cesiumDemo/Source/Scene/TileReplacementQueue.js

import defined from '../Core/defined.js';

    /**
     * A priority queue of tiles to be replaced, if necessary, to make room for new tiles.  The queue
     * is implemented as a linked list.
     *
     * @alias TileReplacementQueue
     * @private
     */
    function TileReplacementQueue() {
        this.head = undefined;
        this.tail = undefined;
        this.count = 0;
        this._lastBeforeStartOfFrame = undefined;
    }

    /**
     * Marks the start of the render frame.  Tiles before (closer to the head) this tile in the
     * list were used last frame and must not be unloaded.
     */
    TileReplacementQueue.prototype.markStartOfRenderFrame = function() {
        this._lastBeforeStartOfFrame = this.head;
    };

    /**
     * Reduces the size of the queue to a specified size by unloading the least-recently used
     * tiles.  Tiles that were used last frame will not be unloaded, even if that puts the number
     * of tiles above the specified maximum.
     *
     * @param {Number} maximumTiles The maximum number of tiles in the queue.
     */
    TileReplacementQueue.prototype.trimTiles = function(maximumTiles) {
        var tileToTrim = this.tail;
        var keepTrimming = true;
        while (keepTrimming &&
               defined(this._lastBeforeStartOfFrame) &&
               this.count > maximumTiles &&
               defined(tileToTrim)) {
            // Stop trimming after we process the last tile not used in the
            // current frame.
            keepTrimming = tileToTrim !== this._lastBeforeStartOfFrame;

            var previous = tileToTrim.replacementPrevious;

            if (tileToTrim.eligibleForUnloading) {
                tileToTrim.freeResources();
                remove(this, tileToTrim);
            }

            tileToTrim = previous;
        }
    };

    function remove(tileReplacementQueue, item) {
        var previous = item.replacementPrevious;
        var next = item.replacementNext;

        if (item === tileReplacementQueue._lastBeforeStartOfFrame) {
            tileReplacementQueue._lastBeforeStartOfFrame = next;
        }

        if (item === tileReplacementQueue.head) {
            tileReplacementQueue.head = next;
        } else {
            previous.replacementNext = next;
        }

        if (item === tileReplacementQueue.tail) {
            tileReplacementQueue.tail = previous;
        } else {
            next.replacementPrevious = previous;
        }

        item.replacementPrevious = undefined;
        item.replacementNext = undefined;

        --tileReplacementQueue.count;
    }

    /**
     * Marks a tile as rendered this frame and moves it before the first tile that was not rendered
     * this frame.
     *
     * @param {TileReplacementQueue} item The tile that was rendered.
     */
    TileReplacementQueue.prototype.markTileRendered = function(item) {
        var head = this.head;
        if (head === item) {
            if (item === this._lastBeforeStartOfFrame) {
                this._lastBeforeStartOfFrame = item.replacementNext;
            }
            return;
        }

        ++this.count;

        if (!defined(head)) {
            // no other tiles in the list
            item.replacementPrevious = undefined;
            item.replacementNext = undefined;
            this.head = item;
            this.tail = item;
            return;
        }

        if (defined(item.replacementPrevious) || defined(item.replacementNext)) {
            // tile already in the list, remove from its current location
            remove(this, item);
        }

        item.replacementPrevious = undefined;
        item.replacementNext = head;
        head.replacementPrevious = item;

        this.head = item;
    };
export default TileReplacementQueue;