/* vim: se cin sw=2 ts=2 et filetype=javascript :
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
 * This module implements the front end behavior for AeroPeek. Starting in
 * Windows Vista, the taskbar began showing live thumbnail previews of windows
 * when the user hovered over the window icon in the taskbar. Starting with
 * Windows 7, the taskbar allows an application to expose its tabbed interface
 * in the taskbar by showing thumbnail previews rather than the default window
 * preview. Additionally, when a user hovers over a thumbnail (tab or window),
 * they are shown a live preview of the window (or tab + its containing window).
 *
 * In Windows 7, a title, icon, close button and optional toolbar are shown for
 * each preview. This feature does not make use of the toolbar. For window
 * previews, the title is the window title and the icon the window icon. For
 * tab previews, the title is the page title and the page's favicon. In both
 * cases, the close button "does the right thing."
 *
 * The primary objects behind this feature are nsITaskbarTabPreview and
 * nsITaskbarPreviewController. Each preview has a controller. The controller
 * responds to the user's interactions on the taskbar and provides the required
 * data to the preview for determining the size of the tab and thumbnail. The
 * PreviewController class implements this interface. The preview will request
 * the controller to provide a thumbnail or preview when the user interacts with
 * the taskbar. To reduce the overhead of drawing the tab area, the controller
 * implementation caches the tab's contents in a <canvas> element. If no
 * previews or thumbnails have been requested for some time, the controller will
 * discard its cached tab contents.
 *
 * Screen real estate is limited so when there are too many thumbnails to fit
 * on the screen, the taskbar stops displaying thumbnails and instead displays
 * just the title, icon and close button in a similar fashion to previous
 * versions of the taskbar. If there are still too many previews to fit on the
 * screen, the taskbar resorts to a scroll up and scroll down button pair to let
 * the user scroll through the list of tabs. Since this is undoubtedly
 * inconvenient for users with many tabs, the AeroPeek objects turns off all of
 * the tab previews. This tells the taskbar to revert to one preview per window.
 * If the number of tabs falls below this magic threshold, the preview-per-tab
 * behavior returns. There is no reliable way to determine when the scroll
 * buttons appear on the taskbar, so a magic pref-controlled number determines
 * when this threshold has been crossed.
 */
import { NetUtil } from "resource://gre/modules/NetUtil.sys.mjs";
import { PlacesUtils } from "resource://gre/modules/PlacesUtils.sys.mjs";
import { PrivateBrowsingUtils } from "resource://gre/modules/PrivateBrowsingUtils.sys.mjs";
import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";

// Pref to enable/disable preview-per-tab
const TOGGLE_PREF_NAME = "browser.taskbar.previews.enable";
// Pref to determine the magic auto-disable threshold
const DISABLE_THRESHOLD_PREF_NAME = "browser.taskbar.previews.max";
// Pref to control the time in seconds that tab contents live in the cache
const CACHE_EXPIRATION_TIME_PREF_NAME = "browser.taskbar.previews.cachetime";

const WINTASKBAR_CONTRACTID = "@mozilla.org/windows-taskbar;1";

const lazy = {};

// Various utility properties
XPCOMUtils.defineLazyServiceGetter(
  lazy,
  "imgTools",
  "@mozilla.org/image/tools;1",
  "imgITools"
);
ChromeUtils.defineESModuleGetters(lazy, {
  PageThumbs: "resource://gre/modules/PageThumbs.sys.mjs",
});

// nsIURI -> imgIContainer
function _imageFromURI(uri, privateMode, callback) {
  let channel = NetUtil.newChannel({
    uri,
    loadUsingSystemPrincipal: true,
    contentPolicyType: Ci.nsIContentPolicy.TYPE_INTERNAL_IMAGE,
  });

  try {
    channel.QueryInterface(Ci.nsIPrivateBrowsingChannel);
    channel.setPrivate(privateMode);
  } catch (e) {
    // Ignore channels which do not support nsIPrivateBrowsingChannel
  }
  NetUtil.asyncFetch(channel, function (inputStream, resultCode) {
    if (!Components.isSuccessCode(resultCode)) {
      return;
    }

    const decodeCallback = {
      onImageReady(image, status) {
        if (!image) {
          // We failed, so use the default favicon (only if this wasn't the
          // default favicon).
          let defaultURI = PlacesUtils.favicons.defaultFavicon;
          if (!defaultURI.equals(uri)) {
            _imageFromURI(defaultURI, privateMode, callback);
            return;
          }
        }

        callback(image);
      },
    };

    try {
      let threadManager = Cc["@mozilla.org/thread-manager;1"].getService();
      lazy.imgTools.decodeImageAsync(
        inputStream,
        channel.contentType,
        decodeCallback,
        threadManager.currentThread
      );
    } catch (e) {
      // We failed, so use the default favicon (only if this wasn't the default
      // favicon).
      let defaultURI = PlacesUtils.favicons.defaultFavicon;
      if (!defaultURI.equals(uri)) {
        _imageFromURI(defaultURI, privateMode, callback);
      }
    }
  });
}

// string? -> imgIContainer
function getFaviconAsImage(iconurl, privateMode, callback) {
  if (iconurl) {
    _imageFromURI(NetUtil.newURI(iconurl), privateMode, callback);
  } else {
    _imageFromURI(PlacesUtils.favicons.defaultFavicon, privateMode, callback);
  }
}

// PreviewController

/*
 * This class manages the behavior of thumbnails and previews. It has the following
 * responsibilities:
 * 1) responding to requests from Windows taskbar for a thumbnail or window
 *    preview.
 * 2) listens for dom events that result in a thumbnail or window preview needing
 *    to be refresh, and communicates this to the taskbar.
 * 3) Handles querying and returning to the taskbar new thumbnail or window
 *    preview images through PageThumbs.
 *
 * @param win
 *        The TabWindow (see below) that owns the preview that this controls
 * @param tab
 *        The <tab> that this preview is associated with
 */
function PreviewController(win, tab) {
  this.win = win;
  this.tab = tab;
  this.linkedBrowser = tab.linkedBrowser;
  this.preview = this.win.createTabPreview(this);

  this.tab.addEventListener("TabAttrModified", this);

  XPCOMUtils.defineLazyGetter(this, "canvasPreview", function () {
    let canvas = lazy.PageThumbs.createCanvas(this.win.win);
    canvas.mozOpaque = true;
    return canvas;
  });
}

PreviewController.prototype = {
  QueryInterface: ChromeUtils.generateQI(["nsITaskbarPreviewController"]),

  destroy() {
    this.tab.removeEventListener("TabAttrModified", this);

    // Break cycles, otherwise we end up leaking the window with everything
    // attached to it.
    delete this.win;
    delete this.preview;
  },

  get wrappedJSObject() {
    return this;
  },

  // Resizes the canvasPreview to 0x0, essentially freeing its memory.
  resetCanvasPreview() {
    this.canvasPreview.width = 0;
    this.canvasPreview.height = 0;
  },

  /**
   * Set the canvas dimensions.
   */
  resizeCanvasPreview(aRequestedWidth, aRequestedHeight) {
    this.canvasPreview.width = aRequestedWidth;
    this.canvasPreview.height = aRequestedHeight;
  },

  get browserDims() {
    return this.tab.linkedBrowser.getBoundingClientRect();
  },

  cacheBrowserDims() {
    let dims = this.browserDims;
    this._cachedWidth = dims.width;
    this._cachedHeight = dims.height;
  },

  testCacheBrowserDims() {
    let dims = this.browserDims;
    return this._cachedWidth == dims.width && this._cachedHeight == dims.height;
  },

  /**
   * Capture a new thumbnail image for this preview. Called by the controller
   * in response to a request for a new thumbnail image.
   */
  updateCanvasPreview(aFullScale) {
    // Update our cached browser dims so that delayed resize
    // events don't trigger another invalidation if this tab becomes active.
    this.cacheBrowserDims();
    AeroPeek.resetCacheTimer();
    return lazy.PageThumbs.captureToCanvas(
      this.linkedBrowser,
      this.canvasPreview,
      {
        fullScale: aFullScale,
      }
    ).catch(console.error);
    // If we're updating the canvas, then we're in the middle of a peek so
    // don't discard the cache of previews.
  },

  updateTitleAndTooltip() {
    let title = this.win.tabbrowser.getWindowTitleForBrowser(
      this.linkedBrowser
    );
    this.preview.title = title;
    this.preview.tooltip = title;
  },

  // nsITaskbarPreviewController

  // window width and height, not browser
  get width() {
    return this.win.width;
  },

  // window width and height, not browser
  get height() {
    return this.win.height;
  },

  get thumbnailAspectRatio() {
    let browserDims = this.browserDims;
    // Avoid returning 0
    let tabWidth = browserDims.width || 1;
    // Avoid divide by 0
    let tabHeight = browserDims.height || 1;
    return tabWidth / tabHeight;
  },

  /**
   * Responds to taskbar requests for window previews. Returns the results asynchronously
   * through updateCanvasPreview.
   *
   * @param aTaskbarCallback nsITaskbarPreviewCallback results callback
   */
  requestPreview(aTaskbarCallback) {
    // Grab a high res content preview
    this.resetCanvasPreview();
    this.updateCanvasPreview(true).then(aPreviewCanvas => {
      let winWidth = this.win.width;
      let winHeight = this.win.height;

      let composite = lazy.PageThumbs.createCanvas(this.win.win);

      // Use transparency, Aero glass is drawn black without it.
      composite.mozOpaque = false;

      let ctx = composite.getContext("2d");
      let scale = this.win.win.devicePixelRatio;

      composite.width = winWidth * scale;
      composite.height = winHeight * scale;

      ctx.save();
      ctx.scale(scale, scale);

      // Draw chrome. Note we currently do not get scrollbars for remote frames
      // in the image above.
      ctx.drawWindow(this.win.win, 0, 0, winWidth, winHeight, "rgba(0,0,0,0)");

      // Draw the content are into the composite canvas at the right location.
      ctx.drawImage(
        aPreviewCanvas,
        this.browserDims.x,
        this.browserDims.y,
        aPreviewCanvas.width,
        aPreviewCanvas.height
      );
      ctx.restore();

      // Deliver the resulting composite canvas to Windows
      this.win.tabbrowser.previewTab(this.tab, function () {
        aTaskbarCallback.done(composite, false);
      });
    });
  },

  /**
   * Responds to taskbar requests for tab thumbnails. Returns the results asynchronously
   * through updateCanvasPreview.
   *
   * Note Windows requests a specific width and height here, if the resulting thumbnail
   * does not match these dimensions thumbnail display will fail.
   *
   * @param aTaskbarCallback nsITaskbarPreviewCallback results callback
   * @param aRequestedWidth width of the requested thumbnail
   * @param aRequestedHeight height of the requested thumbnail
   */
  requestThumbnail(aTaskbarCallback, aRequestedWidth, aRequestedHeight) {
    this.resizeCanvasPreview(aRequestedWidth, aRequestedHeight);
    this.updateCanvasPreview(false).then(aThumbnailCanvas => {
      aTaskbarCallback.done(aThumbnailCanvas, false);
    });
  },

  // Event handling

  onClose() {
    this.win.tabbrowser.removeTab(this.tab);
  },

  onActivate() {
    this.win.tabbrowser.selectedTab = this.tab;

    // Accept activation - this will restore the browser window
    // if it's minimized
    return true;
  },

  // EventListener
  handleEvent(evt) {
    switch (evt.type) {
      case "TabAttrModified":
        this.updateTitleAndTooltip();
        break;
    }
  },
};

// TabWindow

/*
 * This class monitors a browser window for changes to its tabs
 *
 * @param win
 *        The nsIDOMWindow browser window
 */
function TabWindow(win) {
  this.win = win;
  this.tabbrowser = win.gBrowser;

  this.previews = new Map();

  for (let i = 0; i < this.tabEvents.length; i++) {
    this.tabbrowser.tabContainer.addEventListener(this.tabEvents[i], this);
  }

  for (let i = 0; i < this.winEvents.length; i++) {
    this.win.addEventListener(this.winEvents[i], this);
  }

  this.tabbrowser.addTabsProgressListener(this);

  AeroPeek.windows.push(this);
  let tabs = this.tabbrowser.tabs;
  for (let i = 0; i < tabs.length; i++) {
    this.newTab(tabs[i]);
  }

  this.updateTabOrdering();
  AeroPeek.checkPreviewCount();
}

TabWindow.prototype = {
  _enabled: false,
  _cachedWidth: 0,
  _cachedHeight: 0,
  tabEvents: ["TabOpen", "TabClose", "TabSelect", "TabMove"],
  winEvents: ["resize"],

  destroy() {
    this._destroying = true;

    let tabs = this.tabbrowser.tabs;

    this.tabbrowser.removeTabsProgressListener(this);

    for (let i = 0; i < this.winEvents.length; i++) {
      this.win.removeEventListener(this.winEvents[i], this);
    }

    for (let i = 0; i < this.tabEvents.length; i++) {
      this.tabbrowser.tabContainer.removeEventListener(this.tabEvents[i], this);
    }

    for (let i = 0; i < tabs.length; i++) {
      this.removeTab(tabs[i]);
    }

    let idx = AeroPeek.windows.indexOf(this.win.gTaskbarTabGroup);
    AeroPeek.windows.splice(idx, 1);
    AeroPeek.checkPreviewCount();
  },

  get width() {
    return this.win.innerWidth;
  },
  get height() {
    return this.win.innerHeight;
  },

  cacheDims() {
    this._cachedWidth = this.width;
    this._cachedHeight = this.height;
  },

  testCacheDims() {
    return this._cachedWidth == this.width && this._cachedHeight == this.height;
  },

  // Invoked when the given tab is added to this window
  newTab(tab) {
    let controller = new PreviewController(this, tab);
    // It's OK to add the preview now while the favicon still loads.
    this.previews.set(tab, controller.preview);
    AeroPeek.addPreview(controller.preview);
    // updateTitleAndTooltip relies on having controller.preview which is lazily resolved.
    // Now that we've updated this.previews, it will resolve successfully.
    controller.updateTitleAndTooltip();
  },

  createTabPreview(controller) {
    let docShell = this.win.docShell;
    let preview = AeroPeek.taskbar.createTaskbarTabPreview(
      docShell,
      controller
    );
    preview.visible = AeroPeek.enabled;
    let { tab } = controller;
    preview.active = this.tabbrowser.selectedTab == tab;
    this.updateFavicon(tab, tab.getAttribute("image"));
    return preview;
  },

  // Invoked when the given tab is closed
  removeTab(tab) {
    let preview = this.previewFromTab(tab);
    preview.active = false;
    preview.visible = false;
    preview.move(null);
    preview.controller.wrappedJSObject.destroy();

    this.previews.delete(tab);
    AeroPeek.removePreview(preview);
  },

  get enabled() {
    return this._enabled;
  },

  set enabled(enable) {
    this._enabled = enable;
    // Because making a tab visible requires that the tab it is next to be
    // visible, it is far simpler to unset the 'next' tab and recreate them all
    // at once.
    for (let [, preview] of this.previews) {
      preview.move(null);
      preview.visible = enable;
    }
    this.updateTabOrdering();
  },

  previewFromTab(tab) {
    return this.previews.get(tab);
  },

  updateTabOrdering() {
    let previews = this.previews;
    let tabs = this.tabbrowser.tabs;

    // Previews are internally stored using a map, so we need to iterate the
    // tabbrowser's array of tabs to retrieve previews in the same order.
    let inorder = [];
    for (let t of tabs) {
      if (previews.has(t)) {
        inorder.push(previews.get(t));
      }
    }

    // Since the internal taskbar array has not yet been updated we must force
    // on it the sorting order of our local array.  To do so we must walk
    // the local array backwards, otherwise we would send move requests in the
    // wrong order.  See bug 522610 for details.
    for (let i = inorder.length - 1; i >= 0; i--) {
      inorder[i].move(inorder[i + 1] || null);
    }
  },

  // EventListener
  handleEvent(evt) {
    let tab = evt.originalTarget;
    switch (evt.type) {
      case "TabOpen":
        this.newTab(tab);
        this.updateTabOrdering();
        break;
      case "TabClose":
        this.removeTab(tab);
        this.updateTabOrdering();
        break;
      case "TabSelect":
        this.previewFromTab(tab).active = true;
        break;
      case "TabMove":
        this.updateTabOrdering();
        break;
      case "resize":
        if (!AeroPeek._prefenabled) {
          return;
        }
        this.onResize();
        break;
    }
  },

  // Set or reset a timer that will invalidate visible thumbnails soon.
  setInvalidationTimer() {
    if (!this.invalidateTimer) {
      this.invalidateTimer = Cc["@mozilla.org/timer;1"].createInstance(
        Ci.nsITimer
      );
    }
    this.invalidateTimer.cancel();

    // delay 1 second before invalidating
    this.invalidateTimer.initWithCallback(
      () => {
        // invalidate every preview. note the internal implementation of
        // invalidate ignores thumbnails that aren't visible.
        this.previews.forEach(function (aPreview) {
          let controller = aPreview.controller.wrappedJSObject;
          if (!controller.testCacheBrowserDims()) {
            controller.cacheBrowserDims();
            aPreview.invalidate();
          }
        });
      },
      1000,
      Ci.nsITimer.TYPE_ONE_SHOT
    );
  },

  onResize() {
    // Specific to a window.

    // Call invalidate on each tab thumbnail so that Windows will request an
    // updated image. However don't do this repeatedly across multiple resize
    // events triggered during window border drags.

    if (this.testCacheDims()) {
      return;
    }

    // update the window dims on our TabWindow object.
    this.cacheDims();

    // invalidate soon
    this.setInvalidationTimer();
  },

  invalidateTabPreview(aBrowser) {
    for (let [tab, preview] of this.previews) {
      if (aBrowser == tab.linkedBrowser) {
        preview.invalidate();
        break;
      }
    }
  },

  // Browser progress listener

  onLocationChange(aBrowser) {
    // I'm not sure we need this, onStateChange does a really good job
    // of picking up page changes.
    // this.invalidateTabPreview(aBrowser);
  },

  onStateChange(aBrowser, aWebProgress, aRequest, aStateFlags, aStatus) {
    if (
      aStateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
      aStateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK
    ) {
      this.invalidateTabPreview(aBrowser);
    }
  },

  directRequestProtocols: new Set([
    "file",
    "chrome",
    "resource",
    "about",
    "data",
  ]),
  onLinkIconAvailable(aBrowser, aIconURL) {
    let tab = this.win.gBrowser.getTabForBrowser(aBrowser);
    this.updateFavicon(tab, aIconURL);
  },
  updateFavicon(aTab, aIconURL) {
    let requestURL = null;
    if (aIconURL) {
      let shouldRequestFaviconURL = true;
      try {
        let urlObject = NetUtil.newURI(aIconURL);
        shouldRequestFaviconURL = !this.directRequestProtocols.has(
          urlObject.scheme
        );
      } catch (ex) {}

      requestURL = shouldRequestFaviconURL
        ? "moz-anno:favicon:" + aIconURL
        : aIconURL;
    }
    let isDefaultFavicon = !requestURL;
    getFaviconAsImage(
      requestURL,
      PrivateBrowsingUtils.isWindowPrivate(this.win),
      img => {
        // The tab could have closed, and there's no guarantee the icons
        // will have finished fetching 'in order'.
        if (this.win.closed || aTab.closing || !aTab.linkedBrowser) {
          return;
        }
        // Note that bizarrely, we can get to updateFavicon via a sync codepath
        // where the new preview controller hasn't yet been added to the
        // window's map of previews. So `preview` would be null here - except
        // getFaviconAsImage is async so that should never happen, as we add
        // the controller to the preview collection straight after creating it.
        // However, if any of this code ever tries to access this
        // synchronously, that won't work.
        let preview = this.previews.get(aTab);
        if (
          aTab.getAttribute("image") == aIconURL ||
          (!preview.icon && isDefaultFavicon)
        ) {
          preview.icon = img;
        }
      }
    );
  },
};

// AeroPeek

/*
 * This object acts as global storage and external interface for this feature.
 * It maintains the values of the prefs.
 */
export var AeroPeek = {
  available: false,
  // Does the pref say we're enabled?
  __prefenabled: false,

  _enabled: true,

  initialized: false,

  // nsITaskbarTabPreview array
  previews: [],

  // TabWindow array
  windows: [],

  // nsIWinTaskbar service
  taskbar: null,

  // Maximum number of previews
  maxpreviews: 20,

  // Length of time in seconds that previews are cached
  cacheLifespan: 20,

  initialize() {
    if (!(WINTASKBAR_CONTRACTID in Cc)) {
      return;
    }
    this.taskbar = Cc[WINTASKBAR_CONTRACTID].getService(Ci.nsIWinTaskbar);
    this.available = this.taskbar.available;
    if (!this.available) {
      return;
    }

    Services.prefs.addObserver(TOGGLE_PREF_NAME, this, true);
    this.enabled = this._prefenabled =
      Services.prefs.getBoolPref(TOGGLE_PREF_NAME);
    this.initialized = true;
  },

  destroy: function destroy() {
    this._enabled = false;

    if (this.cacheTimer) {
      this.cacheTimer.cancel();
    }
  },

  get enabled() {
    return this._enabled;
  },

  set enabled(enable) {
    if (this._enabled == enable) {
      return;
    }

    this._enabled = enable;

    this.windows.forEach(function (win) {
      win.enabled = enable;
    });
  },

  get _prefenabled() {
    return this.__prefenabled;
  },

  set _prefenabled(enable) {
    if (enable == this.__prefenabled) {
      return;
    }
    this.__prefenabled = enable;

    if (enable) {
      this.enable();
    } else {
      this.disable();
    }
  },

  _observersAdded: false,

  enable() {
    if (!this._observersAdded) {
      Services.prefs.addObserver(DISABLE_THRESHOLD_PREF_NAME, this, true);
      Services.prefs.addObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this, true);
      this._placesListener = this.handlePlacesEvents.bind(this);
      PlacesUtils.observers.addListener(
        ["favicon-changed"],
        this._placesListener
      );
      this._observersAdded = true;
    }

    this.cacheLifespan = Services.prefs.getIntPref(
      CACHE_EXPIRATION_TIME_PREF_NAME
    );

    this.maxpreviews = Services.prefs.getIntPref(DISABLE_THRESHOLD_PREF_NAME);

    // If the user toggled us on/off while the browser was already up
    // (rather than this code running on startup because the pref was
    // already set to true), we must initialize previews for open windows:
    if (this.initialized) {
      for (let win of Services.wm.getEnumerator("navigator:browser")) {
        if (!win.closed) {
          this.onOpenWindow(win);
        }
      }
    }
  },

  disable() {
    while (this.windows.length) {
      // We can't call onCloseWindow here because it'll bail if we're not
      // enabled.
      let tabWinObject = this.windows[0];
      tabWinObject.destroy(); // This will remove us from the array.
      delete tabWinObject.win.gTaskbarTabGroup; // Tidy up the window.
    }
    PlacesUtils.observers.removeListener(
      ["favicon-changed"],
      this._placesListener
    );
  },

  addPreview(preview) {
    this.previews.push(preview);
    this.checkPreviewCount();
  },

  removePreview(preview) {
    let idx = this.previews.indexOf(preview);
    this.previews.splice(idx, 1);
    this.checkPreviewCount();
  },

  checkPreviewCount() {
    if (!this._prefenabled) {
      return;
    }
    this.enabled = this.previews.length <= this.maxpreviews;
  },

  onOpenWindow(win) {
    // This occurs when the taskbar service is not available (xp, vista)
    if (!this.available || !this._prefenabled) {
      return;
    }

    win.gTaskbarTabGroup = new TabWindow(win);
  },

  onCloseWindow(win) {
    // This occurs when the taskbar service is not available (xp, vista)
    if (!this.available || !this._prefenabled) {
      return;
    }

    win.gTaskbarTabGroup.destroy();
    delete win.gTaskbarTabGroup;

    if (!this.windows.length) {
      this.destroy();
    }
  },

  resetCacheTimer() {
    this.cacheTimer.cancel();
    this.cacheTimer.init(
      this,
      1000 * this.cacheLifespan,
      Ci.nsITimer.TYPE_ONE_SHOT
    );
  },

  // nsIObserver
  observe(aSubject, aTopic, aData) {
    if (aTopic == "nsPref:changed" && aData == TOGGLE_PREF_NAME) {
      this._prefenabled = Services.prefs.getBoolPref(TOGGLE_PREF_NAME);
    }
    if (!this._prefenabled) {
      return;
    }
    switch (aTopic) {
      case "nsPref:changed":
        if (aData == CACHE_EXPIRATION_TIME_PREF_NAME) {
          break;
        }

        if (aData == DISABLE_THRESHOLD_PREF_NAME) {
          this.maxpreviews = Services.prefs.getIntPref(
            DISABLE_THRESHOLD_PREF_NAME
          );
        }
        // Might need to enable/disable ourselves
        this.checkPreviewCount();
        break;
      case "timer-callback":
        this.previews.forEach(function (preview) {
          let controller = preview.controller.wrappedJSObject;
          controller.resetCanvasPreview();
        });
        break;
    }
  },

  handlePlacesEvents(events) {
    for (let event of events) {
      switch (event.type) {
        case "favicon-changed": {
          for (let win of this.windows) {
            for (let [tab] of win.previews) {
              if (tab.getAttribute("image") == event.faviconUrl) {
                win.updateFavicon(tab, event.faviconUrl);
              }
            }
          }
        }
      }
    }
  },

  QueryInterface: ChromeUtils.generateQI([
    "nsISupportsWeakReference",
    "nsIObserver",
  ]),
};

XPCOMUtils.defineLazyGetter(AeroPeek, "cacheTimer", () =>
  Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer)
);

AeroPeek.initialize();
