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// Copyright 2013 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 <memory>
#include <queue>
#include <string>
#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/gtest_prod_util.h"
#include "base/macros.h"
#include "base/memory/scoped_refptr.h"
#include "base/message_loop/message_loop_current.h"
#include "base/message_loop/message_pump.h"
#include "base/message_loop/pending_task_queue.h"
#include "base/message_loop/timer_slack.h"
#include "base/observer_list.h"
#include "base/pending_task.h"
#include "base/run_loop.h"
#include "base/synchronization/lock.h"
#include "base/threading/sequence_local_storage_map.h"
#include "base/threading/thread_checker.h"
#include "base/time/time.h"
#include "build/build_config.h"
namespace base {
class SequencedTaskSource;
class ThreadTaskRunnerHandle;
namespace internal {
class MessageLoopTaskRunner;
// A MessageLoop is used to process events for a particular thread. There is
// at most one MessageLoop instance per thread.
// Events include at a minimum Task instances submitted to the MessageLoop's
// TaskRunner. Depending on the type of message pump used by the MessageLoop
// other events such as UI messages may be processed. On Windows APC calls (as
// time permits) and signals sent to a registered set of HANDLEs may also be
// processed.
// The MessageLoop's API should only be used directly by its owner (and users
// which the owner opts to share a MessageLoop* with). Other ways to access
// subsets of the MessageLoop API:
// - base::RunLoop : Drive the MessageLoop from the thread it's bound to.
// - base::Thread/SequencedTaskRunnerHandle : Post back to the MessageLoop
// from a task running on it.
// - SequenceLocalStorageSlot : Bind external state to this MessageLoop.
// - base::MessageLoopCurrent : Access statically exposed APIs of this
// MessageLoop.
// - Embedders may provide their own static accessors to post tasks on
// specific loops (e.g. content::BrowserThreads).
// NOTE: Unless otherwise specified, a MessageLoop's methods may only be called
// on the thread where the MessageLoop's Run method executes.
// NOTE: MessageLoop has task reentrancy protection. This means that if a
// task is being processed, a second task cannot start until the first task is
// finished. Reentrancy can happen when processing a task, and an inner
// message pump is created. That inner pump then processes native messages
// which could implicitly start an inner task. Inner message pumps are created
// with dialogs (DialogBox), common dialogs (GetOpenFileName), OLE functions
// (DoDragDrop), printer functions (StartDoc) and *many* others.
// Sample workaround when inner task processing is needed:
// HRESULT hr;
// {
// MessageLoopCurrent::ScopedNestableTaskAllower allow;
// hr = DoDragDrop(...); // Implicitly runs a modal message loop.
// }
// // Process |hr| (the result returned by DoDragDrop()).
// Please be SURE your task is reentrant (nestable) and all global variables
// are stable and accessible before calling SetNestableTasksAllowed(true).
// TODO(gab): MessageLoop doesn't need to be a MessageLoopCurrent once callers
// that store MessageLoop::current() in a MessageLoop* variable have been
// updated to use a MessageLoopCurrent variable.
class BASE_EXPORT MessageLoop : public MessagePump::Delegate,
public RunLoop::Delegate,
public MessageLoopCurrent {
// TODO(gab): Migrate usage of this class to MessageLoopCurrent and remove
// this forwarded declaration.
using DestructionObserver = MessageLoopCurrent::DestructionObserver;
// A MessageLoop has a particular type, which indicates the set of
// asynchronous events it may process in addition to tasks and timers.
// This type of ML only supports tasks and timers.
// This type of ML also supports native UI events (e.g., Windows messages).
// See also MessageLoopForUI.
// This type of ML also supports asynchronous IO. See also
// MessageLoopForIO.
// This type of ML is backed by a Java message handler which is responsible
// for running the tasks added to the ML. This is only for use on Android.
// TYPE_JAVA behaves in essence like TYPE_UI, except during construction
// where it does not use the main thread specific pump factory.
// MessagePump was supplied to constructor.
enum Type {
#if defined(OS_ANDROID)
#endif // defined(OS_ANDROID)
// Normally, it is not necessary to instantiate a MessageLoop. Instead, it
// is typical to make use of the current thread's MessageLoop instance.
explicit MessageLoop(Type type = TYPE_DEFAULT);
// Creates a TYPE_CUSTOM MessageLoop with the supplied MessagePump, which must
// be non-NULL.
explicit MessageLoop(std::unique_ptr<MessagePump> pump);
~MessageLoop() override;
// TODO(gab): Mass migrate callers to MessageLoopCurrent::Get().
static MessageLoopCurrent current();
using MessagePumpFactory = std::unique_ptr<MessagePump>();
// Uses the given base::MessagePumpForUIFactory to override the default
// MessagePump implementation for 'TYPE_UI'. Returns true if the factory
// was successfully registered.
static bool InitMessagePumpForUIFactory(MessagePumpFactory* factory);
// Creates the default MessagePump based on |type|. Caller owns return
// value.
static std::unique_ptr<MessagePump> CreateMessagePumpForType(Type type);
// Set the timer slack for this message loop.
void SetTimerSlack(TimerSlack timer_slack) {
// Returns true if this loop is |type|. This allows subclasses (especially
// those in tests) to specialize how they are identified.
virtual bool IsType(Type type) const;
// Returns the type passed to the constructor.
Type type() const { return type_; }
// Returns the name of the thread this message loop is bound to. This function
// is only valid when this message loop is running, BindToCurrentThread has
// already been called and has an "happens-before" relationship with this call
// (this relationship is obtained implicitly by the MessageLoop's task posting
// system unless calling this very early).
std::string GetThreadName() const;
// Gets the TaskRunner associated with this message loop.
const scoped_refptr<SingleThreadTaskRunner>& task_runner() const {
return task_runner_;
// Sets a new TaskRunner for this message loop. If the message loop was
// already bound, this must be called on the thread to which it is bound.
void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
// TODO( Remove users of TaskObservers through
// MessageLoop::current() and migrate the type back here.
using TaskObserver = MessageLoopCurrent::TaskObserver;
// These functions can only be called on the same thread that |this| is
// running on.
// These functions must not be called from a TaskObserver callback.
void AddTaskObserver(TaskObserver* task_observer);
void RemoveTaskObserver(TaskObserver* task_observer);
// When this functionality is enabled, the queue time will be recorded for
// posted tasks.
void SetAddQueueTimeToTasks(bool enable);
// Returns true if the message loop is idle (ignoring delayed tasks). This is
// the same condition which triggers DoWork() to return false: i.e.
// out of tasks which can be processed at the current run-level -- there might
// be deferred non-nestable tasks remaining if currently in a nested run
// level.
bool IsIdleForTesting();
// Runs the specified PendingTask.
void RunTask(PendingTask* pending_task);
#if defined(STARBOARD)
// Starboard main requires the ability to force shut down.
void Quit() override;
// For testing purpose only.
void RunUntilIdleForTesting();
std::unique_ptr<MessagePump> pump_;
using MessagePumpFactoryCallback =
// Common protected constructor. Other constructors delegate the
// initialization to this constructor.
// A subclass can invoke this constructor to create a message_loop of a
// specific type with a custom loop. The implementation does not call
// BindToCurrentThread. If this constructor is invoked directly by a subclass,
// then the subclass must subsequently bind the message loop.
MessageLoop(Type type, MessagePumpFactoryCallback pump_factory);
// Configure various members and bind this message loop to the current thread.
void BindToCurrentThread();
friend class MessageLoopCurrent;
friend class MessageLoopCurrentForIO;
friend class MessageLoopCurrentForUI;
friend class ScheduleWorkTest;
friend class Thread;
FRIEND_TEST_ALL_PREFIXES(MessageLoopTest, DeleteUnboundLoop);
class Controller;
// Creates a MessageLoop without binding to a thread.
// If |type| is TYPE_CUSTOM non-null |pump_factory| must be also given
// to create a message pump for this message loop. Otherwise a default
// message pump for the |type| is created.
// It is valid to call this to create a new message loop on one thread,
// and then pass it to the thread where the message loop actually runs.
// The message loop's BindToCurrentThread() method must be called on the
// thread the message loop runs on, before calling Run().
// Before BindToCurrentThread() is called, only Post*Task() functions can
// be called on the message loop.
static std::unique_ptr<MessageLoop> CreateUnbound(
Type type,
MessagePumpFactoryCallback pump_factory);
// Sets the ThreadTaskRunnerHandle for the current thread to point to the
// task runner for this message loop.
void SetThreadTaskRunnerHandle();
// RunLoop::Delegate:
void Run(bool application_tasks_allowed) override;
#if !defined(STARBOARD)
void Quit() override;
void EnsureWorkScheduled() override;
// Called to process any delayed non-nestable tasks.
bool ProcessNextDelayedNonNestableTask();
// Calls RunTask or queues the pending_task on the deferred task list if it
// cannot be run right now. Returns true if the task was run.
bool DeferOrRunPendingTask(PendingTask pending_task);
// Delete tasks that haven't run yet without running them. Used in the
// destructor to make sure all the task's destructors get called.
void DeletePendingTasks();
// Wakes up the message pump. Can be called on any thread. The caller is
// responsible for synchronizing ScheduleWork() calls.
void ScheduleWork();
// Returns |next_run_time| capped at 1 day from |recent_time_|. This is used
// to mitigate where some platforms are unhappy with
// delays > 100,000,000 seconds. In practice, a diagnosis metric showed that
// no sleep > 1 hour ever completes (always interrupted by an earlier
// MessageLoop event) and 99% of completed sleeps are the ones scheduled for
// <= 1 second. Details @
TimeTicks CapAtOneDay(TimeTicks next_run_time);
// MessagePump::Delegate methods:
bool DoWork() override;
bool DoDelayedWork(TimeTicks* next_delayed_work_time) override;
bool DoIdleWork() override;
const Type type_;
#if defined(OS_WIN)
// Tracks if we have requested high resolution timers. Its only use is to
// turn off the high resolution timer upon loop destruction.
bool in_high_res_mode_ = false;
// A recent snapshot of Time::Now(), used to check delayed_work_queue_.
TimeTicks recent_time_;
ObserverList<DestructionObserver>::Unchecked destruction_observers_;
// A boolean which prevents unintentional reentrant task execution (e.g. from
// induced nested message loops). As such, nested message loops will only
// process system messages (not application tasks) by default. A nested loop
// layer must have been explicitly granted permission to be able to execute
// application tasks. This is granted either by
// RunLoop::Type::kNestableTasksAllowed when the loop is driven by the
// application or by a ScopedNestableTaskAllower preceding a system call that
// is known to generate a system-driven nested loop.
bool task_execution_allowed_ = true;
// pump_factory_.Run() is called to create a message pump for this loop
// if type_ is TYPE_CUSTOM and pump_ is null.
MessagePumpFactoryCallback pump_factory_;
// Using an ObserverList adds significant overhead. We use a raw vector and
// require that callers do not attempt to mutate the list during a callback.
std::vector<TaskObserver*> task_observers_;
// Pointer to this MessageLoop's Controller, valid throughout this
// MessageLoop's lifetime (until |underlying_task_runner_| is released at the
// end of ~MessageLoop()).
Controller* const message_loop_controller_;
// The task runner this MessageLoop will extract its tasks from. By default,
// it will also be bound as the ThreadTaskRunnerHandle on the current thread.
// That default can be overridden by SetTaskRunner() but this MessageLoop will
// nonetheless take its tasks from |underlying_task_runner_| (the overrider is
// responsible for doing the routing). This member must be before
// |pending_task_queue| as it must outlive it.
const scoped_refptr<internal::MessageLoopTaskRunner> underlying_task_runner_;
// The source of tasks for this MessageLoop. Currently this is always
// |underlying_task_runner_|. TODO(gab): Make this customizable.
SequencedTaskSource* const sequenced_task_source_;
internal::PendingTaskQueue pending_task_queue_;
// The task runner exposed by this message loop.
scoped_refptr<SingleThreadTaskRunner> task_runner_;
std::unique_ptr<ThreadTaskRunnerHandle> thread_task_runner_handle_;
// Id of the thread this message loop is bound to. Initialized once when the
// MessageLoop is bound to its thread and constant forever after.
PlatformThreadId thread_id_ = kInvalidThreadId;
// Holds data stored through the SequenceLocalStorageSlot API.
internal::SequenceLocalStorageMap sequence_local_storage_map_;
// Enables the SequenceLocalStorageSlot API within its scope.
// Instantiated in BindToCurrentThread().
// Verifies that calls are made on the thread on which BindToCurrentThread()
// was invoked.
#if !defined(OS_NACL)
// MessageLoopForUI extends MessageLoop with methods that are particular to a
// MessageLoop instantiated with TYPE_UI.
// By instantiating a MessageLoopForUI on the current thread, the owner enables
// native UI message pumping.
// MessageLoopCurrentForUI is exposed statically on its thread via
// MessageLoopCurrentForUI::Get() to provide additional functionality.
class BASE_EXPORT MessageLoopForUI : public MessageLoop {
explicit MessageLoopForUI(Type type = TYPE_UI);
// TODO(gab): Mass migrate callers to MessageLoopCurrentForUI::Get()/IsSet().
static MessageLoopCurrentForUI current();
static bool IsCurrent();
#if defined(STARBOARD)
// Starboard needs this function to register message loop to pump.
void Start();
#if defined(OS_IOS)
// On iOS, the main message loop cannot be Run(). Instead call Attach(),
// which connects this MessageLoop to the UI thread's CFRunLoop and allows
// PostTask() to work.
void Attach();
#if defined(OS_ANDROID)
// On Android there are cases where we want to abort immediately without
// calling Quit(), in these cases we call Abort().
void Abort();
// True if this message pump has been aborted.
bool IsAborted();
// Since Run() is never called on Android, and the message loop is run by the
// java Looper, quitting the RunLoop won't join the thread, so we need a
// callback to run when the RunLoop goes idle to let the Java thread know when
// it can safely quit.
void QuitWhenIdle(base::OnceClosure callback);
#if defined(OS_WIN)
// See method of the same name in the Windows MessagePumpForUI implementation.
void EnableWmQuit();
// Do not add any member variables to MessageLoopForUI! This is important b/c
// MessageLoopForUI is often allocated via MessageLoop(TYPE_UI). Any extra
// data that you need should be stored on the MessageLoop's pump_ instance.
static_assert(sizeof(MessageLoop) == sizeof(MessageLoopForUI),
"MessageLoopForUI should not have extra member variables");
#endif // !defined(OS_NACL)
// MessageLoopForIO extends MessageLoop with methods that are particular to a
// MessageLoop instantiated with TYPE_IO.
// By instantiating a MessageLoopForIO on the current thread, the owner enables
// native async IO message pumping.
// MessageLoopCurrentForIO is exposed statically on its thread via
// MessageLoopCurrentForIO::Get() to provide additional functionality.
class BASE_EXPORT MessageLoopForIO : public MessageLoop {
MessageLoopForIO() : MessageLoop(TYPE_IO) {}
static MessageLoopCurrentForIO current();
static bool IsCurrent();
// Do not add any member variables to MessageLoopForIO! This is important b/c
// MessageLoopForIO is often allocated via MessageLoop(TYPE_IO). Any extra
// data that you need should be stored on the MessageLoop's pump_ instance.
static_assert(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
"MessageLoopForIO should not have extra member variables");
} // namespace base