Add Pale Moon

application/palemoon/modules/WindowsPreviewPerTab.jsm

@@ -0,0 +1,708 @@
+/* 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.
+ */
+this.EXPORTED_SYMBOLS = ["AeroPeek"];
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/NetUtil.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+
+// 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";
+
+////////////////////////////////////////////////////////////////////////////////
+//// Various utility properties
+XPCOMUtils.defineLazyServiceGetter(this, "ioSvc",
+                                   "@mozilla.org/network/io-service;1",
+                                   "nsIIOService");
+XPCOMUtils.defineLazyServiceGetter(this, "imgTools",
+                                   "@mozilla.org/image/tools;1",
+                                   "imgITools");
+XPCOMUtils.defineLazyServiceGetter(this, "faviconSvc",
+                                   "@mozilla.org/browser/favicon-service;1",
+                                   "nsIFaviconService");
+
+// nsIURI -> imgIContainer
+function _imageFromURI(uri, privateMode, callback) {
+  let channel = ioSvc.newChannelFromURI(uri);
+  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;
+    try {
+      let out_img = { value: null };
+      imgTools.decodeImageData(inputStream, channel.contentType, out_img);
+      callback(out_img.value);
+    } catch (e) {
+      // We failed, so use the default favicon (only if this wasn't the default
+      // favicon).
+      let defaultURI = faviconSvc.defaultFavicon;
+      if (!defaultURI.equals(uri))
+        _imageFromURI(defaultURI, callback);
+    }
+  });
+}
+
+// string? -> imgIContainer
+function getFaviconAsImage(iconurl, privateMode, callback) {
+  if (iconurl)
+    _imageFromURI(NetUtil.newURI(iconurl), privateMode, callback);
+  else
+    _imageFromURI(faviconSvc.defaultFavicon, privateMode, callback);
+}
+
+// Snaps the given rectangle to be pixel-aligned at the given scale
+function snapRectAtScale(r, scale) {
+  let x = Math.floor(r.x * scale);
+  let y = Math.floor(r.y * scale);
+  let width = Math.ceil((r.x + r.width) * scale) - x;
+  let height = Math.ceil((r.y + r.height) * scale) - y;
+
+  r.x = x / scale;
+  r.y = y / scale;
+  r.width = width / scale;
+  r.height = height / scale;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//// PreviewController
+
+/*
+ * This class manages the behavior of the preview.
+ *
+ * To give greater performance when drawing, the dirty areas of the content
+ * window are tracked and drawn on demand into a canvas of the same size.
+ * This provides a great increase in responsiveness when drawing a preview
+ * for unchanged (or even only slightly changed) tabs.
+ *
+ * @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.linkedBrowser.addEventListener("MozAfterPaint", this, false);
+  this.tab.addEventListener("TabAttrModified", this, false);
+
+  XPCOMUtils.defineLazyGetter(this, "canvasPreview", function () {
+    let canvas = this.win.win.document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
+    canvas.mozOpaque = true;
+    return canvas;
+  });
+
+  XPCOMUtils.defineLazyGetter(this, "dirtyRegion",
+    function () {
+      let dirtyRegion = Cc["@mozilla.org/gfx/region;1"]
+                       .createInstance(Ci.nsIScriptableRegion);
+      dirtyRegion.init();
+      return dirtyRegion;
+    });
+
+  XPCOMUtils.defineLazyGetter(this, "winutils",
+    function () {
+      let win = tab.linkedBrowser.contentWindow;
+      return win.QueryInterface(Ci.nsIInterfaceRequestor)
+                .getInterface(Ci.nsIDOMWindowUtils);
+  });
+}
+
+PreviewController.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsITaskbarPreviewController,
+                                         Ci.nsIDOMEventListener]),
+  destroy: function () {
+    this.tab.removeEventListener("TabAttrModified", this, false);
+    this.linkedBrowser.removeEventListener("MozAfterPaint", this, false);
+
+    // Break cycles, otherwise we end up leaking the window with everything
+    // attached to it.
+    delete this.win;
+    delete this.preview;
+    delete this.dirtyRegion;
+  },
+  get wrappedJSObject() {
+    return this;
+  },
+
+  get dirtyRects() {
+    let rectstream = this.dirtyRegion.getRects();
+    if (!rectstream)
+      return [];
+    let rects = [];
+    for (let i = 0; i < rectstream.length; i+= 4) {
+      let r = {x:      rectstream[i],
+               y:      rectstream[i+1],
+               width:  rectstream[i+2],
+               height: rectstream[i+3]};
+      rects.push(r);
+    }
+    return rects;
+  },
+
+  // Resizes the canvasPreview to 0x0, essentially freeing its memory.
+  // updateCanvasPreview() will detect the size mismatch as a resize event
+  // the next time it is called.
+  resetCanvasPreview: function () {
+    this.canvasPreview.width = 0;
+    this.canvasPreview.height = 0;
+  },
+
+  get zoom() {
+    // Note that winutils.fullZoom accounts for "quantization" of the zoom factor
+    // from nsIMarkupDocumentViewer due to conversion through appUnits.
+    // We do -not- want screenPixelsPerCSSPixel here, because that would -also-
+    // incorporate any scaling that is applied due to hi-dpi resolution options.
+    return this.winutils.fullZoom;
+  },
+
+  // Updates the controller's canvas with the parts of the <browser> that need
+  // to be redrawn.
+  updateCanvasPreview: function () {
+    let win = this.linkedBrowser.contentWindow;
+    let bx = this.linkedBrowser.boxObject;
+    // Check for resize
+    if (bx.width != this.canvasPreview.width ||
+        bx.height != this.canvasPreview.height) {
+      // Invalidate the entire area and repaint
+      this.onTabPaint({left:0, top:0, right:win.innerWidth, bottom:win.innerHeight});
+      this.canvasPreview.width = bx.width;
+      this.canvasPreview.height = bx.height;
+    }
+
+    // Draw dirty regions
+    let ctx = this.canvasPreview.getContext("2d");
+    let scale = this.zoom;
+
+    let flags = this.canvasPreviewFlags;
+    // The dirty region may include parts that are offscreen so we clip to the
+    // canvas area.
+    this.dirtyRegion.intersectRect(0, 0, win.innerWidth, win.innerHeight);
+    this.dirtyRects.forEach(function (r) {
+      // We need to snap the rectangle to be pixel aligned in the destination
+      // coordinate space. Otherwise natively themed widgets might not draw.
+      snapRectAtScale(r, scale);
+      let x = r.x;
+      let y = r.y;
+      let width = r.width;
+      let height = r.height;
+
+      ctx.save();
+      ctx.scale(scale, scale);
+      ctx.translate(x, y);
+      ctx.drawWindow(win, x, y, width, height, "white", flags);
+      ctx.restore();
+    });
+    this.dirtyRegion.setToRect(0,0,0,0);
+
+    // If we're updating the canvas, then we're in the middle of a peek so
+    // don't discard the cache of previews.
+    AeroPeek.resetCacheTimer();
+  },
+
+  onTabPaint: function (rect) {
+    let x = Math.floor(rect.left),
+        y = Math.floor(rect.top),
+        width = Math.ceil(rect.right) - x,
+        height = Math.ceil(rect.bottom) - y;
+    this.dirtyRegion.unionRect(x, y, width, height);
+  },
+
+  updateTitleAndTooltip: function () {
+    let title = this.win.tabbrowser.getWindowTitleForBrowser(this.linkedBrowser);
+    this.preview.title = title;
+    this.preview.tooltip = title;
+  },
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// nsITaskbarPreviewController 
+
+  get width() {
+    return this.win.width;
+  },
+
+  get height() {
+    return this.win.height;
+  },
+
+  get thumbnailAspectRatio() {
+    let boxObject = this.tab.linkedBrowser.boxObject;
+    // Avoid returning 0
+    let tabWidth = boxObject.width || 1;
+    // Avoid divide by 0
+    let tabHeight = boxObject.height || 1;
+    return tabWidth / tabHeight;
+  },
+
+  drawPreview: function (ctx) {
+    let self = this;
+    this.win.tabbrowser.previewTab(this.tab, function () self.previewTabCallback(ctx));
+
+    // We must avoid having the frame drawn around the window. See bug 520807
+    return false;
+  },
+
+  previewTabCallback: function (ctx) {
+    // This will extract the resolution-scale component of the scaling we need,
+    // which should be applied to both chrome and content;
+    // the page zoom component is applied (to content only) within updateCanvasPreview.
+    let scale = this.winutils.screenPixelsPerCSSPixel / this.winutils.fullZoom;
+    ctx.save();
+    ctx.scale(scale, scale);
+    let width = this.win.width;
+    let height = this.win.height;
+    // Draw our toplevel window
+    ctx.drawWindow(this.win.win, 0, 0, width, height, "transparent");
+
+    // XXX (jfkthame): Pending tabs don't seem to draw with the proper scaling
+    // unless we use this block of code; but doing this for "normal" (loaded) tabs
+    // results in blurry rendering on hidpi systems, so we avoid it if possible.
+    // I don't understand why pending and loaded tabs behave differently here...
+    // (see bug 857061).
+    if (this.tab.hasAttribute("pending")) {
+      // Compositor, where art thou?
+      // Draw the tab content on top of the toplevel window
+      this.updateCanvasPreview();
+
+      let boxObject = this.linkedBrowser.boxObject;
+      ctx.translate(boxObject.x, boxObject.y);
+      ctx.drawImage(this.canvasPreview, 0, 0);
+    }
+
+    ctx.restore();
+  },
+
+  drawThumbnail: function (ctx, width, height) {
+    this.updateCanvasPreview();
+
+    let scale = width/this.linkedBrowser.boxObject.width;
+    ctx.scale(scale, scale);
+    ctx.drawImage(this.canvasPreview, 0, 0);
+
+    // Don't draw a frame around the thumbnail
+    return false;
+  },
+
+  onClose: function () {
+    this.win.tabbrowser.removeTab(this.tab);
+  },
+
+  onActivate: function () {
+    this.win.tabbrowser.selectedTab = this.tab;
+
+    // Accept activation - this will restore the browser window
+    // if it's minimized
+    return true;
+  },
+
+  //// nsIDOMEventListener
+  handleEvent: function (evt) {
+    switch (evt.type) {
+      case "MozAfterPaint":
+        if (evt.originalTarget === this.linkedBrowser.contentWindow) {
+          let clientRects = evt.clientRects;
+          let length = clientRects.length;
+          for (let i = 0; i < length; i++) {
+            let r = clientRects.item(i);
+            this.onTabPaint(r);
+          }
+        }
+        let preview = this.preview;
+        if (preview.visible)
+          preview.invalidate();
+        break;
+      case "TabAttrModified":
+        this.updateTitleAndTooltip();
+        break;
+    }
+  }
+};
+
+XPCOMUtils.defineLazyGetter(PreviewController.prototype, "canvasPreviewFlags",
+  function () { let canvasInterface = Ci.nsIDOMCanvasRenderingContext2D;
+                return canvasInterface.DRAWWINDOW_DRAW_VIEW
+                     | canvasInterface.DRAWWINDOW_DRAW_CARET
+                     | canvasInterface.DRAWWINDOW_ASYNC_DECODE_IMAGES
+                     | canvasInterface.DRAWWINDOW_DO_NOT_FLUSH;
+});
+
+////////////////////////////////////////////////////////////////////////////////
+//// 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, false);
+  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,
+  tabEvents: ["TabOpen", "TabClose", "TabSelect", "TabMove"],
+
+  destroy: function () {
+    this._destroying = true;
+
+    let tabs = this.tabbrowser.tabs;
+
+    this.tabbrowser.removeTabsProgressListener(this);
+    for (let i = 0; i < this.tabEvents.length; i++)
+      this.tabbrowser.tabContainer.removeEventListener(this.tabEvents[i], this, false);
+
+    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;
+  },
+
+  // Invoked when the given tab is added to this window
+  newTab: function (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: function (controller) {
+    let docShell = this.win
+                  .QueryInterface(Ci.nsIInterfaceRequestor)
+                  .getInterface(Ci.nsIWebNavigation)
+                  .QueryInterface(Ci.nsIDocShell);
+    let preview = AeroPeek.taskbar.createTaskbarTabPreview(docShell, controller);
+    preview.visible = AeroPeek.enabled;
+    preview.active = this.tabbrowser.selectedTab == controller.tab;
+    // Grab the default favicon
+    getFaviconAsImage(null, PrivateBrowsingUtils.isWindowPrivate(this.win), function (img) {
+      // It is possible that we've already gotten the real favicon, so make sure
+      // we have not set one before setting this default one.
+      if (!preview.icon)
+        preview.icon = img;
+    });
+
+    return preview;
+  },
+
+  // Invoked when the given tab is closed
+  removeTab: function (tab) {
+    let preview = this.previewFromTab(tab);
+    preview.active = false;
+    preview.visible = false;
+    preview.move(null);
+    preview.controller.wrappedJSObject.destroy();
+
+    // We don't want to splice from the array if the tabs aren't being removed
+    // from the tab bar as well (as is the case when the window closes).
+    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 [tab, preview] of this.previews) {
+      preview.move(null);
+      preview.visible = enable;
+    }
+    this.updateTabOrdering();
+  },
+
+  previewFromTab: function (tab) {
+    return this.previews.get(tab);
+  },
+
+  updateTabOrdering: function () {
+    let previews = this.previews;
+    let tabs = this.tabbrowser.tabs;
+
+    // Previews are internally stored using a map, so we need to iterate over
+    // the tabbrowser's array of tabs to retrieve previews in the same order.
+    let inorder = [previews.get(t) for (t of tabs) if (previews.has(t))];
+
+    // Since the internal taskbar array has not yet been updated, we must force
+    // the sorting order of our local array on it.  To do so, we must walk
+    // the local array backwards, because 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);
+    }
+  },
+
+  //// nsIDOMEventListener
+  handleEvent: function (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;
+    }
+  },
+
+  //// Browser progress listener
+  onLinkIconAvailable: function (aBrowser, aIconURL) {
+    let self = this;
+    getFaviconAsImage(aIconURL, PrivateBrowsingUtils.isWindowPrivate(this.win), function (img) {
+      let index = self.tabbrowser.browsers.indexOf(aBrowser);
+      // Only add it if we've found the index.  The tab could have closed!
+      if (index != -1) {
+        let tab = self.tabbrowser.tabs[index];
+        self.previews.get(tab).icon = img;
+      }
+    });
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//// AeroPeek
+
+/*
+ * This object acts as global storage and external interface for this feature.
+ * It maintains the values of the prefs.
+ */
+this.AeroPeek = {
+  available: false,
+  // Does the pref say we're enabled?
+  _prefenabled: true,
+
+  _enabled: true,
+
+  // 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: function () {
+    if (!(WINTASKBAR_CONTRACTID in Cc))
+      return;
+    this.taskbar = Cc[WINTASKBAR_CONTRACTID].getService(Ci.nsIWinTaskbar);
+    this.available = this.taskbar.available;
+    if (!this.available)
+      return;
+
+    this.prefs.addObserver(TOGGLE_PREF_NAME, this, false);
+    this.prefs.addObserver(DISABLE_THRESHOLD_PREF_NAME, this, false);
+    this.prefs.addObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this, false);
+
+    this.cacheLifespan = this.prefs.getIntPref(CACHE_EXPIRATION_TIME_PREF_NAME);
+
+    this.maxpreviews = this.prefs.getIntPref(DISABLE_THRESHOLD_PREF_NAME);
+
+    this.enabled = this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
+  },
+
+  destroy: function destroy() {
+    this._enabled = false;
+
+    this.prefs.removeObserver(TOGGLE_PREF_NAME, this);
+    this.prefs.removeObserver(DISABLE_THRESHOLD_PREF_NAME, this);
+    this.prefs.removeObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this);
+
+    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;
+    });
+  },
+
+  addPreview: function (preview) {
+    this.previews.push(preview);
+    this.checkPreviewCount();
+  },
+
+  removePreview: function (preview) {
+    let idx = this.previews.indexOf(preview);
+    this.previews.splice(idx, 1);
+    this.checkPreviewCount();
+  },
+
+  checkPreviewCount: function () {
+    if (this.previews.length > this.maxpreviews)
+      this.enabled = false;
+    else
+      this.enabled = this._prefenabled;
+  },
+
+  onOpenWindow: function (win) {
+    // This occurs when the taskbar service is not available (xp, vista)
+    if (!this.available)
+      return;
+
+    win.gTaskbarTabGroup = new TabWindow(win);
+  },
+
+  onCloseWindow: function (win) {
+    // This occurs when the taskbar service is not available (xp, vista)
+    if (!this.available)
+      return;
+
+    win.gTaskbarTabGroup.destroy();
+    delete win.gTaskbarTabGroup;
+
+    if (this.windows.length == 0)
+      this.destroy();
+  },
+
+  resetCacheTimer: function () {
+    this.cacheTimer.cancel();
+    this.cacheTimer.init(this, 1000*this.cacheLifespan, Ci.nsITimer.TYPE_ONE_SHOT);
+  },
+
+  //// nsIObserver
+  observe: function (aSubject, aTopic, aData) {
+    switch (aTopic) {
+      case "nsPref:changed":
+        if (aData == CACHE_EXPIRATION_TIME_PREF_NAME)
+          break;
+
+        if (aData == TOGGLE_PREF_NAME)
+          this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
+        else if (aData == DISABLE_THRESHOLD_PREF_NAME)
+          this.maxpreviews = this.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;
+    }
+  }
+};
+
+XPCOMUtils.defineLazyGetter(AeroPeek, "cacheTimer", function ()
+  Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer)
+);
+
+XPCOMUtils.defineLazyServiceGetter(AeroPeek, "prefs",
+                                   "@mozilla.org/preferences-service;1",
+                                   "nsIPrefBranch");
+
+AeroPeek.initialize();