simple_keyed_service_factory.h | Explore in Territory

// Copyright 2019 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef COMPONENTS_KEYED_SERVICE_CORE_SIMPLE_KEYED_SERVICE_FACTORY_H_
#define COMPONENTS_KEYED_SERVICE_CORE_SIMPLE_KEYED_SERVICE_FACTORY_H_

#include <memory>

#include "base/compiler_specific.h"
#include "components/keyed_service/core/keyed_service_export.h"
#include "components/keyed_service/core/keyed_service_factory.h"

class KeyedService;
class SimpleDependencyManager;
class SimpleFactoryKey;

// Base class for Factories that take a SimpleFactoryKey object and return some
// service on a one-to-one mapping. Each factory that derives from this class
// *must* be a Singleton (only unit tests don't do that).
//
// We do this because services depend on each other and we need to control
// shutdown/destruction order. In each derived classes' constructors, the
// implementors must explicitly state on which services they depend.
//
// Note:
// The SimpleKeyedServiceFactory (SKSF) provides a way to create or get a
// SimpleKeyedService before BrowserContext is created.
// BrowserContextKeyedServiceFactories (BCKSFs) can only be converted to
// SKSFs as long as they access only part of Profile properties:
// path, PrefService, and is_off_the_record flag.
//
// An SKSF shouldn't declare DependsOn() of any BCKSF on creation (constructor).
// It is because an SKSF can't depend on any BCKSF when it is created. However,
// dependencies from SKSFs to BCKSFs may exist after full browser launches,
// since some SimpleKeyedServices move from "reduced mode" to "full browser
// mode" when the full browser starts up, which involves injection of
// BrowserContextKeyedService dependencies into the SimpleKeyedService.
//
// If such dependencies exist in a SimpleKeyedService, the service **MUST**
// explicitly reset/clean up the dependencies in KeyedService::Shutdown().
//
// Once the dependencies are reset, the dependencies from the BCKSF dependency
// graph to the SKSF dependency graph are removed. Therefore, we adopt a
// two-phase shutdown:
// - Shutdown of all BCKSFactories
// - Shutdown of all SKSFactories
// - Destruction of all BCKSFactories
// - Destruction of all SKSFactories

// A SimpleKeyedService should *AVOID* full browser inflation whenever it is
// possible. A solution might be splitting the part of the service that
// depends on BrowserContextKeyedService or BrowserContext into a separate
// BrowserContextKeyedService.
//
// See
// https://docs.google.com/document/d/1caWonaPnBhMb6sk4syNe0BbdsQih13S6QmDW237Mcrg/edit?usp=sharing
// for more details.
class KEYED_SERVICE_EXPORT SimpleKeyedServiceFactory
    : public KeyedServiceFactory { … };

#endif  // COMPONENTS_KEYED_SERVICE_CORE_SIMPLE_KEYED_SERVICE_FACTORY_H_
chromium/components/keyed_service/core/simple_keyed_service_factory.h
