Preference Service User Guide

What is the Preference Service?

Preferences, also known as “prefs”, are key-value pairs stored by Chrome. Examples include the settings in chrome://settings, all per-extension metadata, the list of plugins and so on. Individual prefs are keyed by a string and have a type. E.g., the “browser.enable_spellchecking” pref stores a boolean indicating whether spell-checking is enabled.

The pref service persists prefs to disk and communicates updates to prefs between services, including Chrome itself. There is a pref service instance per profile (prefs are persisted on a per-profile basis).

Semantics

Updates made on the PrefService object are reflected immediately in the originating service and eventually in all other services. In other words, updates are eventually consistent.

Registering your preferences

Every pref should be owned by one service. The owning service provides the type and default value for that pref. Owned prefs can be registered as public, meaning other services can read and/or write them, or private (the default). Services that want to access a pref not owned by them must still register those prefs as “foreign” prefs. Registration happens through the PrefRegistry passed to ConnectToPrefService. For example:

//services/my_service/my_service.cc

void MyService::OnStart() {
  auto pref_registry = base::MakeRefCounted<PrefRegistrySimple>();
  pref_registry->RegisterIntegerPref(kKey, kInitialValue, PrefRegistry::PUBLIC);
  prefs::ConnectToPrefService(...);
}

//services/other_service/other_service.cc

void OtherService::OnStart() {
  auto pref_registry = base::MakeRefCounted<PrefRegistrySimple>();
  pref_registry->RegisterForeignPref(kKey);
  prefs::ConnectToPrefService(...);
}

Design

The design doc is here:

https://docs.google.com/document/d/1JU8QUWxMEXWMqgkvFUumKSxr7Z-nfq0YvreSJTkMVmU/edit?usp=sharing