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213 lines
8.4 KiB
C++
213 lines
8.4 KiB
C++
// Copyright 2017 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_TASK_SCHEDULER_LAZY_TASK_RUNNER_H_
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#define BASE_TASK_SCHEDULER_LAZY_TASK_RUNNER_H_
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#include <vector>
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#include "base/atomicops.h"
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#include "base/callback.h"
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#include "base/compiler_specific.h"
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#include "base/lazy_instance.h"
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#include "base/sequenced_task_runner.h"
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#include "base/single_thread_task_runner.h"
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#include "base/task_scheduler/scheduler_lock.h"
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#include "base/task_scheduler/single_thread_task_runner_thread_mode.h"
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#include "base/task_scheduler/task_traits.h"
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#include "build/build_config.h"
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// Lazy(Sequenced|SingleThread|COMSTA)TaskRunner lazily creates a TaskRunner.
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//
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// Lazy(Sequenced|SingleThread|COMSTA)TaskRunner is meant to be instantiated in
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// an anonymous namespace (no static initializer is generated) and used to post
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// tasks to the same sequence/thread from pieces of code that don't have a
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// better way of sharing a TaskRunner. It is important to use this class
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// instead of a self-managed global variable or LazyInstance so that the
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// TaskRunners do not outlive the scope of the ScopedTaskEnvironment in unit
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// tests (otherwise the next test in the same process will die in use-after-
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// frees).
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//
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// IMPORTANT: Only use this API as a last resort. Prefer storing a
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// (Sequenced|SingleThread)TaskRunner returned by
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// base::Create(Sequenced|SingleThread|COMSTA)TaskRunnerWithTraits() as a member
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// on an object accessible by all PostTask() call sites.
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//
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// Example usage 1:
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//
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// namespace {
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// base::LazySequencedTaskRunner g_sequenced_task_runner =
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// LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER(
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// base::TaskTraits(base::MayBlock(),
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// base::TaskPriority::USER_VISIBLE));
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// } // namespace
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//
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// void SequencedFunction() {
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// // Different invocations of this function post to the same
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// // MayBlock() SequencedTaskRunner.
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// g_sequenced_task_runner.Get()->PostTask(FROM_HERE, base::BindOnce(...));
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// }
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//
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// Example usage 2:
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//
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// namespace {
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// base::LazySequencedTaskRunner g_sequenced_task_task_runner =
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// LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER({base::MayBlock()});
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// } // namespace
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//
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// // Code from different files can access the SequencedTaskRunner via this
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// // function.
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// scoped_refptr<base::SequencedTaskRunner> GetTaskRunner() {
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// return g_sequenced_task_runner.Get();
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// }
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namespace base {
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namespace internal {
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template <typename TaskRunnerType, bool com_sta>
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class BASE_EXPORT LazyTaskRunner;
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} // namespace internal
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// Lazy SequencedTaskRunner.
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using LazySequencedTaskRunner =
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internal::LazyTaskRunner<SequencedTaskRunner, false>;
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// Lazy SingleThreadTaskRunner.
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using LazySingleThreadTaskRunner =
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internal::LazyTaskRunner<SingleThreadTaskRunner, false>;
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#if defined(OS_WIN)
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// Lazy COM-STA enabled SingleThreadTaskRunner.
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using LazyCOMSTATaskRunner =
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internal::LazyTaskRunner<SingleThreadTaskRunner, true>;
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#endif
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// Helper macros to generate a variable name by concatenation.
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#define LAZY_TASK_RUNNER_CONCATENATE_INTERNAL2(a, b) a##b
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#define LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(a, b) \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL2(a, b)
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// Use the macros below to initialize a LazyTaskRunner. These macros verify that
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// their arguments are constexpr, which is important to prevent the generation
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// of a static initializer.
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// |traits| are TaskTraits used when creating the SequencedTaskRunner.
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#define LAZY_SEQUENCED_TASK_RUNNER_INITIALIZER(traits) \
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base::LazySequencedTaskRunner::CreateInternal(traits); \
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ALLOW_UNUSED_TYPE constexpr base::TaskTraits \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(kVerifyTraitsAreConstexpr, \
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__LINE__) = traits
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// |traits| are TaskTraits used when creating the SingleThreadTaskRunner.
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// |thread_mode| specifies whether the SingleThreadTaskRunner can share its
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// thread with other SingleThreadTaskRunners.
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#define LAZY_SINGLE_THREAD_TASK_RUNNER_INITIALIZER(traits, thread_mode) \
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base::LazySingleThreadTaskRunner::CreateInternal(traits, thread_mode); \
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ALLOW_UNUSED_TYPE constexpr base::TaskTraits \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(kVerifyTraitsAreConstexpr, \
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__LINE__) = traits; \
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ALLOW_UNUSED_TYPE constexpr base::SingleThreadTaskRunnerThreadMode \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(kVerifyThreadModeIsConstexpr, \
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__LINE__) = thread_mode
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// |traits| are TaskTraits used when creating the COM STA
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// SingleThreadTaskRunner. |thread_mode| specifies whether the COM STA
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// SingleThreadTaskRunner can share its thread with other
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// SingleThreadTaskRunners.
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#define LAZY_COM_STA_TASK_RUNNER_INITIALIZER(traits, thread_mode) \
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base::LazyCOMSTATaskRunner::CreateInternal(traits, thread_mode); \
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ALLOW_UNUSED_TYPE constexpr base::TaskTraits \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(kVerifyTraitsAreConstexpr, \
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__LINE__) = traits; \
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ALLOW_UNUSED_TYPE constexpr base::SingleThreadTaskRunnerThreadMode \
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LAZY_TASK_RUNNER_CONCATENATE_INTERNAL(kVerifyThreadModeIsConstexpr, \
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__LINE__) = thread_mode
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namespace internal {
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template <typename TaskRunnerType, bool com_sta>
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class BASE_EXPORT LazyTaskRunner {
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public:
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// Use the macros above rather than a direct call to this.
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//
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// |traits| are TaskTraits to use to create the TaskRunner. If this
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// LazyTaskRunner is specialized to create a SingleThreadTaskRunner,
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// |thread_mode| specifies whether the SingleThreadTaskRunner can share its
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// thread with other SingleThreadTaskRunner. Otherwise, it is unused.
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static constexpr LazyTaskRunner CreateInternal(
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const TaskTraits& traits,
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SingleThreadTaskRunnerThreadMode thread_mode =
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SingleThreadTaskRunnerThreadMode::SHARED) {
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return LazyTaskRunner(traits, thread_mode);
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}
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// Returns the TaskRunner held by this instance. Creates it if it didn't
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// already exist. Thread-safe.
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scoped_refptr<TaskRunnerType> Get();
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private:
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constexpr LazyTaskRunner(const TaskTraits& traits,
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SingleThreadTaskRunnerThreadMode thread_mode =
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SingleThreadTaskRunnerThreadMode::SHARED)
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: traits_(traits), thread_mode_(thread_mode) {}
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// Releases the TaskRunner held by this instance.
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void Reset();
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// Creates and returns a new TaskRunner.
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scoped_refptr<TaskRunnerType> Create();
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// TaskTraits to create the TaskRunner.
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const TaskTraits traits_;
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// SingleThreadTaskRunnerThreadMode to create the TaskRunner.
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const SingleThreadTaskRunnerThreadMode thread_mode_;
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// Can have 3 states:
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// - This instance does not hold a TaskRunner: 0
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// - This instance is creating a TaskRunner: kLazyInstanceStateCreating
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// - This instance holds a TaskRunner: Pointer to the TaskRunner.
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// LazyInstance's internals are reused to handle transition between states.
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subtle::AtomicWord state_ = 0;
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// No DISALLOW_COPY_AND_ASSIGN since that prevents static initialization with
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// Visual Studio (warning C4592: 'symbol will be dynamically initialized
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// (implementation limitation))'.
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};
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// When a LazyTaskRunner becomes active (invokes Get()), it adds a callback to
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// the current ScopedLazyTaskRunnerListForTesting, if any. Callbacks run when
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// the ScopedLazyTaskRunnerListForTesting is destroyed. In a test process, a
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// ScopedLazyTaskRunnerListForTesting must be instantiated before any
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// LazyTaskRunner becomes active.
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class BASE_EXPORT ScopedLazyTaskRunnerListForTesting {
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public:
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ScopedLazyTaskRunnerListForTesting();
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~ScopedLazyTaskRunnerListForTesting();
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private:
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friend class LazyTaskRunner<SequencedTaskRunner, false>;
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friend class LazyTaskRunner<SingleThreadTaskRunner, false>;
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#if defined(OS_WIN)
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friend class LazyTaskRunner<SingleThreadTaskRunner, true>;
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#endif
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// Add |callback| to the list of callbacks to run on destruction.
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void AddCallback(OnceClosure callback);
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// Synchronizes accesses to |callbacks_|.
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SchedulerLock lock_;
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// List of callbacks to run on destruction.
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std::vector<OnceClosure> callbacks_;
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DISALLOW_COPY_AND_ASSIGN(ScopedLazyTaskRunnerListForTesting);
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};
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} // namespace internal
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} // namespace base
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#endif // BASE_TASK_SCHEDULER_LAZY_TASK_RUNNER_H_
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