Only show Last N Pages in the UI when the corresponding tab is visible.

components/offline_pages/core/recent_tabs/recent_tabs_ui_adapter_delegate.h

Index: components/offline_pages/core/recent_tabs/recent_tabs_ui_adapter_delegate.h
diff --git a/components/offline_pages/core/recent_tabs/recent_tabs_ui_adapter_delegate.h b/components/offline_pages/core/recent_tabs/recent_tabs_ui_adapter_delegate.h
new file mode 100644
index 0000000000000000000000000000000000000000..bd166e44896c7b1e1c866f2b39f3a7476fb337a9
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+++ b/components/offline_pages/core/recent_tabs/recent_tabs_ui_adapter_delegate.h
@@ -0,0 +1,85 @@
+// Copyright 2016 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.
+
+#ifndef COMPONENTS_OFFLINE_PAGES_CORE_RECENT_TABS_RECENT_TABS_UI_ADAPTER_DELEGATE_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_RECENT_TABS_RECENT_TABS_UI_ADAPTER_DELEGATE_H_
+
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "base/observer_list.h"
+#include "base/supports_user_data.h"
+#include "components/offline_pages/core/client_namespace_constants.h"
+#include "components/offline_pages/core/downloads/download_ui_adapter.h"
+#include "components/offline_pages/core/offline_page_model.h"
+#include "components/offline_pages/core/offline_page_types.h"
+#include "url/gurl.h"
+
+namespace offline_pages {
+
+// Keeps track of all the tabs open for the profile, and restricts its view of
+// the offline pages to only those that have an open tab.  This begins
+// observation early, as soon as the first Tab is created, and even before that
+// tab gets a web contents.
+//
+// In this UI adapter, |guid| represents the tab ID for a given page.
+class RecentTabsUIAdapterDelegate : public DownloadUIAdapter::Delegate {
+ public:
+  explicit RecentTabsUIAdapterDelegate(OfflinePageModel* model);
+  ~RecentTabsUIAdapterDelegate() override;
+
+  static DownloadUIAdapter* GetOrCreateRecentTabsUIAdapter(
+      OfflinePageModel* offline_page_model,
+      RequestCoordinator* request_coordinator);
+  static RecentTabsUIAdapterDelegate* FromDownloadUIAdapter(
+      DownloadUIAdapter* adapter);
+  // This extracts the tab ID out of the |id| field of the client ID using a
+  // string conversion.  Crashes if conversion fails.
+  static int TabIdFromClientId(const ClientId& client_id);
+
+  // This override returns true if the client ID is shown on the NTP as a recent
+  // tab.  This is determined by policy.
+  bool IsVisibleInUI(const ClientId& client_id) override;
+  // This returns true if there does not exist a tab ID for the given client ID.
+  bool IsTemporarilyHiddenInUI(const ClientId& client_id) override;
+  // Sets our reference to the UI adapter so we can notify it of visibility
+  // changes.
+  void SetUIAdapter(DownloadUIAdapter* ui_adapter) override;
+
+  // Register/UnregisterTab add and remove tab IDs from the list.  These
+  // functions can be called before a page actually exists with the given tab
+  // ID.
+  // Note that these change temporary visibility and therefore |set_ui_adapter|
+  // must be called before either of these functions.
+  void RegisterTab(int tab_id);
+  void UnregisterTab(int tab_id);
+
+ private:
+  // Constructs the client ID assuming that we use the Last N namespace.
+  //
+  // TODO(dewittj): Make this more resilient to adding more namespaces.
+  static ClientId ClientIdFromTabId(int tab_id);
+
+  // Checks a client ID for proper namespace and ID format to be shown in the
+  // Downloads Home UI.
+  bool IsRecentTab(const ClientId& page);
+
+  // Always valid, this class is owned by DownloadUIAdapter, which is owned by
+  // Offline Page Model.
+  OfflinePageModel* model_;
+
+  // This is set by the UI adapter itself.
+  DownloadUIAdapter* ui_adapter_ = nullptr;
+
+  // The tabs we've seen.
+  std::unordered_set<int> active_tabs_;
+
+  DISALLOW_COPY_AND_ASSIGN(RecentTabsUIAdapterDelegate);
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_RECENT_TABS_RECENT_TABS_UI_ADAPTER_DELEGATE_H_