base/prefs/scoped_user_pref_update.h

   1 // Copyright 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4 //
   5 // A helper class that assists preferences in firing notifications when lists
   6 // or dictionaries are changed.
   7
   8 #ifndef BASE_PREFS_SCOPED_USER_PREF_UPDATE_H_
   9 #define BASE_PREFS_SCOPED_USER_PREF_UPDATE_H_
  10
  11 #include <string>
  12
  13 #include "base/basictypes.h"
  14 #include "base/prefs/base_prefs_export.h"
  15 #include "base/prefs/pref_service.h"
  16 #include "base/threading/non_thread_safe.h"
  17 #include "base/values.h"
  18
  19 class PrefService;
  20
  21 namespace base {
  22 class DictionaryValue;
  23 class ListValue;
  24 }
  25
  26 namespace subtle {
  27
  28 // Base class for ScopedUserPrefUpdateTemplate that contains the parts
  29 // that do not depend on ScopedUserPrefUpdateTemplate's template parameter.
  30 //
  31 // We need this base class mostly for making it a friend of PrefService
  32 // and getting access to PrefService::GetMutableUserPref and
  33 // PrefService::ReportUserPrefChanged.
  34 class BASE_PREFS_EXPORT ScopedUserPrefUpdateBase : public base::NonThreadSafe {
  35  protected:
  36   ScopedUserPrefUpdateBase(PrefService* service, const char* path);
  37
  38   // Calls Notify().
  39   ~ScopedUserPrefUpdateBase();
  40
  41   // Sets |value_| to |service_|->GetMutableUserPref and returns it.
  42   base::Value* GetValueOfType(base::Value::Type type);
  43
  44  private:
  45   // If |value_| is not null, triggers a notification of PrefObservers and
  46   // resets |value_|.
  47   void Notify();
  48
  49   // Weak pointer.
  50   PrefService* service_;
  51   // Path of the preference being updated.
  52   std::string path_;
  53   // Cache of value from user pref store (set between Get() and Notify() calls).
  54   base::Value* value_;
  55
  56   DISALLOW_COPY_AND_ASSIGN(ScopedUserPrefUpdateBase);
  57 };
  58
  59 }  // namespace subtle
  60
  61 // Class to support modifications to DictionaryValues and ListValues while
  62 // guaranteeing that PrefObservers are notified of changed values.
  63 //
  64 // This class may only be used on the UI thread as it requires access to the
  65 // PrefService.
  66 template <typename T, base::Value::Type type_enum_value>
  67 class ScopedUserPrefUpdate : public subtle::ScopedUserPrefUpdateBase {
  68  public:
  69   ScopedUserPrefUpdate(PrefService* service, const char* path)
  70       : ScopedUserPrefUpdateBase(service, path) {}
  71
  72   // Triggers an update notification if Get() was called.
  73   virtual ~ScopedUserPrefUpdate() {}
  74
  75   // Returns a mutable |T| instance that
  76   // - is already in the user pref store, or
  77   // - is (silently) created and written to the user pref store if none existed
  78   //   before.
  79   //
  80   // Calling Get() implies that an update notification is necessary at
  81   // destruction time.
  82   //
  83   // The ownership of the return value remains with the user pref store.
  84   // Virtual so it can be overriden in subclasses that transform the value
  85   // before returning it (for example to return a subelement of a dictionary).
  86   virtual T* Get() {
  87     return static_cast<T*>(GetValueOfType(type_enum_value));
  88   }
  89
  90   T& operator*() {
  91     return *Get();
  92   }
  93
  94   T* operator->() {
  95     return Get();
  96   }
  97
  98  private:
  99   DISALLOW_COPY_AND_ASSIGN(ScopedUserPrefUpdate);
 100 };
 101
 102 typedef ScopedUserPrefUpdate<base::DictionaryValue,
 103                              base::Value::TYPE_DICTIONARY>
 104     DictionaryPrefUpdate;
 105 typedef ScopedUserPrefUpdate<base::ListValue, base::Value::TYPE_LIST>
 106     ListPrefUpdate;
 107
 108 #endif  // BASE_PREFS_SCOPED_USER_PREF_UPDATE_H_