
@@ -318,6 +319,8 @@ class CESIUM3DTILESSELECTION_API Tileset final {
   CesiumAsync::Future<SampleHeightResult> sampleHeightMostDetailed(
       const std::vector<CesiumGeospatial::Cartographic>& positions);
 
+  TilesetViewGroup createViewGroup();
+
   Tileset(const Tileset& rhs) = delete;
   Tileset& operator=(const Tileset& rhs) = delete;
 
@@ -380,7 +383,7 @@ class CESIUM3DTILESSELECTION_API Tileset final {
    * passed on through the traversal.
    */
   struct FrameState {
-    ViewGroup& viewGroup;
+    TilesetViewGroup& viewGroup;
     const std::vector<ViewState>& frustums;
     std::vector<double> fogDensities;
     int32_t lastFrameNumber;
@@ -478,7 +481,6 @@ class CESIUM3DTILESSELECTION_API Tileset final {
   void _processWorkerThreadLoadQueue();
   void _processMainThreadLoadQueue();
 
-  void _clearChildrenRecursively(Tile* pTile) noexcept;
   void _unloadCachedTiles(double timeBudget) noexcept;
   void _markTileVisited(Tile& tile) noexcept;
 
@@ -565,7 +567,7 @@ class CESIUM3DTILESSELECTION_API Tileset final {
 
   std::list<TilesetHeightRequest> _heightRequests;
 
-  ViewGroup _defaultViewGroup;
+  TilesetViewGroup _defaultViewGroup;
 
   void addTileToLoadQueue(
       Tile& tile,

Cesium3DTilesSelection/include/Cesium3DTilesSelection/TilesetViewGroup.h

@@ -0,0 +1,84 @@
+#pragma once
+
+#include <Cesium3DTilesSelection/TileSelectionState.h>
+#include <CesiumUtility/IntrusivePointer.h>
+
+#include <unordered_map>
+
+namespace Cesium3DTilesSelection {
+
+class Tile;
+class Tileset;
+class TilesetContentManager;
+
+/**
+ * @brief A group of views that select tiles from a particular {@link Tileset}
+ * together.
+ *
+ * Create an instance of this class by calling {@link Tileset::createViewGroup}.
+ */
+class CESIUM3DTILESSELECTION_API TilesetViewGroup final {
+public:
+  TilesetViewGroup(const TilesetViewGroup& rhs) noexcept;
+  TilesetViewGroup(TilesetViewGroup&& rhs) noexcept;
+  ~TilesetViewGroup() noexcept;
+
+  /**
+   * @brief Returns the previous {@link TileSelectionState} of this tile last
+   * time this view group was updated.
+   *
+   * This function is not supposed to be called by clients.
+   *
+   * @return The previous selection state
+   */
+  TileSelectionState getPreviousSelectionState(const Tile& tile) const noexcept;
+
+  /**
+   * @brief Returns the current {@link TileSelectionState} of this tile during
+   * the current update of this view group.
+   *
+   * This function is not supposed to be called by clients.
+   *
+   * @return The current selection state
+   */
+  TileSelectionState getCurrentSelectionState(const Tile& tile) const noexcept;
+
+  /**
+   * @brief Set the {@link TileSelectionState} of this tile.
+   *
+   * This function is not supposed to be called by clients.
+   *
+   * @param newState The new state
+   */
+  void setCurrentSelectionState(
+      const Tile& tile,
+      const TileSelectionState& newState) noexcept;
+
+  void kick(const Tile& tile) noexcept;
+
+  /**
+   * @brief Finishes the current frame by making the current tile selection
+   * state the previous one and releasing references to tiles in the old previous
+   * one.
+   */
+  void finishFrame();
+
+private:
+  /**
+   * @brief Constructs a new instance.
+   *
+   * @param tileset The tileset that this view group views.
+   */
+  TilesetViewGroup(const CesiumUtility::IntrusivePointer<TilesetContentManager>&
+                       pTilesetContentManager);
+
+  CesiumUtility::IntrusivePointer<TilesetContentManager>
+      _pTilesetContentManager;
+  std::unordered_map<const Tile*, TileSelectionState> _previousSelectionStates;
+  std::unordered_map<const Tile*, TileSelectionState> _currentSelectionStates;
+
+  // So that the Tileset can create instances of this class.
+  friend class Tileset;
+};
+
+} // namespace Cesium3DTilesSelection

Cesium3DTilesSelection/src/Tile.cpp

@@ -362,4 +362,15 @@ void Tile::decrementDoNotUnloadSubtreeCount(
   }
 }
 
+bool Tile::addViewGroupReference() const noexcept {
+  ++this->_viewGroupReferences;
+  return this->_viewGroupReferences == 1;
+}
+
+bool Tile::releaseViewGroupReference() const noexcept {
+  CESIUM_ASSERT(this->_viewGroupReferences > 0);
+  --this->_viewGroupReferences;
+  return this->_viewGroupReferences == 0;
+}
+
 } // namespace Cesium3DTilesSelection