pdf.js/web/text_layer_builder.js

/* Copyright 2012 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.
 */

// eslint-disable-next-line max-len
/** @typedef {import("../src/display/display_utils").PageViewport} PageViewport */
/** @typedef {import("../src/display/api").TextContent} TextContent */
/** @typedef {import("./text_highlighter").TextHighlighter} TextHighlighter */
// eslint-disable-next-line max-len
/** @typedef {import("./text_accessibility.js").TextAccessibilityManager} TextAccessibilityManager */

import { renderTextLayer, updateTextLayer } from "pdfjs-lib";

/**
 * @typedef {Object} TextLayerBuilderOptions
 * @property {TextHighlighter} highlighter - Optional object that will handle
 *   highlighting text from the find controller.
 * @property {TextAccessibilityManager} [accessibilityManager]
 * @property {boolean} [isOffscreenCanvasSupported] - Allows to use an
 *   OffscreenCanvas if needed.
 */

/**
 * The text layer builder provides text selection functionality for the PDF.
 * It does this by creating overlay divs over the PDF's text. These divs
 * contain text that matches the PDF text they are overlaying.
 */
class TextLayerBuilder {
  #rotation = 0;

  #scale = 0;

  #textContentSource = null;

  constructor({
    highlighter = null,
    accessibilityManager = null,
    isOffscreenCanvasSupported = true,
  }) {
    this.textContentItemsStr = [];
    this.renderingDone = false;
    this.textDivs = [];
    this.textDivProperties = new WeakMap();
    this.textLayerRenderTask = null;
    this.highlighter = highlighter;
    this.accessibilityManager = accessibilityManager;
    this.isOffscreenCanvasSupported = isOffscreenCanvasSupported;

    this.div = document.createElement("div");
    this.div.className = "textLayer";
    this.hide();
  }

  #finishRendering() {
    this.renderingDone = true;

    const endOfContent = document.createElement("div");
    endOfContent.className = "endOfContent";
    this.div.append(endOfContent);

    this.#bindMouse();
  }

  get numTextDivs() {
    return this.textDivs.length;
  }

  /**
   * Renders the text layer.
   * @param {PageViewport} viewport
   */
  async render(viewport) {
    if (!this.#textContentSource) {
      throw new Error('No "textContentSource" parameter specified.');
    }

    const scale = viewport.scale * (globalThis.devicePixelRatio || 1);
    if (this.renderingDone) {
      const { rotation } = viewport;
      const mustRotate = rotation !== this.#rotation;
      const mustRescale = scale !== this.#scale;
      if (mustRotate || mustRescale) {
        this.hide();
        updateTextLayer({
          container: this.div,
          viewport,
          textDivs: this.textDivs,
          textDivProperties: this.textDivProperties,
          isOffscreenCanvasSupported: this.isOffscreenCanvasSupported,
          mustRescale,
          mustRotate,
        });
        this.show();
        this.#scale = scale;
        this.#rotation = rotation;
      }
      return;
    }

    this.cancel();
    this.highlighter?.setTextMapping(this.textDivs, this.textContentItemsStr);
    this.accessibilityManager?.setTextMapping(this.textDivs);

    this.textLayerRenderTask = renderTextLayer({
      textContentSource: this.#textContentSource,
      container: this.div,
      viewport,
      textDivs: this.textDivs,
      textDivProperties: this.textDivProperties,
      textContentItemsStr: this.textContentItemsStr,
      isOffscreenCanvasSupported: this.isOffscreenCanvasSupported,
    });

    await this.textLayerRenderTask.promise;
    this.#finishRendering();
    this.#scale = scale;
    this.accessibilityManager?.enable();
    this.show();
  }

  hide() {
    // We turn off the highlighter in order to avoid to scroll into view an
    // element of the text layer which could be hidden.
    this.highlighter?.disable();
    this.div.hidden = true;
  }

  show() {
    this.div.hidden = false;
    this.highlighter?.enable();
  }

  /**
   * Cancel rendering of the text layer.
   */
  cancel() {
    if (this.textLayerRenderTask) {
      this.textLayerRenderTask.cancel();
      this.textLayerRenderTask = null;
    }
    this.highlighter?.disable();
    this.accessibilityManager?.disable();
    this.textContentItemsStr.length = 0;
    this.textDivs.length = 0;
    this.textDivProperties = new WeakMap();
  }

  /**
   * @param {ReadableStream | TextContent} source
   */
  setTextContentSource(source) {
    this.cancel();
    this.#textContentSource = source;
  }

  /**
   * Improves text selection by adding an additional div where the mouse was
   * clicked. This reduces flickering of the content if the mouse is slowly
   * dragged up or down.
   */
  #bindMouse() {
    const { div } = this;

    div.addEventListener("mousedown", evt => {
      const end = div.querySelector(".endOfContent");
      if (!end) {
        return;
      }
      if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
        // On non-Firefox browsers, the selection will feel better if the height
        // of the `endOfContent` div is adjusted to start at mouse click
        // location. This avoids flickering when the selection moves up.
        // However it does not work when selection is started on empty space.
        let adjustTop = evt.target !== div;
        if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
          adjustTop &&=
            getComputedStyle(end).getPropertyValue("-moz-user-select") !==
            "none";
        }
        if (adjustTop) {
          const divBounds = div.getBoundingClientRect();
          const r = Math.max(0, (evt.pageY - divBounds.top) / divBounds.height);
          end.style.top = (r * 100).toFixed(2) + "%";
        }
      }
      end.classList.add("active");
    });

    div.addEventListener("mouseup", () => {
      const end = div.querySelector(".endOfContent");
      if (!end) {
        return;
      }
      if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
        end.style.top = "";
      }
      end.classList.remove("active");
    });
  }
}

export { TextLayerBuilder };