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// Copyright (c) 2012 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 <stddef.h>
#include <memory>
#include <string>
#include "base/base_export.h"
#include "base/callback.h"
#include "base/macros.h"
#include "base/message_loop/message_loop.h"
#include "base/message_loop/timer_slack.h"
#include "base/single_thread_task_runner.h"
#include "base/synchronization/lock.h"
#include "base/synchronization/waitable_event.h"
#include "base/threading/platform_thread.h"
#include "build/build_config.h"
namespace base {
class MessagePump;
// A simple thread abstraction that establishes a MessageLoop on a new thread.
// The consumer uses the MessageLoop of the thread to cause code to execute on
// the thread. When this object is destroyed the thread is terminated. All
// pending tasks queued on the thread's message loop will run to completion
// before the thread is terminated.
// After the thread is stopped, the destruction sequence is:
// (1) Thread::CleanUp()
// (2) MessageLoop::~MessageLoop
// (3.b) MessageLoop::DestructionObserver::WillDestroyCurrentMessageLoop
class BASE_EXPORT Thread : PlatformThread::Delegate {
struct BASE_EXPORT Options {
typedef Callback<std::unique_ptr<MessagePump>()> MessagePumpFactory;
Options(MessageLoop::Type type, size_t size);
Options(const Options& other);
// Specifies the type of message loop that will be allocated on the thread.
// This is ignored if message_pump_factory.is_null() is false.
MessageLoop::Type message_loop_type;
// Specifies timer slack for thread message loop.
TimerSlack timer_slack;
// Used to create the MessagePump for the MessageLoop. The callback is Run()
// on the thread. If message_pump_factory.is_null(), then a MessagePump
// appropriate for |message_loop_type| is created. Setting this forces the
// MessageLoop::Type to TYPE_CUSTOM.
MessagePumpFactory message_pump_factory;
// Specifies the maximum stack size that the thread is allowed to use.
// This does not necessarily correspond to the thread's initial stack size.
// A value of 0 indicates that the default maximum should be used.
size_t stack_size;
// Specifies the initial thread priority.
ThreadPriority priority;
// Constructor.
// name is a display string to identify the thread.
explicit Thread(const std::string& name);
// Destroys the thread, stopping it if necessary.
// guarantee Stop() is explicitly called before the subclass is destroyed).
// This is required to avoid a data race between the destructor modifying the
// vtable, and the thread's ThreadMain calling the virtual method Run(). It
// also ensures that the CleanUp() virtual method is called on the subclass
// before it is destructed.
~Thread() override;
#if defined(OS_WIN)
// Causes the thread to initialize COM. This must be called before calling
// Start() or StartWithOptions(). If |use_mta| is false, the thread is also
// started with a TYPE_UI message loop. It is an error to call
// init_com_with_mta(false) and then StartWithOptions() with any message loop
// type other than TYPE_UI.
void init_com_with_mta(bool use_mta) {
com_status_ = use_mta ? MTA : STA;
// Starts the thread. Returns true if the thread was successfully started;
// otherwise, returns false. Upon successful return, the message_loop()
// getter will return non-null.
// Note: This function can't be called on Windows with the loader lock held;
// i.e. during a DllMain, global object construction or destruction, atexit()
// callback.
bool Start();
// Starts the thread. Behaves exactly like Start in addition to allow to
// override the default options.
// Note: This function can't be called on Windows with the loader lock held;
// i.e. during a DllMain, global object construction or destruction, atexit()
// callback.
bool StartWithOptions(const Options& options);
// Starts the thread and wait for the thread to start and run initialization
// before returning. It's same as calling Start() and then
// WaitUntilThreadStarted().
// Note that using this (instead of Start() or StartWithOptions() causes
// jank on the calling thread, should be used only in testing code.
bool StartAndWaitForTesting();
// Blocks until the thread starts running. Called within StartAndWait().
// Note that calling this causes jank on the calling thread, must be used
// carefully for production code.
bool WaitUntilThreadStarted() const;
// Signals the thread to exit and returns once the thread has exited. After
// this method returns, the Thread object is completely reset and may be used
// as if it were newly constructed (i.e., Start may be called again).
// Stop may be called multiple times and is simply ignored if the thread is
// already stopped.
// NOTE: If you are a consumer of Thread, it is not necessary to call this
// before deleting your Thread objects, as the destructor will do it.
void Stop();
// Signals the thread to exit in the near future.
// WARNING: This function is not meant to be commonly used. Use at your own
// risk. Calling this function will cause message_loop() to become invalid in
// the near future. This function was created to workaround a specific
// deadlock on Windows with printer worker thread. In any other case, Stop()
// should be used.
// StopSoon should not be called multiple times as it is risky to do so. It
// could cause a timing issue in message_loop() access. Call Stop() to reset
// the thread object once it is known that the thread has quit.
void StopSoon();
// Returns the message loop for this thread. Use the MessageLoop's
// PostTask methods to execute code on the thread. This only returns
// non-null after a successful call to Start. After Stop has been called,
// this will return nullptr.
// NOTE: You must not call this MessageLoop's Quit method directly. Use
// the Thread's Stop method instead.
MessageLoop* message_loop() const { return message_loop_; }
// Returns a TaskRunner for this thread. Use the TaskRunner's PostTask
// methods to execute code on the thread. Returns nullptr if the thread is not
// running (e.g. before Start or after Stop have been called). Callers can
// hold on to this even after the thread is gone; in this situation, attempts
// to PostTask() will fail.
scoped_refptr<SingleThreadTaskRunner> task_runner() const {
return message_loop_ ? message_loop_->task_runner() : nullptr;
// Returns the name of this thread (for display in debugger too).
const std::string& thread_name() const { return name_; }
// The native thread handle.
PlatformThreadHandle thread_handle() { return thread_; }
// Returns the thread ID. Should not be called before the first Start*()
// call. Keeps on returning the same ID even after a Stop() call. The next
// Start*() call renews the ID.
// WARNING: This function will block if the thread hasn't started yet.
PlatformThreadId GetThreadId() const;
// Returns true if the thread has been started, and not yet stopped.
bool IsRunning() const;
// Called just prior to starting the message loop
virtual void Init() {}
// Called to start the message loop
virtual void Run(MessageLoop* message_loop);
// Called just after the message loop ends
virtual void CleanUp() {}
static void SetThreadWasQuitProperly(bool flag);
static bool GetThreadWasQuitProperly();
void set_message_loop(MessageLoop* message_loop) {
message_loop_ = message_loop;
#if defined(OS_WIN)
enum ComStatus {
// PlatformThread::Delegate methods:
void ThreadMain() override;
#if defined(OS_WIN)
// Whether this thread needs to initialize COM, and if so, in what mode.
ComStatus com_status_;
// If true, we're in the middle of stopping, and shouldn't access
// |message_loop_|. It may non-nullptr and invalid.
// Should be written on the thread that created this thread. Also read data
// could be wrong on other threads.
bool stopping_;
// True while inside of Run().
bool running_;
mutable base::Lock running_lock_; // Protects |running_|.
// The thread's handle.
PlatformThreadHandle thread_;
mutable base::Lock thread_lock_; // Protects |thread_|.
// The thread's id once it has started.
PlatformThreadId id_;
mutable WaitableEvent id_event_; // Protects |id_|.
// The thread's message loop. Valid only while the thread is alive. Set
// by the created thread.
MessageLoop* message_loop_;
// Stores Options::timer_slack_ until the message loop has been bound to
// a thread.
TimerSlack message_loop_timer_slack_;
// The name of the thread. Used for debugging purposes.
std::string name_;
// Signaled when the created thread gets ready to use the message loop.
mutable WaitableEvent start_event_;
friend void ThreadQuitHelper();
} // namespace base