blob: 523fd137b66557e7146f1774b661b8b8e3cf6d5e [file] [log] [blame]
// Copyright 2016 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.
#include <stdint.h>
#include <iosfwd>
#include "base/base_export.h"
#include "build/build_config.h"
namespace base {
// Valid priorities supported by the task scheduler. Note: internal algorithms
// depend on priorities being expressed as a continuous zero-based list from
// lowest to highest priority. Users of this API shouldn't otherwise care about
// nor use the underlying values.
enum class TaskPriority {
// This will always be equal to the lowest priority available.
// User won't notice if this task takes an arbitrarily long time to complete.
// This task affects UI or responsiveness of future user interactions. It is
// not an immediate response to a user interaction.
// Examples:
// - Updating the UI to reflect progress on a long task.
// - Loading data that might be shown in the UI after a future user
// interaction.
// This task affects UI immediately after a user interaction.
// Example: Generating data shown in the UI immediately after a click.
// This will always be equal to the highest priority available.
// Valid shutdown behaviors supported by the task scheduler.
enum class TaskShutdownBehavior {
// Tasks posted with this mode which have not started executing before
// shutdown is initiated will never run. Tasks with this mode running at
// shutdown will be ignored (the worker thread will not be joined).
// This option provides a nice way to post stuff you don't want blocking
// shutdown. For example, you might be doing a slow DNS lookup and if it's
// blocked on the OS, you may not want to stop shutdown, since the result
// doesn't really matter at that point.
// However, you need to be very careful what you do in your callback when you
// use this option. Since the thread will continue to run until the OS
// terminates the process, the app can be in the process of tearing down when
// you're running. This means any singletons or global objects you use may
// suddenly become invalid out from under you. For this reason, it's best to
// use this only for slow but simple operations like the DNS example.
// Tasks posted with this mode that have not started executing at
// shutdown will never run. However, any task that has already begun
// executing when shutdown is invoked will be allowed to continue and
// will block shutdown until completion.
// Note: Because TaskScheduler::Shutdown() may block while these tasks are
// executing, care must be taken to ensure that they do not block on the
// thread that called TaskScheduler::Shutdown(), as this may lead to deadlock.
// Tasks posted with this mode before shutdown is complete will block shutdown
// until they're executed. Generally, this should be used only to save
// critical user data.
// Note: Tasks with BACKGROUND priority that block shutdown will be promoted
// to USER_VISIBLE priority during shutdown.
// Describes metadata for a single task or a group of tasks.
class BASE_EXPORT TaskTraits {
// Constructs a default TaskTraits for tasks with
// (1) no I/O,
// (2) low priority, and
// (3) may block shutdown or be skipped on shutdown.
// Tasks that require stricter guarantees should highlight those by requesting
// explicit traits below.
TaskTraits(const TaskTraits& other) = default;
TaskTraits& operator=(const TaskTraits& other) = default;
// Allows tasks with these traits to do file I/O.
TaskTraits& WithFileIO();
// Applies |priority| to tasks with these traits.
TaskTraits& WithPriority(TaskPriority priority);
// Applies |shutdown_behavior| to tasks with these traits.
TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
// Returns true if file I/O is allowed by these traits.
bool with_file_io() const { return with_file_io_; }
// Returns the priority of tasks with these traits.
TaskPriority priority() const { return priority_; }
// Returns the shutdown behavior of tasks with these traits.
TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
bool with_file_io_;
TaskPriority priority_;
TaskShutdownBehavior shutdown_behavior_;
// Describes how tasks are executed by a task runner.
enum class ExecutionMode {
// Can execute multiple tasks at a time in any order.
// Executes one task at a time in posting order. The sequence’s priority is
// equivalent to the highest priority pending task in the sequence.
// Executes one task at a time on a single thread in posting order.
// Stream operators so TaskPriority and TaskShutdownBehavior can be used in
// DCHECK statements.
BASE_EXPORT std::ostream& operator<<(std::ostream& os,
const TaskPriority& shutdown_behavior);
BASE_EXPORT std::ostream& operator<<(
std::ostream& os,
const TaskShutdownBehavior& shutdown_behavior);
} // namespace base