Add a cache for FillFontFamilyMap.

chrome/browser/font_family_cache.h

+// Copyright 2014 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 CHROME_BROWSER_FONT_FAMILY_CACHE_H_
+#define CHROME_BROWSER_FONT_FAMILY_CACHE_H_
+
+#include "base/containers/hash_tables.h"
+#include "base/gtest_prod_util.h"
+#include "base/prefs/pref_change_registrar.h"
+#include "base/strings/string16.h"
+#include "base/supports_user_data.h"
+#include "content/public/common/web_preferences.h"
+
+class PrefService;
+class Profile;
+
+FORWARD_DECLARE_TEST(FontFamilyCacheTest, Caching);
+
+// Caches font family preferences associated with a PrefService. This class
+// relies on the assumption that each concatenation of map_name + '.' + script
+// is a unique string. It also relies on the assumption that the (const char*)
+// keys used in both inner and outer hash_maps are compile time constants.
+class FontFamilyCache : public base::SupportsUserData::Data {
+ public:
+  explicit FontFamilyCache(PrefService* prefs);
+  virtual ~FontFamilyCache();
+
+  // Gets or creates the relevant FontFamilyCache, and then fills |map|.
+  static void FillFontFamilyMap(Profile* profile,
+                                const char* map_name,
+                                content::ScriptFontFamilyMap* map);
+
+  // Fills |map| with font family preferences.
+  void FillFontFamilyMap(const char* map_name,
+                         content::ScriptFontFamilyMap* map);
+
+ protected:
+  // Exposed and virtual for testing.
+  // Fetches the font without checking the cache.
+  virtual base::string16 FetchFont(const char* script, const char* map_name);
+
+ private:
+  FRIEND_TEST_ALL_PREFIXES(::FontFamilyCacheTest, Caching);
+
+  // Map from script to font.
+  // Key comparison uses pointer equality.
+  typedef base::hash_map<const char*, base::string16> ScriptFontMap;
+
+  // Map from font family to ScriptFontMap.
+  // Key comparison uses pointer equality.
+  typedef base::hash_map<const char*, ScriptFontMap> FontFamilyMap;
+
+  // Checks the cache for the font. If not present, fetches the font and stores
+  // the result in the cache.
+  // This method needs to be very fast, because it's called ~20,000 times on a
+  // fresh launch with an empty profile. It's important to avoid unnecessary
+  // object construction, hence the heavy use of const char* and the minimal use
+  // of std::string.
+  // |script| and |map_name| must be compile time constants. Two behaviors rely
+  // on this: key comparison uses pointer equality, and keys must outlive the
+  // hash_maps.
+  base::string16 FetchAndCacheFont(const char* script, const char* map_name);
+
+  // Called when font family preferences changed.
+  // Invalidates the cached entry, and removes the relevant observer.
+  // Note: It is safe to remove the observer from the pref change callback.
+  void OnPrefsChanged(const std::string& pref_name);
+
+  // Cache of font family preferences.
+  FontFamilyMap font_family_map_;
+
+  // Weak reference.
+  // Note: The lifetime of this object is tied to the lifetime of the
+  // PrefService, so there is no worry about an invalid pointer.
+  const PrefService* prefs_;
+
+  // Reacts to profile changes.
+  PrefChangeRegistrar profile_pref_registrar_;
+
+  DISALLOW_COPY_AND_ASSIGN(FontFamilyCache);
+};
+
+#endif  // CHROME_BROWSER_FONT_FAMILY_CACHE_H_