Introduce Image loader extension.

chrome/browser/resources/image_loader/client.js

Index: chrome/browser/resources/image_loader/client.js
diff --git a/chrome/browser/resources/image_loader/client.js b/chrome/browser/resources/image_loader/client.js
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+// Copyright 2013 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.
+
+var ImageLoader = ImageLoader || {};
+
+/**
+ * Image loader's extension id.
+ * @const
+ * @type {string}
+ */
+ImageLoader.EXTENSION_ID = 'pmfjbimdmchhbnneeidfognadeopoehp';
+
+/**
+ * Client used to connect to the remote ImageLoader extension. Client class runs
+ * in the extension, where the client.js is included (eg. Files.app).
+ * It sends remote requests using IPC to the ImageLoader class and forwards
+ * its responses.
+ *
+ * Implements cache, which is stored in the calling extension.
+ *
+ * @constructor
+ */
+ImageLoader.Client = function() {
+  /**
+   * @type {Port}
+   * @private
+   */
+  this.port_ = chrome.extension.connect(ImageLoader.EXTENSION_ID);
+  this.port_.onMessage.addListener(this.handleMessage_.bind(this));
+
+  /**
+   * Hash array with active tasks.
+   * @type {Object}
+   * @private
+   */
+  this.tasks_ = {};
+
+  /**
+   * @type {number}
+   * @private
+   */
+  this.lastTaskId_ = 0;
+
+  /**
+   * LRU cache for images.
+   * @type {ImageLoader.Client.Cache}
+   * @private
+   */
+  this.cache_ = new ImageLoader.Client.Cache();
+};
+
+/**
+ * Returns a singleton instance.
+ * @return {ImageLoader.Client} ImageLoader.Client instance.
+ */
+ImageLoader.Client.getInstance = function() {
+  if (!ImageLoader.Client.instance_)
+    ImageLoader.Client.instance_ = new ImageLoader.Client();
+  return ImageLoader.Client.instance_;
+};
+
+/**
+ * Handles a message from the remote image loader and calls the registered
+ * callback to pass the response back to the requester.
+ *
+ * @param {Object} message Response message as a hash array.
+ * @private
+ */
+ImageLoader.Client.prototype.handleMessage_ = function(message) {
+  if (!(message.taskId in this.tasks_)) {
+    // This task has been canceled, but was already fetched, so it's result
+    // should be discarded anyway.
+    return;
+  }
+
+  var task = this.tasks_[message.taskId];
+
+  // Check if the task is still valid.
+  if (task.isValid())
+    task.accept(message);
+
+  delete this.tasks_[message.taskId];
+};
+
+/**
+ * Loads and resizes and image. Use opt_isValid to easily cancel requests
+ * which are not valid anymore, which will reduce cpu consumption.
+ *
+ * @param {string} url Url of the requested image.
+ * @param {function} callback Callback used to return response.
+ * @param {Object=} opt_options Loader options, such as: scale, maxHeight,
+ *     width, height and/or cache.
+ * @param {function=} opt_isValid Function returning false in case
+ *     a request is not valid anymore, eg. parent node has been detached.
+ * @return {?number} Remote task id or null if loaded from cache.
+ */
+ImageLoader.Client.prototype.load = function(
+    url, callback, opt_options, opt_isValid) {
+  opt_options = opt_options || {};
+  opt_isValid = opt_isValid || function() { return true; };
+
+  // Cancel old, invalid tasks.
+  var taskKeys = Object.keys(this.tasks_);
+  for (var index = 0; index < taskKeys.length; index++) {
+    var taskKey = taskKeys[index];
+    var task = this.tasks_[taskKey];
+    if (!task.isValid()) {
+      // Cancel this task since it is not valid anymore.
+      this.cancel(taskKey);
+      delete this.tasks_[taskKey];
+    }
+  }
+
+  // Replace the extension id.
+  var sourceId = chrome.i18n.getMessage('@@extension_id');
+  var targetId = ImageLoader.EXTENSION_ID;
+
+  url = url.replace('filesystem:chrome-extension://' + sourceId,
+                    'filesystem:chrome-extension://' + targetId);
+
+  // Try to load from cache, if available.
+  var cacheKey = ImageLoader.Client.Cache.createKey(url, opt_options);
+  if (opt_options.cache) {
+    // Load from cache.
+    // TODO(mtomasz): Add cache invalidating if the file has changed.
+    var cachedData = this.cache_.loadImage(cacheKey);
+    if (cachedData) {
+      callback({ status: 'success', data: cachedData });
+      return null;
+    }
+  } else {
+    // Remove from cache.
+    this.cache_.removeImage(cacheKey);
+  }
+
+  // Not available in cache, performing a request to a remote extension.
+  request = opt_options;
+  this.lastTaskId_++;
+  var task = { isValid: opt_isValid, accept: function(result) {
+    // Save to cache.
+    if (result.status == 'success' && opt_options.cache)
+      this.cache_.saveImage(cacheKey, result.data);
+    callback(result);
+  }.bind(this) };
+  this.tasks_[this.lastTaskId_] = task;
+
+  request.url = url;
+  request.taskId = this.lastTaskId_;
+
+  this.port_.postMessage(request);
+  return request.taskId;
+};
+
+/**
+ * Cancels the request.
+ * @param {number} taskId Task id returned by ImageLoader.Client.load().
+ */
+ImageLoader.Client.prototype.cancel = function(taskId) {
+  this.port_.postMessage({ taskId: taskId, cancel: true });
+};
+
+/**
+ * Prints the cache usage statistics.
+ */
+ImageLoader.Client.prototype.stat = function() {
+  this.cache_.stat();
+};
+
+/**
+ * Least Recently Used (LRU) cache implementation to be used by
+ * ImageLoader.Client class. It has memory constraints, so it will never
+ * exceed specified memory limit defined in MEMORY_LIMIT.
+ *
+ * @constructor
+ */
+ImageLoader.Client.Cache = function() {
+  this.images_ = [];
+  this.size_ = 0;
+};
+
+/**
+ * Memory limit for images data in bytes.
+ *
+ * @const
+ * @type {number}
+ */
+ImageLoader.Client.Cache.MEMORY_LIMIT = 100 * 1024 * 1024;  // 100 MB.
+
+/**
+ * Creates a cache key.
+ *
+ * @param {string} url Image url.
+ * @param {Object=} opt_options Loader options as a hash array.
+ * @return {string} Cache key.
+ */
+ImageLoader.Client.Cache.createKey = function(url, opt_options) {
+  var array = opt_options || {};
+  array.url = url;
+  return JSON.stringify(array);
+};
+
+/**
+ * Evicts the least used elements in cache to make space for a new image.
+ *
+ * @param {number} size Requested size.
+ * @private
+ */
+ImageLoader.Client.Cache.prototype.evictCache_ = function(size) {
+  // Sort from the most recent to the oldest.
+  this.images_.sort(function(a, b) {
+    return b.timestamp - a.timestamp;
+  });
+
+  while (this.images_.length > 0 &&
+         (ImageLoader.Client.Cache.MEMORY_LIMIT - this.size_ < size)) {
+    var entry = this.images_.pop();
+    this.size_ -= entry.data.length;
+  }
+};
+
+/**
+ * Saves an image in the cache.
+ *
+ * @param {string} key Cache key.
+ * @param {string} data Image data.
+ */
+ImageLoader.Client.Cache.prototype.saveImage = function(key, data) {
+  this.evictCache_(data.length);
+  if (ImageLoader.Client.Cache.MEMORY_LIMIT - this.size_ >= data.length) {
+    this.images_[key] = { timestamp: Date.now(), data: data };
+    this.size_ += data.length;
+  }
+};
+
+/**
+ * Loads an image from the cache (if available) or returns null.
+ *
+ * @param {string} key Cache key.
+ * @return {?string} Data of the loaded image or null.
+ */
+ImageLoader.Client.Cache.prototype.loadImage = function(key) {
+  if (!(key in this.images_))
+    return null;
+
+  var entry = this.images_[key];
+  entry.timestamp = Date.now();
+  return entry.data;
+};
+
+/**
+ * Prints the cache usage stats.
+ */
+ImageLoader.Client.Cache.prototype.stat = function() {
+  console.log('Cache entries: ' + Object.keys(this.images_).length);
+  console.log('Usage: ' + Math.round(this.size_ /
+              ImageLoader.Client.Cache.MEMORY_LIMIT * 100.0) + '%');
+};
+
+/**
+ * Removes the image from the cache.
+ * @param {string} key Cache key.
+ */
+ImageLoader.Client.Cache.prototype.removeImage = function(key) {
+  if (!(key in this.images_))
+    return;
+
+  var entry = this.images_[key];
+  this.size_ -= entry.data.length;
+  delete this.images_[key];
+};
+
+// Helper functions.
+
+/**
+ * Loads and resizes and image. Use opt_isValid to easily cancel requests
+ * which are not valid anymore, which will reduce cpu consumption.
+ *
+ * @param {string} url Url of the requested image.
+ * @param {Image} image Image node to load the requested picture into.
+ * @param {Object} options Loader options, such as: scale, maxHeight, width,
+ *     height and/or cache.
+ * @param {function=} onSuccess Callback for success.
+ * @param {function=} onError Callback for failure.
+ * @param {function=} opt_isValid Function returning false in case
+ *     a request is not valid anymore, eg. parent node has been detached.
+ * @return {?number} Remote task id or null if loaded from cache.
+ */
+ImageLoader.Client.loadToImage = function(url, image, options, onSuccess,
+    onError, opt_isValid) {
+  var callback = function(result) {
+    if (result.status == 'error') {
+      onError();
+      return;
+    }
+    image.src = result.data;
+    onSuccess();
+  };
+
+  return ImageLoader.Client.getInstance().load(
+      url, callback, options, opt_isValid);
+};