Cesium中的QuadtreePrimitive.js类

function QuadtreePrimitive(options) {
  //>>includeStart('debug', pragmas.debug);
  if (!defined(options) || !defined(options.tileProvider)) {
    throw new DeveloperError("options.tileProvider is required.");
  }
  if (defined(options.tileProvider.quadtree)) {
    throw new DeveloperError(
      "A QuadtreeTileProvider can only be used with a single QuadtreePrimitive"
    );
  }
  //>>includeEnd('debug');

  this._tileProvider = options.tileProvider;
  this._tileProvider.quadtree = this;

  this._debug = {
    enableDebugOutput: false,

    maxDepth: 0,
    maxDepthVisited: 0,
    tilesVisited: 0,
    tilesCulled: 0,
    tilesRendered: 0,
    tilesWaitingForChildren: 0,

    lastMaxDepth: -1,
    lastMaxDepthVisited: -1,
    lastTilesVisited: -1,
    lastTilesCulled: -1,
    lastTilesRendered: -1,
    lastTilesWaitingForChildren: -1,

    suspendLodUpdate: false,
  };

  var tilingScheme = this._tileProvider.tilingScheme;
  var ellipsoid = tilingScheme.ellipsoid;

  this._tilesToRender = [];
  this._tileLoadQueueHigh = []; // high priority tiles are preventing refinement
  this._tileLoadQueueMedium = []; // medium priority tiles are being rendered
  this._tileLoadQueueLow = []; // low priority tiles were refined past or are non-visible parts of quads.
  this._tileReplacementQueue = new TileReplacementQueue();
  this._levelZeroTiles = undefined;
  this._loadQueueTimeSlice = 5.0;
  this._tilesInvalidated = false;

  this._addHeightCallbacks = [];
  this._removeHeightCallbacks = [];

  this._tileToUpdateHeights = [];
  this._lastTileIndex = 0;
  this._updateHeightsTimeSlice = 2.0;

  // If a culled tile contains _cameraPositionCartographic or _cameraReferenceFrameOriginCartographic, it will be marked
  // TileSelectionResult.CULLED_BUT_NEEDED and added to the list of tiles to update heights,
  // even though it is not rendered.
  // These are updated each frame in `selectTilesForRendering`.
  this._cameraPositionCartographic = undefined;
  this._cameraReferenceFrameOriginCartographic = undefined;

  /**
   * Gets or sets the maximum screen-space error, in pixels, that is allowed.
   * A higher maximum error will render fewer tiles and improve performance, while a lower
   * value will improve visual quality.
   * @type {Number}
   * @default 2
   */
  this.maximumScreenSpaceError = defaultValue(
    options.maximumScreenSpaceError,
    2
  );

  /**
   * Gets or sets the maximum number of tiles that will be retained in the tile cache.
   * Note that tiles will never be unloaded if they were used for rendering the last
   * frame, so the actual number of resident tiles may be higher.  The value of
   * this property will not affect visual quality.
   * @type {Number}
   * @default 100
   */
  this.tileCacheSize = defaultValue(options.tileCacheSize, 100);

  /**
   * Gets or sets the number of loading descendant tiles that is considered "too many".
   * If a tile has too many loading descendants, that tile will be loaded and rendered before any of
   * its descendants are loaded and rendered. This means more feedback for the user that something
   * is happening at the cost of a longer overall load time. Setting this to 0 will cause each
   * tile level to be loaded successively, significantly increasing load time. Setting it to a large
   * number (e.g. 1000) will minimize the number of tiles that are loaded but tend to make
   * detail appear all at once after a long wait.
   * @type {Number}
   * @default 20
   */
  this.loadingDescendantLimit = 20;

  /**
   * Gets or sets a value indicating whether the ancestors of rendered tiles should be preloaded.
   * Setting this to true optimizes the zoom-out experience and provides more detail in
   * newly-exposed areas when panning. The down side is that it requires loading more tiles.
   * @type {Boolean}
   * @default true
   */
  this.preloadAncestors = true;

  /**
   * Gets or sets a value indicating whether the siblings of rendered tiles should be preloaded.
   * Setting this to true causes tiles with the same parent as a rendered tile to be loaded, even
   * if they are culled. Setting this to true may provide a better panning experience at the
   * cost of loading more tiles.
   * @type {Boolean}
   * @default false
   */
  this.preloadSiblings = false;

  this._occluders = new QuadtreeOccluders({
    ellipsoid: ellipsoid,
  });

  this._tileLoadProgressEvent = new Event();
  this._lastTileLoadQueueLength = 0;

  this._lastSelectionFrameNumber = undefined;
}