ui/file_manager/file_manager/foreground/js/list_thumbnail_loader.js - chromium/src - Git at Google

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 /**
  * A thumbnail loader for list style UI.
  *
  * ListThumbnailLoader is a thubmanil loader designed for list style ui. List
  * thumbnail loader loads thumbnail in a viewport of the UI. ListThumbnailLoader
  * is responsible to return dataUrls of thumbnails and fetch them with proper
  * priority.
  */
 class ListThumbnailLoader extends cr.EventTarget {
   /**
    * @param {!DirectoryModel} directoryModel A directory model.
    * @param {!ThumbnailModel} thumbnailModel Thumbnail metadata model.
    * @param {!VolumeManager} volumeManager Volume manager.
    * @param {Function=} opt_thumbnailLoaderConstructor A constructor of
    *     thumbnail loader. This argument is used for testing.
    */
   constructor(
       directoryModel, thumbnailModel, volumeManager,
       opt_thumbnailLoaderConstructor) {
     super();
     /**
      * @private {!DirectoryModel}
      */
     this.directoryModel_ = directoryModel;

     /**
      * @private {!ThumbnailModel}
      */
     this.thumbnailModel_ = thumbnailModel;

     /**
      * @private {!VolumeManager}
      */
     this.volumeManager_ = volumeManager;

     /**
      * Constructor of thumbnail loader.
      * @private {!Function}
      */
     this.thumbnailLoaderConstructor_ =
         opt_thumbnailLoaderConstructor || ThumbnailLoader;

     /**
      * @private {!Object<!ListThumbnailLoader.Task>}
      */
     this.active_ = {};

     /**
      * @private {LRUCache<!ListThumbnailLoader.ThumbnailData>}
      */
     this.cache_ = new LRUCache(ListThumbnailLoader.CACHE_SIZE);

     /**
      * @private {number}
      */
     this.beginIndex_ = 0;

     /**
      * @private {number}
      */
     this.endIndex_ = 0;

     /**
      * Cursor.
      * @private {number}
      */
     this.cursor_ = 0;

     /**
      * Current volume type.
      * @private {?ListThumbnailLoader.VolumeType}
      */
     this.currentVolumeType_ = null;

     /**
      * @private {!FileListModel}
      */
     this.dataModel_ = assert(this.directoryModel_.getFileList());

     this.directoryModel_.addEventListener(
         'scan-completed', this.onScanCompleted_.bind(this));
     this.dataModel_.addEventListener('splice', this.onSplice_.bind(this));
     this.dataModel_.addEventListener('sorted', this.onSorted_.bind(this));
     this.dataModel_.addEventListener('change', this.onChange_.bind(this));
   }

   /**
    * Gets number of prefetch requests. This number changes based on current
    * volume type.
    * @return {number} Number of prefetch requests.
    * @private
    */
   getNumOfPrefetch_() {
     switch (/** @type {?ListThumbnailLoader.VolumeType} */
             (this.currentVolumeType_)) {
       case VolumeManagerCommon.VolumeType.MTP:
         return 0;
       case ListThumbnailLoader.TEST_VOLUME_TYPE:
         return 1;
       default:
         return 20;
     }
   }

   /**
    * Gets maximum number of active thumbnail fetch tasks. This number changes
    * based on current volume type.
    * @return {number} Maximum number of active thumbnail fetch tasks.
    * @private
    */
   getNumOfMaxActiveTasks_() {
     switch (/** @type {?ListThumbnailLoader.VolumeType} */
             (this.currentVolumeType_)) {
       case VolumeManagerCommon.VolumeType.MTP:
         return 1;
       case ListThumbnailLoader.TEST_VOLUME_TYPE:
         return ListThumbnailLoader.numOfMaxActiveTasksForTest;
       default:
         return 10;
     }
   }

   /**
    * An event handler for scan-completed event of directory model. When
    * directory scan is running, we don't fetch thumbnail in order not to block
    * IO for directory scan. i.e. modification events during directory scan is
    * ignored. We need to check thumbnail loadings after directory scan is
    * completed.
    *
    * @param {!Event} event Event
    */
   onScanCompleted_(event) {
     this.cursor_ = this.beginIndex_;
     this.continue_();
   }

   /**
    * An event handler for splice event of data model. When list is changed,
    * start to rescan items.
    *
    * @param {!Event} event Event
    */
   onSplice_(event) {
     this.cursor_ = this.beginIndex_;
     this.continue_();
   }

   /**
    * An event handler for sorted event of data model. When list is sorted, start
    * to rescan items.
    *
    * @param {!Event} event Event
    */
   onSorted_(event) {
     this.cursor_ = this.beginIndex_;
     this.continue_();
   }

   /**
    * An event handler for change event of data model.
    *
    * @param {!Event} event Event
    */
   onChange_(event) {
     // Mark the thumbnail in cache as invalid.
     const entry = this.dataModel_.item(event.index);
     const cachedThumbnail = this.cache_.peek(entry.toURL());
     if (cachedThumbnail) {
       cachedThumbnail.outdated = true;
     }

     this.cursor_ = this.beginIndex_;
     this.continue_();
   }

   /**
    * Sets high priority range in the list.
    *
    * @param {number} beginIndex Begin index of the range, inclusive.
    * @param {number} endIndex End index of the range, exclusive.
    */
   setHighPriorityRange(beginIndex, endIndex) {
     if (!(beginIndex < endIndex)) {
       return;
     }

     this.beginIndex_ = beginIndex;
     this.endIndex_ = endIndex;
     this.cursor_ = this.beginIndex_;

     this.continue_();
   }

   /**
    * Returns a thumbnail of an entry if it is in cache. This method returns
    * thumbnail even if the thumbnail is outdated.
    *
    * @return {ListThumbnailLoader.ThumbnailData} If the thumbnail is not in
    *     cache, this returns null.
    */
   getThumbnailFromCache(entry) {
     // Since we want to evict cache based on high priority range, we use peek
     // here instead of get.
     return this.cache_.peek(entry.toURL()) || null;
   }

   /**
    * Enqueues tasks if available.
    */
   continue_() {
     // If directory scan is running or all items are scanned, do nothing.
     if (this.directoryModel_.isScanning() ||
         !(this.cursor_ < this.dataModel_.length)) {
       return;
     }

     const entry = /** @type {Entry} */ (this.dataModel_.item(this.cursor_));

     // Check volume type for optimizing the parameters.
     const volumeInfo = this.volumeManager_.getVolumeInfo(assert(entry));
     this.currentVolumeType_ = volumeInfo ? volumeInfo.volumeType : null;

     // If tasks are running full or all items are scanned, do nothing.
     if (!(Object.keys(this.active_).length < this.getNumOfMaxActiveTasks_()) ||
         !(this.cursor_ < this.endIndex_ + this.getNumOfPrefetch_())) {
       return;
     }

     // If the entry is a directory, already in cache as valid or fetching, skip.
     const thumbnail = this.cache_.get(entry.toURL());
     if (entry.isDirectory || (thumbnail && !thumbnail.outdated) ||
         this.active_[entry.toURL()]) {
       this.cursor_++;
       this.continue_();
       return;
     }

     this.enqueue_(this.cursor_, entry);
     this.cursor_++;
     this.continue_();
   }

   /**
    * Enqueues a thumbnail fetch task for an entry.
    *
    * @param {number} index Index of an entry in current data model.
    * @param {!Entry} entry An entry.
    */
   enqueue_(index, entry) {
     const task = new ListThumbnailLoader.Task(
         entry, this.volumeManager_, this.thumbnailModel_,
         this.thumbnailLoaderConstructor_);

     const url = entry.toURL();
     this.active_[url] = task;

     task.fetch().then(thumbnail => {
       delete this.active_[url];
       this.cache_.put(url, thumbnail);
       this.dispatchThumbnailLoaded_(index, thumbnail);
       this.continue_();
     });
   }

   /**
    * Dispatches thumbnail loaded event.
    *
    * @param {number} index Index of an original image in the data model.
    * @param {!ListThumbnailLoader.ThumbnailData} thumbnail Thumbnail.
    */
   dispatchThumbnailLoaded_(index, thumbnail) {
     // Update index if it's already invalid, i.e. index may be invalid if some
     // change had happened in the data model during thumbnail fetch.
     const item = this.dataModel_.item(index);
     if (item && item.toURL() !== thumbnail.fileUrl) {
       index = -1;
       for (let i = 0; i < this.dataModel_.length; i++) {
         if (this.dataModel_.item(i).toURL() === thumbnail.fileUrl) {
           index = i;
           break;
         }
       }
     }

     if (index > -1) {
       this.dispatchEvent(
           new ListThumbnailLoader.ThumbnailLoadedEvent(index, thumbnail));
     }
   }
 }

 /**
  * Cache size. Cache size must be larger than sum of high priority range size
  * and number of prefetch tasks.
  * @const {number}
  */
 ListThumbnailLoader.CACHE_SIZE = 500;

 /**
  * Volume type for testing.
  * @const {string}
  */
 ListThumbnailLoader.TEST_VOLUME_TYPE = 'test_volume_type';

 /**
  * Number of maximum active tasks for testing.
  * @type {number}
  */
 ListThumbnailLoader.numOfMaxActiveTasksForTest = 2;

 /**
  * @typedef {(VolumeManagerCommon.VolumeType|string)}
  */
 ListThumbnailLoader.VolumeType;

 /**
  * Thumbnail loaded event.
  */
 ListThumbnailLoader.ThumbnailLoadedEvent =
     class ThumbnailLoadedEvent extends Event {
   /**
    * @param {number} index Index of an original image in the current data
    *     model.
    * @param {!ListThumbnailLoader.ThumbnailData} thumbnail Thumbnail.
    */
   constructor(index, thumbnail) {
     super('thumbnailLoaded');

     /** @type {number} */
     this.index = index;

     /** @type {string}*/
     this.fileUrl = thumbnail.fileUrl;

     /** @type {?string} */
     this.dataUrl = thumbnail.dataUrl;

     /** @type {?number} */
     this.width = thumbnail.width;

     /** @type {?number}*/
     this.height = thumbnail.height;
   }
 };

 /**
  * A class to represent thumbnail data.
  */
 ListThumbnailLoader.ThumbnailData = class {
   /**
    * @param {string} fileUrl File url of an original image.
    * @param {?string} dataUrl Data url of thumbnail.
    * @param {?number} width Width of thumbnail.
    * @param {?number} height Height of thumbnail.
    */
   constructor(fileUrl, dataUrl, width, height) {
     /**
      * @const {string}
      */
     this.fileUrl = fileUrl;

     /**
      * @const {?string}
      */
     this.dataUrl = dataUrl;

     /**
      * @const {?number}
      */
     this.width = width;

     /**
      * @const {?number}
      */
     this.height = height;

     /**
      * @type {boolean}
      */
     this.outdated = false;
   }
 };

 /**
  * A task to load thumbnail.
  */
 ListThumbnailLoader.Task = class {
   /**
    *
    * @param {!Entry} entry An entry.
    * @param {!VolumeManager} volumeManager Volume manager.
    * @param {!ThumbnailModel} thumbnailModel Metadata cache.
    * @param {!Function} thumbnailLoaderConstructor A constructor of thumbnail
    *     loader.
    */
   constructor(
       entry, volumeManager, thumbnailModel, thumbnailLoaderConstructor) {
     this.entry_ = entry;
     this.volumeManager_ = volumeManager;
     this.thumbnailModel_ = thumbnailModel;
     this.thumbnailLoaderConstructor_ = thumbnailLoaderConstructor;
   }

   /**
    * Fetches thumbnail.
    *
    * @return {!Promise<!ListThumbnailLoader.ThumbnailData>} A promise which is
    *     resolved when thumbnail data is fetched with either a success or an
    *     error.
    */
   fetch() {
     let ioError = false;
     return this.thumbnailModel_.get([this.entry_])
         .then(metadatas => {
           // When it failed to read exif header with an IO error, do not
           // generate thumbnail at this time since it may success in the second
           // try. If it failed to read at 0 byte, it would be an IO error.
           if (metadatas[0].thumbnail.urlError &&
               metadatas[0].thumbnail.urlError.errorDescription ===
                   'Error: Unexpected EOF @0') {
             ioError = true;
             return Promise.reject();
           }
           return metadatas[0];
         })
         .then(metadata => {
           const loadTargets = [
             ThumbnailLoader.LoadTarget.CONTENT_METADATA,
             ThumbnailLoader.LoadTarget.EXTERNAL_METADATA
           ];

           // If the file is on a network filesystem, don't generate thumbnails
           // from file entry, as it could cause very high network traffic.
           // Allow Drive to do so however, as ThumbnailLoader tries to generate
           // thumbnails of Drive files from file entry only if cached locally.
           const volumeInfo = this.volumeManager_.getVolumeInfo(this.entry_);
           if (volumeInfo &&
               (volumeInfo.source !== VolumeManagerCommon.Source.NETWORK ||
                volumeInfo.volumeType ===
                    VolumeManagerCommon.VolumeType.DRIVE)) {
             loadTargets.push(ThumbnailLoader.LoadTarget.FILE_ENTRY);
           }

           return new this
               .thumbnailLoaderConstructor_(
                   this.entry_, ThumbnailLoader.LoaderType.IMAGE, metadata,
                   undefined /* opt_mediaType */, loadTargets)
               .loadAsDataUrl(ThumbnailLoader.FillMode.OVER_FILL);
         })
         .then(result => {
           return new ListThumbnailLoader.ThumbnailData(
               this.entry_.toURL(), result.data, result.width, result.height);
         })
         .catch(() => {
           // If an error happens during generating of a thumbnail, then return
           // an empty object, so we don't retry the thumbnail over and over
           // again.
           const thumbnailData = new ListThumbnailLoader.ThumbnailData(
               this.entry_.toURL(), null, null, null);
           if (ioError) {
             // If fetching a thumbnail from EXIF fails due to an IO error, then
             // try to refetch it in the future, but not earlier than in 3
             // second.
             setTimeout(() => {
               thumbnailData.outdated = true;
             }, ListThumbnailLoader.Task.EXIF_IO_ERROR_DELAY);
           }
           return thumbnailData;
         });
   }
 };

 /**
  * Minimum delay of milliseconds before another retry for fetching a
  * thumbnmail from EXIF after failing with an IO error. In milliseconds.
  *
  * @type {number}
  */
 ListThumbnailLoader.Task.EXIF_IO_ERROR_DELAY = 3000;
	// Copyright 2015 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	/**
	* A thumbnail loader for list style UI.
	*
	* ListThumbnailLoader is a thubmanil loader designed for list style ui. List
	* thumbnail loader loads thumbnail in a viewport of the UI. ListThumbnailLoader
	* is responsible to return dataUrls of thumbnails and fetch them with proper
	* priority.
	*/
	class ListThumbnailLoader extends cr.EventTarget {
	/**
	* @param {!DirectoryModel} directoryModel A directory model.
	* @param {!ThumbnailModel} thumbnailModel Thumbnail metadata model.
	* @param {!VolumeManager} volumeManager Volume manager.
	* @param {Function=} opt_thumbnailLoaderConstructor A constructor of
	* thumbnail loader. This argument is used for testing.
	*/
	constructor(
	directoryModel, thumbnailModel, volumeManager,
	opt_thumbnailLoaderConstructor) {
	super();
	/**
	* @private {!DirectoryModel}
	*/
	this.directoryModel_ = directoryModel;

	/**
	* @private {!ThumbnailModel}
	*/
	this.thumbnailModel_ = thumbnailModel;

	/**
	* @private {!VolumeManager}
	*/
	this.volumeManager_ = volumeManager;

	/**
	* Constructor of thumbnail loader.
	* @private {!Function}
	*/
	this.thumbnailLoaderConstructor_ =
	opt_thumbnailLoaderConstructor \|\| ThumbnailLoader;

	/**
	* @private {!Object<!ListThumbnailLoader.Task>}
	*/
	this.active_ = {};

	/**
	* @private {LRUCache<!ListThumbnailLoader.ThumbnailData>}
	*/
	this.cache_ = new LRUCache(ListThumbnailLoader.CACHE_SIZE);

	/**
	* @private {number}
	*/
	this.beginIndex_ = 0;

	/**
	* @private {number}
	*/
	this.endIndex_ = 0;

	/**
	* Cursor.
	* @private {number}
	*/
	this.cursor_ = 0;

	/**
	* Current volume type.
	* @private {?ListThumbnailLoader.VolumeType}
	*/
	this.currentVolumeType_ = null;

	/**
	* @private {!FileListModel}
	*/
	this.dataModel_ = assert(this.directoryModel_.getFileList());

	this.directoryModel_.addEventListener(
	'scan-completed', this.onScanCompleted_.bind(this));
	this.dataModel_.addEventListener('splice', this.onSplice_.bind(this));
	this.dataModel_.addEventListener('sorted', this.onSorted_.bind(this));
	this.dataModel_.addEventListener('change', this.onChange_.bind(this));
	}

	/**
	* Gets number of prefetch requests. This number changes based on current
	* volume type.
	* @return {number} Number of prefetch requests.
	* @private
	*/
	getNumOfPrefetch_() {
	switch (/** @type {?ListThumbnailLoader.VolumeType} */
	(this.currentVolumeType_)) {
	case VolumeManagerCommon.VolumeType.MTP:
	return 0;
	case ListThumbnailLoader.TEST_VOLUME_TYPE:
	return 1;
	default:
	return 20;
	}
	}

	/**
	* Gets maximum number of active thumbnail fetch tasks. This number changes
	* based on current volume type.
	* @return {number} Maximum number of active thumbnail fetch tasks.
	* @private
	*/
	getNumOfMaxActiveTasks_() {
	switch (/** @type {?ListThumbnailLoader.VolumeType} */
	(this.currentVolumeType_)) {
	case VolumeManagerCommon.VolumeType.MTP:
	return 1;
	case ListThumbnailLoader.TEST_VOLUME_TYPE:
	return ListThumbnailLoader.numOfMaxActiveTasksForTest;
	default:
	return 10;
	}
	}

	/**
	* An event handler for scan-completed event of directory model. When
	* directory scan is running, we don't fetch thumbnail in order not to block
	* IO for directory scan. i.e. modification events during directory scan is
	* ignored. We need to check thumbnail loadings after directory scan is
	* completed.
	*
	* @param {!Event} event Event
	*/
	onScanCompleted_(event) {
	this.cursor_ = this.beginIndex_;
	this.continue_();
	}

	/**
	* An event handler for splice event of data model. When list is changed,
	* start to rescan items.
	*
	* @param {!Event} event Event
	*/
	onSplice_(event) {
	this.cursor_ = this.beginIndex_;
	this.continue_();
	}

	/**
	* An event handler for sorted event of data model. When list is sorted, start
	* to rescan items.
	*
	* @param {!Event} event Event
	*/
	onSorted_(event) {
	this.cursor_ = this.beginIndex_;
	this.continue_();
	}

	/**
	* An event handler for change event of data model.
	*
	* @param {!Event} event Event
	*/
	onChange_(event) {
	// Mark the thumbnail in cache as invalid.
	const entry = this.dataModel_.item(event.index);
	const cachedThumbnail = this.cache_.peek(entry.toURL());
	if (cachedThumbnail) {
	cachedThumbnail.outdated = true;
	}

	this.cursor_ = this.beginIndex_;
	this.continue_();
	}

	/**
	* Sets high priority range in the list.
	*
	* @param {number} beginIndex Begin index of the range, inclusive.
	* @param {number} endIndex End index of the range, exclusive.
	*/
	setHighPriorityRange(beginIndex, endIndex) {
	if (!(beginIndex < endIndex)) {
	return;
	}

	this.beginIndex_ = beginIndex;
	this.endIndex_ = endIndex;
	this.cursor_ = this.beginIndex_;

	this.continue_();
	}

	/**
	* Returns a thumbnail of an entry if it is in cache. This method returns
	* thumbnail even if the thumbnail is outdated.
	*
	* @return {ListThumbnailLoader.ThumbnailData} If the thumbnail is not in
	* cache, this returns null.
	*/
	getThumbnailFromCache(entry) {
	// Since we want to evict cache based on high priority range, we use peek
	// here instead of get.
	return this.cache_.peek(entry.toURL()) \|\| null;
	}

	/**
	* Enqueues tasks if available.
	*/
	continue_() {
	// If directory scan is running or all items are scanned, do nothing.
	if (this.directoryModel_.isScanning() \|\|
	!(this.cursor_ < this.dataModel_.length)) {
	return;
	}

	const entry = /** @type {Entry} */ (this.dataModel_.item(this.cursor_));

	// Check volume type for optimizing the parameters.
	const volumeInfo = this.volumeManager_.getVolumeInfo(assert(entry));
	this.currentVolumeType_ = volumeInfo ? volumeInfo.volumeType : null;

	// If tasks are running full or all items are scanned, do nothing.
	if (!(Object.keys(this.active_).length < this.getNumOfMaxActiveTasks_()) \|\|
	!(this.cursor_ < this.endIndex_ + this.getNumOfPrefetch_())) {
	return;
	}

	// If the entry is a directory, already in cache as valid or fetching, skip.
	const thumbnail = this.cache_.get(entry.toURL());
	if (entry.isDirectory \|\| (thumbnail && !thumbnail.outdated) \|\|
	this.active_[entry.toURL()]) {
	this.cursor_++;
	this.continue_();
	return;
	}

	this.enqueue_(this.cursor_, entry);
	this.cursor_++;
	this.continue_();
	}

	/**
	* Enqueues a thumbnail fetch task for an entry.
	*
	* @param {number} index Index of an entry in current data model.
	* @param {!Entry} entry An entry.
	*/
	enqueue_(index, entry) {
	const task = new ListThumbnailLoader.Task(
	entry, this.volumeManager_, this.thumbnailModel_,
	this.thumbnailLoaderConstructor_);

	const url = entry.toURL();
	this.active_[url] = task;

	task.fetch().then(thumbnail => {
	delete this.active_[url];
	this.cache_.put(url, thumbnail);
	this.dispatchThumbnailLoaded_(index, thumbnail);
	this.continue_();
	});
	}

	/**
	* Dispatches thumbnail loaded event.
	*
	* @param {number} index Index of an original image in the data model.
	* @param {!ListThumbnailLoader.ThumbnailData} thumbnail Thumbnail.
	*/
	dispatchThumbnailLoaded_(index, thumbnail) {
	// Update index if it's already invalid, i.e. index may be invalid if some
	// change had happened in the data model during thumbnail fetch.
	const item = this.dataModel_.item(index);
	if (item && item.toURL() !== thumbnail.fileUrl) {
	index = -1;
	for (let i = 0; i < this.dataModel_.length; i++) {
	if (this.dataModel_.item(i).toURL() === thumbnail.fileUrl) {
	index = i;
	break;
	}
	}
	}

	if (index > -1) {
	this.dispatchEvent(
	new ListThumbnailLoader.ThumbnailLoadedEvent(index, thumbnail));
	}
	}
	}

	/**
	* Cache size. Cache size must be larger than sum of high priority range size
	* and number of prefetch tasks.
	* @const {number}
	*/
	ListThumbnailLoader.CACHE_SIZE = 500;

	/**
	* Volume type for testing.
	* @const {string}
	*/
	ListThumbnailLoader.TEST_VOLUME_TYPE = 'test_volume_type';

	/**
	* Number of maximum active tasks for testing.
	* @type {number}
	*/
	ListThumbnailLoader.numOfMaxActiveTasksForTest = 2;

	/**
	* @typedef {(VolumeManagerCommon.VolumeType\|string)}
	*/
	ListThumbnailLoader.VolumeType;

	/**
	* Thumbnail loaded event.
	*/
	ListThumbnailLoader.ThumbnailLoadedEvent =
	class ThumbnailLoadedEvent extends Event {
	/**
	* @param {number} index Index of an original image in the current data
	* model.
	* @param {!ListThumbnailLoader.ThumbnailData} thumbnail Thumbnail.
	*/
	constructor(index, thumbnail) {
	super('thumbnailLoaded');

	/** @type {number} */
	this.index = index;

	/** @type {string}*/
	this.fileUrl = thumbnail.fileUrl;

	/** @type {?string} */
	this.dataUrl = thumbnail.dataUrl;

	/** @type {?number} */
	this.width = thumbnail.width;

	/** @type {?number}*/
	this.height = thumbnail.height;
	}
	};

	/**
	* A class to represent thumbnail data.
	*/
	ListThumbnailLoader.ThumbnailData = class {
	/**
	* @param {string} fileUrl File url of an original image.
	* @param {?string} dataUrl Data url of thumbnail.
	* @param {?number} width Width of thumbnail.
	* @param {?number} height Height of thumbnail.
	*/
	constructor(fileUrl, dataUrl, width, height) {
	/**
	* @const {string}
	*/
	this.fileUrl = fileUrl;

	/**
	* @const {?string}
	*/
	this.dataUrl = dataUrl;

	/**
	* @const {?number}
	*/
	this.width = width;

	/**
	* @const {?number}
	*/
	this.height = height;

	/**
	* @type {boolean}
	*/
	this.outdated = false;
	}
	};

	/**
	* A task to load thumbnail.
	*/
	ListThumbnailLoader.Task = class {
	/**
	*
	* @param {!Entry} entry An entry.
	* @param {!VolumeManager} volumeManager Volume manager.
	* @param {!ThumbnailModel} thumbnailModel Metadata cache.
	* @param {!Function} thumbnailLoaderConstructor A constructor of thumbnail
	* loader.
	*/
	constructor(
	entry, volumeManager, thumbnailModel, thumbnailLoaderConstructor) {
	this.entry_ = entry;
	this.volumeManager_ = volumeManager;
	this.thumbnailModel_ = thumbnailModel;
	this.thumbnailLoaderConstructor_ = thumbnailLoaderConstructor;
	}

	/**
	* Fetches thumbnail.
	*
	* @return {!Promise<!ListThumbnailLoader.ThumbnailData>} A promise which is
	* resolved when thumbnail data is fetched with either a success or an
	* error.
	*/
	fetch() {
	let ioError = false;
	return this.thumbnailModel_.get([this.entry_])
	.then(metadatas => {
	// When it failed to read exif header with an IO error, do not
	// generate thumbnail at this time since it may success in the second
	// try. If it failed to read at 0 byte, it would be an IO error.
	if (metadatas[0].thumbnail.urlError &&
	metadatas[0].thumbnail.urlError.errorDescription ===
	'Error: Unexpected EOF @0') {
	ioError = true;
	return Promise.reject();
	}
	return metadatas[0];
	})
	.then(metadata => {
	const loadTargets = [
	ThumbnailLoader.LoadTarget.CONTENT_METADATA,
	ThumbnailLoader.LoadTarget.EXTERNAL_METADATA
	];

	// If the file is on a network filesystem, don't generate thumbnails
	// from file entry, as it could cause very high network traffic.
	// Allow Drive to do so however, as ThumbnailLoader tries to generate
	// thumbnails of Drive files from file entry only if cached locally.
	const volumeInfo = this.volumeManager_.getVolumeInfo(this.entry_);
	if (volumeInfo &&
	(volumeInfo.source !== VolumeManagerCommon.Source.NETWORK \|\|
	volumeInfo.volumeType ===
	VolumeManagerCommon.VolumeType.DRIVE)) {
	loadTargets.push(ThumbnailLoader.LoadTarget.FILE_ENTRY);
	}

	return new this
	.thumbnailLoaderConstructor_(
	this.entry_, ThumbnailLoader.LoaderType.IMAGE, metadata,
	undefined /* opt_mediaType */, loadTargets)
	.loadAsDataUrl(ThumbnailLoader.FillMode.OVER_FILL);
	})
	.then(result => {
	return new ListThumbnailLoader.ThumbnailData(
	this.entry_.toURL(), result.data, result.width, result.height);
	})
	.catch(() => {
	// If an error happens during generating of a thumbnail, then return
	// an empty object, so we don't retry the thumbnail over and over
	// again.
	const thumbnailData = new ListThumbnailLoader.ThumbnailData(
	this.entry_.toURL(), null, null, null);
	if (ioError) {
	// If fetching a thumbnail from EXIF fails due to an IO error, then
	// try to refetch it in the future, but not earlier than in 3
	// second.
	setTimeout(() => {
	thumbnailData.outdated = true;
	}, ListThumbnailLoader.Task.EXIF_IO_ERROR_DELAY);
	}
	return thumbnailData;
	});
	}
	};

	/**
	* Minimum delay of milliseconds before another retry for fetching a
	* thumbnmail from EXIF after failing with an IO error. In milliseconds.
	*
	* @type {number}
	*/
	ListThumbnailLoader.Task.EXIF_IO_ERROR_DELAY = 3000;