Chrome Extensions chrome.experimental.offscreenTabs.* API implementation, docs, and test.

chrome/browser/extensions/extension_offscreen_tabs_module.cc

+// Copyright (c) 2011 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.
+
+#include "chrome/browser/extensions/extension_offscreen_tabs_module.h"
+
+#include <algorithm>
+#include <vector>
+
+#include "base/base64.h"
+#include "base/json/json_writer.h"
+#include "base/memory/ref_counted_memory.h"
+#include "base/stl_util.h"
+#include "base/string_number_conversions.h"
+#include "base/string_util.h"
+#include "base/values.h"
+#include "chrome/browser/extensions/extension_event_router.h"
+#include "chrome/browser/extensions/extension_function_dispatcher.h"
+#include "chrome/browser/extensions/extension_host.h"
+#include "chrome/browser/extensions/extension_message_service.h"
+#include "chrome/browser/extensions/extension_offscreen_tabs_module_constants.h"
+#include "chrome/browser/extensions/extension_service.h"
+#include "chrome/browser/extensions/extension_tabs_module.h"
+#include "chrome/browser/net/url_fixer_upper.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/browser_navigator.h"
+#include "chrome/browser/ui/browser_window.h"
+#include "chrome/browser/ui/tab_contents/tab_contents_wrapper.h"
+#include "chrome/browser/ui/window_sizer.h"
+#include "chrome/common/chrome_notification_types.h"
+#include "chrome/common/extensions/extension.h"
+#include "chrome/common/extensions/extension_error_utils.h"
+#include "chrome/common/extensions/extension_messages.h"
+#include "chrome/common/pref_names.h"
+#include "chrome/common/url_constants.h"
+#include "content/browser/renderer_host/backing_store.h"
+#include "content/browser/renderer_host/render_view_host.h"
+#include "content/browser/renderer_host/render_view_host_delegate.h"
+#include "content/browser/tab_contents/navigation_entry.h"
+#include "content/browser/tab_contents/tab_contents.h"
+#include "content/browser/tab_contents/tab_contents_view.h"
+#include "content/common/content_client.h"
+#include "content/common/notification_service.h"
+#include "skia/ext/image_operations.h"
+#include "skia/ext/platform_canvas.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+#include "ui/gfx/codec/jpeg_codec.h"
+#include "ui/gfx/codec/png_codec.h"
+
+using WebKit::WebInputEvent;
+
+namespace keys = extension_offscreen_tabs_module_constants;
+
+namespace {
+
+class Tab;
+
+// Offscreen Tab ---------------------------------------------------------------
+
+// This class is responsible for the creation and destruction of offscreen tabs,
+// as well as dispatching an onUpdated event.
+class OffscreenTab : public NotificationObserver {

[sky, 2011/09/16 16:29:21]

You have way too many things named tab in this file to the point that it makes
it hard to follow. See Map::RemoveOffscreenTab as an example. Here is what it
has:

tab_id: the id off the offscreen tab.
offscreen_tab: yes, this name is perfect!
tab: the parent tab, doesn't relate to tab_id.

In other places you use 'tab' to mean OffscreenTab or even TabContentsWrapper.
Oy! Be consistent. The following would make it a lot easier to read this code:

OffscreenTab (great name!), use variables offscreen_tab and offscreen_tab_id to
match.
Tab: very ambiguous name, maybe it should be ParentTab to match the SetParentTab
method you have. Then use variables named parent_tab and parent_tab_id.
TabContents/TabContentsWrapper I'm fine with using tab for these, as long as you
don't use tab for Tab or OffscreenTab.

[alexbost, 2011/09/16 20:18:14]

I completely agree :) I was trying to keep names short - not a good trade-off in
this case. Done.

+ public:
+  OffscreenTab();
+  virtual ~OffscreenTab();
+  void Init(const GURL& url,
+            const int width,
+            const int height,
+            Profile* profile);
+
+  TabContents* contents() { return tab_.get()->tab_contents(); }
+  Tab* parent_tab() { return parent_tab_; }
+  DictionaryValue* CreateValue();  // The caller owns the returned value.

[sky, 2011/09/16 16:29:21]

Document what this does too. Write comments assuming someone is looking at this
code for the first time.

[alexbost, 2011/09/16 20:18:14]

Done.

+
+  void SetURL(const GURL& url);
+  void SetSize(int width, int height);
+  void SetParentTab(Tab* tab);
+
+ private:
+  virtual void Observe(int type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details) OVERRIDE;
+
+  NotificationRegistrar registrar_;
+
+  scoped_ptr<TabContentsWrapper> tab_;  // TabContentsWrapper associated with
+                                        // this offscreen tab.
+  Tab* parent_tab_;
+
+  DISALLOW_COPY_AND_ASSIGN(OffscreenTab);
+};
+
+typedef std::vector<OffscreenTab*> TabVector;
+typedef TabVector::iterator TabIterator;
+
+// Tab -------------------------------------------------------------------------
+
+// Holds info about a tab that has spawned at least one offscreen tab.
+// Each Tab keeps track of its child offscreen tabs. The Tab is also responsible
+// for killing its children when it navigates away or gets closed.
+class Tab : public NotificationObserver {
+ public:
+  Tab();
+  virtual ~Tab();
+  void Init(TabContents* tab_contents, const std::string& extension_id);
+
+  TabContents* contents() { return tab_->tab_contents(); }
+  const TabVector& offscreen_tabs() { return offscreen_tabs_; }
+  const std::string& extension_id() const { return extension_id_; }
+
+  // Tab takes ownership of OffscreenTab.
+  void AddOffscreenTab(OffscreenTab *tab);
+  // Caller assumes ownership of OffscreenTab.
+  void RemoveOffscreenTab(OffscreenTab *tab);
+
+ private:
+  virtual void Observe(int type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details) OVERRIDE;
+
+  NotificationRegistrar registrar_;
+
+  TabContentsWrapper* tab_;  // TabContentsWrapper associated with this tab.
+  TabVector offscreen_tabs_;  // Offscreen tabs spawned by this tab.
+  std::string extension_id_;  // Id of the extension running in this tab.
+
+  DISALLOW_COPY_AND_ASSIGN(Tab);
+};
+
+// Map -------------------------------------------------------------------------
+
+// This map keeps track of all tabs that are happy parents of offscreen tabs.
+class Map {
+ public:
+  Map();
+  ~Map();
+
+  // Gets an offscreen tab by ID.
+  bool GetOffscreenTab(const int tab_id,
+                       ExtensionFunctionDispatcher* dispatcher,
+                       Profile* profile,
+                       OffscreenTab** offscreen_tab,
+                       std::string* error_message);
+  // Gets a parent tab by ID.
+  bool GetTab(const int tab_id,
+              Tab** tab,
+              std::string* error_message);
+  // Creates a mapping between a parent tab and an offscreen tab.
+  bool AddOffscreenTab(OffscreenTab* offscreen_tab,
+                       ExtensionFunctionDispatcher* dispatcher,
+                       Profile* profile,
+                       const std::string& ext_id,
+                       std::string* error_message);
+  // Removes the mapping between a parent tab and an offscreen tab.
+  // May cause the Tab object associated with the parent to be deleted.
+  bool RemoveOffscreenTab(const int tab_id,
+                          ExtensionFunctionDispatcher* dispatcher,
+                          Profile* profile,
+                          std::string* error_message);
+  // Removes a parent tab from the map along with its child offscreen tabs.
+  bool RemoveTab(const int tab_id, std::string* error_message);

[sky, 2011/09/16 16:29:21]

Document ownership.

[alexbost, 2011/09/16 20:18:14]

Done.

+
+ private:
+  typedef base::hash_map<int, Tab*> TabMap;
+  TabMap map;
+
+  DISALLOW_COPY_AND_ASSIGN(Map);
+};
+
+// Variables -------------------------------------------------------------------
+
+Map* map = NULL;
+
+// We are assuming that offscreen tabs will not be created by background pages
+// with the exception of the API test background page. We keep track of the
+// offscreen tabs associated with the test API background page via this variable
+// These tab contents are created just for convenience and do not do anything.
+// TODO(alexbost): Think about handling multiple background pages each spawning
+// offscreen tabs. Would background pages want to spawn offscreen tabs?
+TabContents* background_page_tab_contents = NULL;
+
+// Util ------------------------------------------------------------------------
+
+// Gets the map of parent tabs to offscreen tabs.
+Map* GetMap() {
+  if (map == NULL)
+    map = new Map();
+
+  return map;
+}
+
+// Gets the TabContents associated with the test API background page.
+TabContents* GetBackgroundPageTabContents(Profile* profile) {
+  if (background_page_tab_contents == NULL) {
+    background_page_tab_contents =
+        new TabContents(profile, NULL, MSG_ROUTING_NONE, NULL, NULL);
+    new TabContentsWrapper(background_page_tab_contents);
+  }
+
+  return background_page_tab_contents;
+}
+
+// Gets the contents of the tab that instantiated the extension API call.
+// In the case of background pages we use tab contents created by us.
+bool GetCurrentTab(ExtensionFunctionDispatcher* dispatcher,

[sky, 2011/09/16 16:29:21]

Name GetCurrentTabContents to match GetBackgroundPageTabContents.

[alexbost, 2011/09/16 20:18:14]

Done.

+                   Profile* profile,
+                   TabContents** tab_contents,
+                   std::string* error_message) {
+  *tab_contents = dispatcher->delegate()->GetAssociatedTabContents();
+
+  // Background page (no associated tab contents).
+  if (!*tab_contents)
+    *tab_contents = GetBackgroundPageTabContents(profile);

[sky, 2011/09/16 16:29:21]

Are we sure this is only going to get hit for testing? Can you add a CHECK to
that effect?

[alexbost, 2011/09/16 20:18:14]

It does get hit because the test runs in a background page which does not have
associated tab contents.

[sky, 2011/09/16 20:23:28]

What prevents a non-test background page from doing this?

[alexbost, 2011/09/16 21:46:53]

Nothing right now. How can I distinguish between the two cases?

[sky, 2011/09/16 21:51:45]

I don't know enough about background extensions and TabContents to know for
sure. You'll need to talk with someone on the extensions team to resolve this.

+
+  if (*tab_contents)
+    return true;
+
+  *error_message = keys::kCurrentTabNotFound;
+  return false;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+// Takes |url_string| and returns a GURL which is either valid and absolute
+// or invalid. If |url_string| is not directly interpretable as a valid (it is
+// likely a relative URL) an attempt is made to resolve it. |extension| is
+// provided so it can be resolved relative to its extension base
+// (chrome-extension://<id>/). Using the source frame url would be more correct,
+// but because the api shipped with urls resolved relative to their extension
+// base, we decided it wasn't worth breaking existing extensions to fix.
+GURL ResolvePossiblyRelativeURL(const std::string& url_string,
+                                const Extension* extension) {
+  GURL url = GURL(url_string);
+  if (!url.is_valid())
+    url = extension->GetResourceURL(url_string);
+
+  return url;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+bool IsCrashURL(const GURL& url) {
+  // Check a fixed-up URL, to normalize the scheme and parse hosts correctly.
+  GURL fixed_url =
+      URLFixerUpper::FixupURL(url.possibly_invalid_spec(), std::string());
+  return (fixed_url.SchemeIs(chrome::kChromeUIScheme) &&
+          (fixed_url.host() == chrome::kChromeUIBrowserCrashHost ||
+           fixed_url.host() == chrome::kChromeUICrashHost));
+}
+
+// Offscreen Tab ---------------------------------------------------------------
+
+OffscreenTab::OffscreenTab() {}

[sky, 2011/09/16 16:29:21]

member initialize parent_tab_.

[alexbost, 2011/09/16 20:18:14]

Done.

+OffscreenTab::~OffscreenTab() {}
+
+void OffscreenTab::Init(const GURL& url,
+                        const int width,
+                        const int height,
+                        Profile* profile) {
+  // Create the offscreen tab.
+  TabContents* tab_contents = new TabContents(
+      profile, NULL, MSG_ROUTING_NONE, NULL, NULL);
+  tab_.reset(new TabContentsWrapper(tab_contents));
+
+  SetSize(width, height);  // Setting the size starts the renderer.
+  SetURL(url);
+
+  // Register for tab notifications.
+  registrar_.Add(this,
+                 content::NOTIFICATION_NAV_ENTRY_COMMITTED,
+                 Source<NavigationController>(&contents()->controller()));
+}
+
+DictionaryValue* OffscreenTab::CreateValue() {
+  DictionaryValue* result = new DictionaryValue();
+
+  result->SetInteger(keys::kIdKey, ExtensionTabUtil::GetTabId(contents()));
+  result->SetString(keys::kUrlKey, contents()->GetURL().spec());
+  result->SetInteger(keys::kWidthKey,
+      contents()->view()->GetContainerSize().width());
+  result->SetInteger(keys::kHeightKey,
+      contents()->view()->GetContainerSize().height());
+
+  return result;
+}
+
+void OffscreenTab::SetURL(const GURL& url) {
+  contents()->controller().LoadURL(
+      url, GURL(), PageTransition::LINK, std::string());
+}
+
+void OffscreenTab::SetSize(int width, int height) {
+  contents()->view()->SizeContents(gfx::Size(width, height));
+}
+
+void OffscreenTab::SetParentTab(Tab* tab) {

[sky, 2011/09/16 16:29:21]

Change this to set_parent_tab and inline it (like you did for parent_tab()).

[alexbost, 2011/09/16 20:18:14]

Done.

+  parent_tab_ = tab;
+}
+
+void OffscreenTab::Observe(int type,
+                           const NotificationSource& source,
+                           const NotificationDetails& details) {
+  CHECK_EQ(content::NOTIFICATION_NAV_ENTRY_COMMITTED, type);
+
+  DictionaryValue* changed_properties = new DictionaryValue();
+  changed_properties->SetString(keys::kUrlKey, contents()->GetURL().spec());
+
+  ListValue args;
+  args.Append(
+      Value::CreateIntegerValue(ExtensionTabUtil::GetTabId(contents())));
+  args.Append(changed_properties);
+  args.Append(CreateValue());
+  std::string json_args;
+  base::JSONWriter::Write(&args, false, &json_args);
+
+  ListValue event_args;
+  event_args.Set(0, Value::CreateStringValue(keys::kEventOnUpdated));
+  event_args.Set(1, Value::CreateStringValue(json_args));
+
+  // Dispatch an onUpdated event.
+  // The primary use case for broadcasting the event is
+  // when the offscreen tab is generated by a test API background page.
+  if (parent_tab_->contents() == background_page_tab_contents) {
+    Profile* profile = TabContentsWrapper::GetCurrentWrapperForContents(
+        parent_tab_->contents())->profile();
+    profile->GetExtensionEventRouter()->DispatchEventToRenderers(
+        keys::kEventOnUpdated, json_args, profile, GURL());
+  } else {
+    // Send a routed event directly to the parent tab.
+    parent_tab_->contents()->render_view_host()->process()->Send(

[sky, 2011/09/16 16:29:21]

I'm not sure what happens if the tab crashes. The process() may be null in that
case. You'll have to try it and make sure this code doesn't crash if that
happens.

[alexbost, 2011/09/16 20:18:14]

Done.

+        new ExtensionMsg_MessageInvoke(
+            parent_tab_->contents()->render_view_host()->routing_id(),
+            parent_tab_->extension_id(),
+            keys::kDispatchEvent,
+            event_args,
+            GURL()));
+  }
+}
+
+// Tab -------------------------------------------------------------------------
+
+Tab::Tab() {
+  tab_ = NULL;

[sky, 2011/09/16 16:29:21]

Use member initializer list.

[alexbost, 2011/09/16 20:18:14]

Done.

+}
+
+Tab::~Tab() {
+  // Kill child offscreen tabs.
+  STLDeleteElements(&offscreen_tabs_);
+
+  bool removed = GetMap()->RemoveTab(
+      ExtensionTabUtil::GetTabId(contents()), new std::string());
+  CHECK(removed);
+}
+
+void Tab::Init(TabContents* tab_contents, const std::string& extension_id) {
+  CHECK(tab_contents);
+
+  tab_ = TabContentsWrapper::GetCurrentWrapperForContents(tab_contents);
+  CHECK(tab_);
+
+  extension_id_ = extension_id;
+
+  // Register for tab notifications.
+  registrar_.Add(this,
+                 content::NOTIFICATION_NAV_ENTRY_COMMITTED,
+                 Source<NavigationController>(&contents()->controller()));
+
+  registrar_.Add(this,
+                 content::NOTIFICATION_TAB_CONTENTS_DESTROYED,
+                 Source<TabContents>(contents()));
+}
+
+void Tab::AddOffscreenTab(OffscreenTab *tab) {
+  offscreen_tabs_.push_back(tab);
+}
+
+void Tab::RemoveOffscreenTab(OffscreenTab *tab) {
+  TabIterator it_tab = std::find(
+      offscreen_tabs_.begin(), offscreen_tabs_.end(), tab);
+  offscreen_tabs_.erase(it_tab);
+}
+
+void Tab::Observe(int type,
+                  const NotificationSource& source,
+                  const NotificationDetails& details) {
+  CHECK(type == content::NOTIFICATION_NAV_ENTRY_COMMITTED ||
+        type == content::NOTIFICATION_TAB_CONTENTS_DESTROYED);
+
+  delete this;
+}
+
+// Map -------------------------------------------------------------------------
+
+Map::Map() {}
+Map::~Map() {}
+
+bool Map::GetOffscreenTab(const int tab_id,
+                          ExtensionFunctionDispatcher* dispatcher,
+                          Profile* profile,
+                          OffscreenTab** offscreen_tab,
+                          std::string* error_message) {
+  // Ensure that the current tab is the parent of the offscreen tab.
+  TabContents* tab_contents = NULL;
+  if (!GetCurrentTab(dispatcher, profile, &tab_contents, error_message))
+    return false;
+
+  Tab* tab = NULL;
+  if (!GetTab(ExtensionTabUtil::GetTabId(tab_contents), &tab, error_message))
+    return false;
+  TabVector offscreen_tabs = tab->offscreen_tabs();
+
+  for (TabIterator it = offscreen_tabs.begin();
+      it != offscreen_tabs.end(); ++it) {
+    if (ExtensionTabUtil::GetTabId((*it)->contents()) == tab_id) {
+      *offscreen_tab = *it;
+      return true;
+    }
+  }
+
+  *error_message = ExtensionErrorUtils::FormatErrorMessage(
+      keys::kOffscreenTabNotFoundError, base::IntToString(tab_id));
+  return false;
+}
+
+bool Map::GetTab(const int tab_id, Tab** tab, std::string* error_message) {
+  TabMap::iterator it = map.find(tab_id);
+
+  if (it == map.end()) {
+    *error_message = ExtensionErrorUtils::FormatErrorMessage(
+        keys::kTabNotFoundError, base::IntToString(tab_id));
+    return false;
+  }
+
+  *tab = it->second;
+
+  return true;
+}
+
+bool Map::AddOffscreenTab(OffscreenTab* offscreen_tab,
+                          ExtensionFunctionDispatcher* dispatcher,
+                          Profile* profile,
+                          const std::string& ext_id,
+                          std::string* error_message) {
+  // Get parent tab.
+  TabContents* tab_contents = NULL;
+  if (!GetCurrentTab(dispatcher, profile, &tab_contents, error_message))
+    return false;
+  int tab_id = ExtensionTabUtil::GetTabId(tab_contents);
+
+  Tab* tab = NULL;
+  if (!GetTab(tab_id, &tab, error_message)) {
+    tab = map[tab_id] = new Tab();
+    tab->Init(tab_contents, ext_id);
+  }
+
+  // Add child to parent.
+  tab->AddOffscreenTab(offscreen_tab);
+  // Add parent to child.
+  offscreen_tab->SetParentTab(tab);

[sky, 2011/09/16 16:29:21]

Shouldn't the parent be passed to Init? Otherwise it implies the parent can
change, which you don't really want, right?

[alexbost, 2011/09/16 20:18:14]

Done.

+
+  return true;
+}
+
+bool Map::RemoveOffscreenTab(const int tab_id,
+                             ExtensionFunctionDispatcher* dispatcher,
+                             Profile* profile,
+                             std::string* error_message) {
+  OffscreenTab* offscreen_tab = NULL;
+  if (!GetOffscreenTab(tab_id, dispatcher, profile,
+                       &offscreen_tab, error_message))
+    return false;
+
+  Tab* tab = offscreen_tab->parent_tab();
+
+  tab->RemoveOffscreenTab(offscreen_tab);
+
+  // If this was the last offscreen tab for the parent tab, remove the parent.
+  if (tab->offscreen_tabs().empty())
+    delete tab;  // Causes tab to be removed from the map!
+
+  return true;

[sky, 2011/09/16 16:29:21]

Who takes ownership of offscreen_tab?

[alexbost, 2011/09/16 20:18:14]

Done.

+}
+
+bool Map::RemoveTab(const int tab_id, std::string* error_message) {
+  if (map.find(tab_id) == map.end()) {
+    *error_message = ExtensionErrorUtils::FormatErrorMessage(
+        keys::kTabNotFoundError, base::IntToString(tab_id));
+    return false;
+  }
+
+  map.erase(tab_id);
+
+  return true;
+}
+
+}  // namespace
+
+// API functions ---------------------------------------------------------------
+
+// create ----------------------------------------------------------------------
+
+CreateOffscreenTabFunction::CreateOffscreenTabFunction() {}
+CreateOffscreenTabFunction::~CreateOffscreenTabFunction() {}
+
+bool CreateOffscreenTabFunction::RunImpl() {
+  DictionaryValue* create_props;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetDictionary(0, &create_props));
+
+  std::string url_string;
+  GURL url;
+  EXTENSION_FUNCTION_VALIDATE(
+      create_props->GetString(keys::kUrlKey, &url_string));
+  url = ResolvePossiblyRelativeURL(url_string, GetExtension());
+  if (!url.is_valid()) {
+    error_ = ExtensionErrorUtils::FormatErrorMessage(
+        keys::kInvalidUrlError, url_string);
+    return false;
+  }
+  if (IsCrashURL(url)) {
+    error_ = keys::kNoCrashBrowserError;
+    return false;
+  }
+
+  gfx::Rect window_bounds;
+  bool maximized;
+  if (!create_props->HasKey(keys::kWidthKey) ||
+      !create_props->HasKey(keys::kHeightKey)) {
+    Browser* browser = GetCurrentBrowser();
+    if (!browser) {
+      error_ = keys::kNoCurrentWindowError;
+      return false;
+    }
+
+    WindowSizer::GetBrowserWindowBounds(std::string(), gfx::Rect(),
+                                        browser, &window_bounds,
+                                        &maximized);
+  }
+
+  int width = window_bounds.width();
+  if (create_props->HasKey(keys::kWidthKey))
+    EXTENSION_FUNCTION_VALIDATE(
+        create_props->GetInteger(keys::kWidthKey, &width));
+
+  int height = window_bounds.height();
+  if (create_props->HasKey(keys::kHeightKey))
+    EXTENSION_FUNCTION_VALIDATE(
+        create_props->GetInteger(keys::kHeightKey, &height));
+
+  OffscreenTab* tab = new OffscreenTab();
+  tab->Init(url, width, height, profile_);
+
+  // Add the offscreen tab to the map so we don't lose track of it.
+  if (!GetMap()->AddOffscreenTab(tab, dispatcher(), profile_,
+                                 extension_id(), &error_)) {
+    delete tab;  // Prevent leaking of offscreen tab.
+    return false;
+  }
+
+  if (has_callback()) {
+    result_.reset(tab->CreateValue());
+    SendResponse(true);
+  }
+
+  return true;
+}
+
+// get -------------------------------------------------------------------------
+
+GetOffscreenTabFunction::GetOffscreenTabFunction() {}
+GetOffscreenTabFunction::~GetOffscreenTabFunction() {}
+
+bool GetOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->
+      GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_)) {
+    error_ = ExtensionErrorUtils::FormatErrorMessage(
+        keys::kOffscreenTabNotFoundError, base::IntToString(tab_id));
+    return false;
+  }
+
+  result_.reset(tab->CreateValue());
+  SendResponse(true);
+
+  return true;
+}
+
+// getAll ----------------------------------------------------------------------
+
+GetAllOffscreenTabFunction::GetAllOffscreenTabFunction() {}
+GetAllOffscreenTabFunction::~GetAllOffscreenTabFunction() {}
+
+bool GetAllOffscreenTabFunction::RunImpl() {
+  TabContents* tab_contents = NULL;
+  if (!GetCurrentTab(dispatcher(), profile_, &tab_contents, &error_))
+    return false;
+
+  Tab* tab = NULL;
+  if (!GetMap()->
+      GetTab(ExtensionTabUtil::GetTabId(tab_contents), &tab, &error_))
+    return false;
+
+  TabVector offscreen_tabs = tab->offscreen_tabs();
+
+  ListValue* tab_list = new ListValue();
+  for (TabIterator it_tab = offscreen_tabs.begin();
+      it_tab != offscreen_tabs.end(); ++it_tab)
+    tab_list->Append((*it_tab)->CreateValue());
+
+  result_.reset(tab_list);
+  SendResponse(true);
+
+  return true;
+}
+
+// remove ----------------------------------------------------------------------
+
+RemoveOffscreenTabFunction::RemoveOffscreenTabFunction() {}
+RemoveOffscreenTabFunction::~RemoveOffscreenTabFunction() {}
+
+bool RemoveOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_))
+    return false;
+
+  if (!GetMap()->RemoveOffscreenTab(tab_id, dispatcher(), profile_, &error_))
+    return false;
+
+  delete tab;
+
+  return true;
+}
+
+// sendKeyboardEvent -----------------------------------------------------------
+
+SendKeyboardEventOffscreenTabFunction::
+    SendKeyboardEventOffscreenTabFunction() {}
+SendKeyboardEventOffscreenTabFunction::
+    ~SendKeyboardEventOffscreenTabFunction() {}
+
+bool SendKeyboardEventOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_))
+    return false;
+
+  // JavaScript KeyboardEvent.
+  DictionaryValue* js_keyboard_event = NULL;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetDictionary(1, &js_keyboard_event));
+
+  NativeWebKeyboardEvent keyboard_event;
+
+  std::string type;
+  if (js_keyboard_event->HasKey(keys::kKeyboardEventTypeKey)) {
+    EXTENSION_FUNCTION_VALIDATE(
+        js_keyboard_event->GetString(keys::kKeyboardEventTypeKey, &type));
+  } else {
+    error_ = keys::kInvalidKeyboardEventObjectError;
+    return false;
+  }
+
+  if (type.compare(keys::kKeyboardEventTypeValueKeypress) == 0) {
+    keyboard_event.type = WebInputEvent::Char;
+  } else if (type.compare(keys::kKeyboardEventTypeValueKeydown) == 0) {
+    keyboard_event.type = WebInputEvent::KeyDown;
+  } else if (type.compare(keys::kKeyboardEventTypeValueKeyup) == 0) {
+    keyboard_event.type = WebInputEvent::KeyUp;
+  } else {
+    error_ = keys::kInvalidKeyboardEventObjectError;
+    return false;
+  }
+
+  int key_code;
+  if (js_keyboard_event->HasKey(keys::kKeyboardEventKeyCodeKey)) {
+    EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+        GetInteger(keys::kKeyboardEventKeyCodeKey, &key_code));
+  } else {
+    error_ = keys::kInvalidKeyboardEventObjectError;
+    return false;
+  }
+
+  keyboard_event.nativeKeyCode = key_code;
+  keyboard_event.windowsKeyCode = key_code;
+  keyboard_event.setKeyIdentifierFromWindowsKeyCode();
+
+  // Keypress = type character
+  if (type.compare(keys::kKeyboardEventTypeValueKeypress) == 0) {
+    int char_code;
+    if (js_keyboard_event->HasKey(keys::kKeyboardEventCharCodeKey)) {
+      EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+          GetInteger(keys::kKeyboardEventCharCodeKey, &char_code));
+      keyboard_event.text[0] = char_code;
+      keyboard_event.unmodifiedText[0] = char_code;
+    } else {
+      error_ = keys::kInvalidKeyboardEventObjectError;
+      return false;
+    }
+  }
+
+  bool alt_key;
+  if (js_keyboard_event->HasKey(keys::kKeyboardEventAltKeyKey))
+    EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+        GetBoolean(keys::kKeyboardEventAltKeyKey, &alt_key));
+  if (alt_key)
+    keyboard_event.modifiers |= WebInputEvent::AltKey;
+
+  bool ctrl_key;
+  if (js_keyboard_event->HasKey(keys::kKeyboardEventCtrlKeyKey))
+    EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+        GetBoolean(keys::kKeyboardEventCtrlKeyKey, &ctrl_key));
+  if (ctrl_key)
+    keyboard_event.modifiers |= WebInputEvent::ControlKey;
+
+  bool meta_key = false;
+  if (js_keyboard_event->HasKey(keys::kMouseEventMetaKeyKey))
+    EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+        GetBoolean(keys::kMouseEventMetaKeyKey, &meta_key));
+  if (meta_key)
+    keyboard_event.modifiers |= WebInputEvent::MetaKey;
+
+  bool shift_key;
+  if (js_keyboard_event->HasKey(keys::kKeyboardEventShiftKeyKey))
+    EXTENSION_FUNCTION_VALIDATE(js_keyboard_event->
+        GetBoolean(keys::kKeyboardEventShiftKeyKey, &shift_key));
+  if (shift_key)
+    keyboard_event.modifiers |= WebInputEvent::ShiftKey;
+
+  // Forward the event.
+  tab->contents()->render_view_host()->
+      ForwardKeyboardEvent(keyboard_event);
+
+  if (has_callback()) {
+    result_.reset(tab->CreateValue());
+    SendResponse(true);
+  }
+
+  return true;
+}
+
+// sendMouseEvent --------------------------------------------------------------
+
+SendMouseEventOffscreenTabFunction::SendMouseEventOffscreenTabFunction() {}
+SendMouseEventOffscreenTabFunction::~SendMouseEventOffscreenTabFunction() {}
+
+bool SendMouseEventOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_))
+    return false;
+
+  // JavaScript MouseEvent.
+  DictionaryValue* js_mouse_event = NULL;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetDictionary(1, &js_mouse_event));
+
+  std::string type;
+  if (js_mouse_event->HasKey(keys::kMouseEventTypeKey)) {
+    EXTENSION_FUNCTION_VALIDATE(
+        js_mouse_event->GetString(keys::kMouseEventTypeKey, &type));
+  } else {
+    error_ = keys::kInvalidMouseEventObjectError;
+    return false;
+  }
+
+  if (type.compare(keys::kMouseEventTypeValueMousewheel) == 0) {
+    WebKit::WebMouseWheelEvent wheel_event;
+
+    wheel_event.type = WebInputEvent::MouseWheel;
+
+    if (js_mouse_event->HasKey(keys::kMouseEventWheelDeltaXKey) &&
+        js_mouse_event->HasKey(keys::kMouseEventWheelDeltaYKey)) {
+      int delta_x, delta_y;
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetInteger(keys::kMouseEventWheelDeltaXKey, &delta_x));
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetInteger(keys::kMouseEventWheelDeltaYKey, &delta_y));
+      wheel_event.deltaX = delta_x;
+      wheel_event.deltaY = delta_y;
+    } else {
+      error_ = keys::kInvalidMouseEventObjectError;
+      return false;
+    }
+
+    // Forward the event.
+    tab->contents()->render_view_host()->ForwardWheelEvent(wheel_event);
+  } else {
+    WebKit::WebMouseEvent mouse_event;
+
+    if (type.compare(keys::kMouseEventTypeValueMousedown) == 0 ||
+        type.compare(keys::kMouseEventTypeValueClick) == 0) {
+      mouse_event.type = WebKit::WebInputEvent::MouseDown;
+    } else if (type.compare(keys::kMouseEventTypeValueMouseup) == 0) {
+      mouse_event.type = WebKit::WebInputEvent::MouseUp;
+    } else if (type.compare(keys::kMouseEventTypeValueMousemove) == 0) {
+      mouse_event.type = WebKit::WebInputEvent::MouseMove;
+    } else {
+      error_ = keys::kInvalidMouseEventObjectError;
+      return false;
+    }
+
+    int button;
+    if (js_mouse_event->HasKey(keys::kMouseEventButtonKey)) {
+      EXTENSION_FUNCTION_VALIDATE(
+          js_mouse_event->GetInteger(keys::kMouseEventButtonKey, &button));
+    } else {
+      error_ = keys::kInvalidMouseEventObjectError;
+      return false;
+    }
+
+    if (button == keys::kMouseEventButtonValueLeft) {
+      mouse_event.button = WebKit::WebMouseEvent::ButtonLeft;
+    } else if (button == keys::kMouseEventButtonValueMiddle) {
+      mouse_event.button = WebKit::WebMouseEvent::ButtonMiddle;
+    } else if (button == keys::kMouseEventButtonValueRight) {
+      mouse_event.button = WebKit::WebMouseEvent::ButtonRight;
+    } else {
+      error_ = keys::kInvalidMouseEventObjectError;
+      return false;
+    }
+
+    if (HasOptionalArgument(2) && HasOptionalArgument(3)) {
+      EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(2, &mouse_event.x));
+      EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(3, &mouse_event.y));
+    } else {
+      error_ = keys::kNoMouseCoordinatesError;
+      return false;
+    }
+
+    bool alt_key = false;
+    if (js_mouse_event->HasKey(keys::kMouseEventAltKeyKey))
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetBoolean(keys::kMouseEventAltKeyKey, &alt_key));
+    if (alt_key)
+      mouse_event.modifiers |= WebInputEvent::AltKey;
+
+    bool ctrl_key = false;
+    if (js_mouse_event->HasKey(keys::kMouseEventCtrlKeyKey))
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetBoolean(keys::kMouseEventCtrlKeyKey, &ctrl_key));
+    if (ctrl_key)
+      mouse_event.modifiers |= WebInputEvent::ControlKey;
+
+    bool meta_key = false;
+    if (js_mouse_event->HasKey(keys::kMouseEventMetaKeyKey))
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetBoolean(keys::kMouseEventMetaKeyKey, &meta_key));
+    if (meta_key)
+      mouse_event.modifiers |= WebInputEvent::MetaKey;
+
+    bool shift_key = false;
+    if (js_mouse_event->HasKey(keys::kMouseEventShiftKeyKey))
+      EXTENSION_FUNCTION_VALIDATE(js_mouse_event->
+          GetBoolean(keys::kMouseEventShiftKeyKey, &shift_key));
+    if (shift_key)
+      mouse_event.modifiers |= WebInputEvent::ShiftKey;
+
+    mouse_event.clickCount = 1;
+
+    // Forward the event.
+    tab->contents()->render_view_host()->ForwardMouseEvent(mouse_event);
+
+    // If the event is a click,
+    // fire a mouseup event in addition to the mousedown.
+    if (type.compare(keys::kMouseEventTypeValueClick) == 0) {
+      mouse_event.type = WebKit::WebInputEvent::MouseUp;
+      tab->contents()->render_view_host()->ForwardMouseEvent(mouse_event);
+    }
+  }
+
+  if (has_callback()) {
+    result_.reset(tab->CreateValue());
+    SendResponse(true);
+  }
+
+  return true;
+}
+
+// toDataUrl -------------------------------------------------------------------
+
+ToDataUrlOffscreenTabFunction::ToDataUrlOffscreenTabFunction() {}
+ToDataUrlOffscreenTabFunction::~ToDataUrlOffscreenTabFunction() {}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+// TODO(alexbost): We want to optimize this function in order to get more image
+// updates on the browser side. One improvement would be to implement another
+// hash map in order to get offscreen tabs in O(1).
+bool ToDataUrlOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_))
+    return false;
+
+  image_format_ = FORMAT_JPEG;  // Default format is JPEG.
+  image_quality_ = kDefaultQuality;  // Default quality setting.
+
+  if (HasOptionalArgument(1)) {
+    DictionaryValue* options;
+    EXTENSION_FUNCTION_VALIDATE(args_->GetDictionary(1, &options));
+
+    if (options->HasKey(keys::kFormatKey)) {
+      std::string format;
+      EXTENSION_FUNCTION_VALIDATE(
+          options->GetString(keys::kFormatKey, &format));
+
+      if (format == keys::kFormatValueJpeg) {
+        image_format_ = FORMAT_JPEG;
+      } else if (format == keys::kFormatValuePng) {
+        image_format_ = FORMAT_PNG;
+      } else {
+        // Schema validation should make this unreachable.
+        EXTENSION_FUNCTION_VALIDATE(0);
+      }
+    }
+
+    if (options->HasKey(keys::kQualityKey)) {
+      EXTENSION_FUNCTION_VALIDATE(
+          options->GetInteger(keys::kQualityKey, &image_quality_));
+    }
+  }
+
+  // captureVisibleTab() can return an image containing sensitive information
+  // that the browser would otherwise protect.  Ensure the extension has
+  // permission to do this.
+  if (!GetExtension()->
+      CanCaptureVisiblePage(tab->contents()->GetURL(), &error_))
+    return false;
+
+// The backing store approach works on Linux but not on Mac.
+// TODO(alexbost): Test on Windows
+#if !defined(OS_MACOSX)
+  RenderViewHost* render_view_host = tab->contents()->render_view_host();
+
+  // If a backing store is cached for the tab we want to capture,
+  // and it can be copied into a bitmap, then use it to generate the image.
+  BackingStore* backing_store = render_view_host->GetBackingStore(false);
+  if (backing_store && CaptureSnapshotFromBackingStore(backing_store))
+    return true;
+#endif
+
+  // Ask the renderer for a snapshot of the tab.
+  TabContentsWrapper* tab_wrapper =
+      TabContentsWrapper::GetCurrentWrapperForContents(tab->contents());
+  tab_wrapper->CaptureSnapshot();
+  registrar_.Add(this,
+               chrome::NOTIFICATION_TAB_SNAPSHOT_TAKEN,
+               Source<TabContentsWrapper>(tab_wrapper));
+
+  AddRef();  // Balanced in ToDataUrlOffscreenTabFunction::Observe().
+
+  return true;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+// Build the image of a tab's contents out of a backing store.
+// This may fail if we can not copy a backing store into a bitmap.
+// For example, some uncommon X11 visual modes are not supported by
+// CopyFromBackingStore().
+bool ToDataUrlOffscreenTabFunction::CaptureSnapshotFromBackingStore(
+    BackingStore* backing_store) {
+
+  skia::PlatformCanvas temp_canvas;
+  if (!backing_store->CopyFromBackingStore(gfx::Rect(backing_store->size()),
+                                           &temp_canvas)) {
+    return false;
+  }
+  VLOG(1) << "captureVisibleTab() got image from backing store.";
+
+  SendResultFromBitmap(
+      skia::GetTopDevice(temp_canvas)->accessBitmap(false));
+  return true;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+void ToDataUrlOffscreenTabFunction::Observe(int type,
+                                           const NotificationSource& source,
+                                           const NotificationDetails& details) {
+  CHECK(type == chrome::NOTIFICATION_TAB_SNAPSHOT_TAKEN);
+
+  const SkBitmap *screen_capture = Details<const SkBitmap>(details).ptr();
+  const bool error = screen_capture->empty();
+
+  if (error) {
+    error_ = keys::kInternalVisibleTabCaptureError;
+    SendResponse(false);
+  } else {
+    VLOG(1) << "Got image from renderer.";
+    SendResultFromBitmap(*screen_capture);
+  }
+
+  Release();  // Balanced in ToDataUrlOffscreenTabFunction::RunImpl().
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+// Turn a bitmap of the screen into an image, set that image as the result,
+// and call SendResponse().
+void ToDataUrlOffscreenTabFunction::SendResultFromBitmap(
+    const SkBitmap& screen_capture) {
+  std::vector<unsigned char> data;
+  SkAutoLockPixels screen_capture_lock(screen_capture);
+  bool encoded = false;
+  std::string mime_type;
+  switch (image_format_) {
+    case FORMAT_JPEG:
+      encoded = gfx::JPEGCodec::Encode(
+          reinterpret_cast<unsigned char*>(screen_capture.getAddr32(0, 0)),
+          gfx::JPEGCodec::FORMAT_SkBitmap,
+          screen_capture.width(),
+          screen_capture.height(),
+          static_cast<int>(screen_capture.rowBytes()),
+          image_quality_,
+          &data);
+      mime_type = keys::kMimeTypeJpeg;
+      break;
+    case FORMAT_PNG:
+      encoded = gfx::PNGCodec::EncodeBGRASkBitmap(
+          screen_capture,
+          true,  // Discard transparency.
+          &data);
+      mime_type = keys::kMimeTypePng;
+      break;
+    default:
+      NOTREACHED() << "Invalid image format.";
+  }
+
+  if (!encoded) {
+    error_ = keys::kInternalVisibleTabCaptureError;
+    SendResponse(false);
+    return;
+  }
+
+  std::string base64_result;
+  base::StringPiece stream_as_string(
+      reinterpret_cast<const char*>(vector_as_array(&data)), data.size());
+
+  base::Base64Encode(stream_as_string, &base64_result);
+  base64_result.insert(0, base::StringPrintf("data:%s;base64,",
+                                             mime_type.c_str()));
+  result_.reset(new StringValue(base64_result));
+  SendResponse(true);
+}
+
+// update ----------------------------------------------------------------------
+
+UpdateOffscreenTabFunction::UpdateOffscreenTabFunction() {}
+UpdateOffscreenTabFunction::~UpdateOffscreenTabFunction() {}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+bool UpdateOffscreenTabFunction::RunImpl() {
+  int tab_id;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetInteger(0, &tab_id));
+
+  OffscreenTab* tab = NULL;
+  if (!GetMap()->GetOffscreenTab(tab_id, dispatcher(), profile_, &tab, &error_))
+    return false;
+
+  DictionaryValue* update_props;
+  EXTENSION_FUNCTION_VALIDATE(args_->GetDictionary(1, &update_props));
+
+  // Url
+  if (update_props->HasKey(keys::kUrlKey)) {
+    std::string url_string;
+    GURL url;
+    EXTENSION_FUNCTION_VALIDATE(
+        update_props->GetString(keys::kUrlKey, &url_string));
+    url = ResolvePossiblyRelativeURL(url_string, GetExtension());
+    if (!url.is_valid()) {
+      error_ = ExtensionErrorUtils::FormatErrorMessage(
+          keys::kInvalidUrlError, url_string);
+      return false;
+    }
+    if (IsCrashURL(url)) {
+      error_ = keys::kNoCrashBrowserError;
+      return false;
+    }
+
+    // JavaScript URLs can do the same kinds of things as cross-origin XHR, so
+    // we need to check host permissions before allowing them.
+    if (url.SchemeIs(chrome::kJavaScriptScheme)) {
+      if (!GetExtension()->CanExecuteScriptOnPage(
+          tab->contents()->GetURL(), NULL, &error_)) {
+        return false;
+      }
+
+      ExtensionMsg_ExecuteCode_Params params;
+      params.request_id = request_id();
+      params.extension_id = extension_id();
+      params.is_javascript = true;
+      params.code = url.path();
+      params.all_frames = false;
+      params.in_main_world = true;
+
+      RenderViewHost* render_view_host =
+          tab->contents()->render_view_host();
+      render_view_host->Send(
+          new ExtensionMsg_ExecuteCode(render_view_host->routing_id(),
+                                       params));
+
+      Observe(tab->contents());
+      AddRef();  // balanced in Observe()
+
+      return true;
+    }
+
+    tab->SetURL(url);
+
+    // The URL of a tab contents never actually changes to a JavaScript URL, so
+    // this check only makes sense in other cases.
+    if (!url.SchemeIs(chrome::kJavaScriptScheme))
+      CHECK_EQ(url.spec(), tab->contents()->GetURL().spec());
+  }
+
+  // Width and height
+  if (update_props->HasKey(keys::kWidthKey) ||
+      update_props->HasKey(keys::kHeightKey)) {
+    int width;
+    if (update_props->HasKey(keys::kWidthKey))
+      EXTENSION_FUNCTION_VALIDATE(
+          update_props->GetInteger(keys::kWidthKey, &width));
+    else
+      tab->contents()->view()->GetContainerSize().width();
+
+    int height;
+    if (update_props->HasKey(keys::kHeightKey))
+      EXTENSION_FUNCTION_VALIDATE(
+          update_props->GetInteger(keys::kHeightKey, &height));
+    else
+      tab->contents()->view()->GetContainerSize().height();
+
+    tab->SetSize(width, height);
+  }
+
+  // Callback
+  if (has_callback()) {
+    result_.reset(tab->CreateValue());
+    SendResponse(true);
+  }
+
+  return true;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+bool UpdateOffscreenTabFunction::OnMessageReceived(
+    const IPC::Message& message) {
+  if (message.type() != ExtensionHostMsg_ExecuteCodeFinished::ID)
+    return false;
+
+  int message_request_id;
+  void* iter = NULL;
+  if (!message.ReadInt(&iter, &message_request_id)) {
+    NOTREACHED() << "malformed extension message";
+    return true;
+  }
+
+  if (message_request_id != request_id())
+    return false;
+
+  IPC_BEGIN_MESSAGE_MAP(UpdateOffscreenTabFunction, message)
+    IPC_MESSAGE_HANDLER(ExtensionHostMsg_ExecuteCodeFinished,
+                        OnExecuteCodeFinished)
+  IPC_END_MESSAGE_MAP()
+  return true;
+}
+
+// TODO(alexbost): Needs refactoring. Similar method in extension_tabs_module.
+void UpdateOffscreenTabFunction::
+    OnExecuteCodeFinished(int request_id,
+                          bool success,
+                          const std::string& error) {
+  if (!error.empty()) {
+    CHECK(!success);
+    error_ = error;
+  }
+
+  SendResponse(success);
+
+  Observe(NULL);
+  Release();  // balanced in Execute()
+}
+