src/core/object_loader.js - master - githubmirror/pdf.js

src/core/object_loader.js
153 строки5 KB
Jonas Jenwald
Simplify how the `ObjectLoader` is used
06 май 2025, 16:49
06 май 2025, 16:4962009ff
/* Copyright 2021 Mozilla Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import { Dict, Ref, RefSet } from "./primitives.js";
import { BaseStream } from "./base_stream.js";
import { MissingDataException } from "./core_utils.js";
import { warn } from "../shared/util.js";

function mayHaveChildren(value) {
  return (
    value instanceof Ref ||
    value instanceof Dict ||
    value instanceof BaseStream ||
    Array.isArray(value)
  );
}

function addChildren(node, nodesToVisit) {
  if (node instanceof Dict) {
    node = node.getRawValues();
  } else if (node instanceof BaseStream) {
    node = node.dict.getRawValues();
  } else if (!Array.isArray(node)) {
    return;
  }
  for (const rawValue of node) {
    if (mayHaveChildren(rawValue)) {
      nodesToVisit.push(rawValue);
    }
  }
}

/**
 * A helper for loading missing data in `Dict` graphs. It traverses the graph
 * depth first and queues up any objects that have missing data. Once it has
 * has traversed as many objects that are available it attempts to bundle the
 * missing data requests and then resume from the nodes that weren't ready.
 *
 * NOTE: It provides protection from circular references by keeping track of
 * loaded references. However, you must be careful not to load any graphs
 * that have references to the catalog or other pages since that will cause the
 * entire PDF document object graph to be traversed.
 */
class ObjectLoader {
  refSet = new RefSet();

  constructor(dict, keys, xref) {
    this.dict = dict;
    this.keys = keys;
    this.xref = xref;
  }

  async load() {
    const { keys, dict } = this;
    // Setup the initial nodes to visit.
    const nodesToVisit = [];
    for (const key of keys) {
      const rawValue = dict.getRaw(key);
      // Skip nodes that are guaranteed to be empty.
      if (rawValue !== undefined) {
        nodesToVisit.push(rawValue);
      }
    }
    await this.#walk(nodesToVisit);

    this.refSet = null; // Everything is loaded, clear the cache.
  }

  async #walk(nodesToVisit) {
    const nodesToRevisit = [];
    const pendingRequests = [];
    // DFS walk of the object graph.
    while (nodesToVisit.length) {
      let currentNode = nodesToVisit.pop();

      // Only references or chunked streams can cause missing data exceptions.
      if (currentNode instanceof Ref) {
        // Skip nodes that have already been visited.
        if (this.refSet.has(currentNode)) {
          continue;
        }
        try {
          this.refSet.put(currentNode);
          currentNode = this.xref.fetch(currentNode);
        } catch (ex) {
          if (!(ex instanceof MissingDataException)) {
            warn(`ObjectLoader.#walk - requesting all data: "${ex}".`);

            await this.xref.stream.manager.requestAllChunks();
            return;
          }
          nodesToRevisit.push(currentNode);
          pendingRequests.push({ begin: ex.begin, end: ex.end });
        }
      }
      if (currentNode instanceof BaseStream) {
        const baseStreams = currentNode.getBaseStreams();
        if (baseStreams) {
          let foundMissingData = false;
          for (const stream of baseStreams) {
            if (stream.isDataLoaded) {
              continue;
            }
            foundMissingData = true;
            pendingRequests.push({ begin: stream.start, end: stream.end });
          }
          if (foundMissingData) {
            nodesToRevisit.push(currentNode);
          }
        }
      }

      addChildren(currentNode, nodesToVisit);
    }

    if (pendingRequests.length) {
      await this.xref.stream.manager.requestRanges(pendingRequests);

      for (const node of nodesToRevisit) {
        // Remove any reference nodes from the current `RefSet` so they
        // aren't skipped when we revist them.
        if (node instanceof Ref) {
          this.refSet.remove(node);
        }
      }
      await this.#walk(nodesToRevisit);
    }
  }

  static async load(obj, keys, xref) {
    // Don't walk the graph if all the data is already loaded.
    if (xref.stream.isDataLoaded) {
      return;
    }
    // eslint-disable-next-line no-restricted-syntax
    const objLoader = new ObjectLoader(obj, keys, xref);
    await objLoader.load();
  }
}

export { ObjectLoader };