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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.
#ifndef BASE_RUN_LOOP_H_
#define BASE_RUN_LOOP_H_
#include <utility>
#include <vector>
#include "base/base_export.h"
#include "base/callback.h"
#include "base/containers/stack.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/memory/weak_ptr.h"
#include "base/observer_list.h"
#include "base/sequence_checker.h"
#include "base/threading/thread_checker.h"
#include "build/build_config.h"
namespace base {
#if defined(OS_ANDROID)
class MessagePumpForUI;
#if defined(OS_IOS)
class MessagePumpUIApplication;
class SingleThreadTaskRunner;
// Helper class to run the RunLoop::Delegate associated with the current thread.
// A RunLoop::Delegate must have been bound to this thread (ref.
// RunLoop::RegisterDelegateForCurrentThread()) prior to using any of RunLoop's
// member and static methods unless explicitly indicated otherwise (e.g.
// IsRunning/IsNestedOnCurrentThread()). RunLoop::Run can only be called once
// per RunLoop lifetime. Create a RunLoop on the stack and call Run/Quit to run
// a nested RunLoop but please do not use nested loops in production code!
class BASE_EXPORT RunLoop {
// The type of RunLoop: a kDefault RunLoop at the top-level (non-nested) will
// process system and application tasks assigned to its Delegate. When nested
// however a kDefault RunLoop will only process system tasks while a
// kNestableTasksAllowed RunLoop will continue to process application tasks
// even if nested.
// This is relevant in the case of recursive RunLoops. Some unwanted run loops
// may occur when using common controls or printer functions. By default,
// recursive task processing is disabled.
// In general, nestable RunLoops are to be avoided. They are dangerous and
// difficult to get right, so please use with extreme caution. To further
// protect this: kNestableTasksAllowed RunLoops are only allowed on threads
// where IsNestingAllowedOnCurrentThread().
// A specific example where this makes a difference is:
// - The thread is running a RunLoop.
// - It receives a task #1 and executes it.
// - The task #1 implicitly starts a RunLoop, like a MessageBox in the unit
// test. This can also be StartDoc or GetSaveFileName.
// - The thread receives a task #2 before or while in this second RunLoop.
// - With a kNestableTasksAllowed RunLoop, the task #2 will run right away.
// Otherwise, it will get executed right after task #1 completes in the main
// RunLoop.
enum class Type {
RunLoop(Type type = Type::kDefault);
// Run the current RunLoop::Delegate. This blocks until Quit is called. Before
// calling Run, be sure to grab the QuitClosure in order to stop the
// RunLoop::Delegate asynchronously.
void Run();
// Run the current RunLoop::Delegate until it doesn't find any tasks or
// messages in its queue (it goes idle). WARNING: This may never return! Only
// use this when repeating tasks such as animated web pages have been shut
// down.
void RunUntilIdle();
bool running() const {
return running_;
// Quit() quits an earlier call to Run() immediately. QuitWhenIdle() quits an
// earlier call to Run() when there aren't any tasks or messages in the queue.
// These methods are thread-safe but note that Quit() is best-effort when
// called from another thread (will quit soon but tasks that were already
// queued on this RunLoop will get to run first).
// There can be other nested RunLoops servicing the same task queue. Quitting
// one RunLoop has no bearing on the others. Quit() and QuitWhenIdle() can be
// called before, during or after Run(). If called before Run(), Run() will
// return immediately when called. Calling Quit() or QuitWhenIdle() after the
// RunLoop has already finished running has no effect.
// WARNING: You must NEVER assume that a call to Quit() or QuitWhenIdle() will
// terminate the targetted message loop. If a nested RunLoop continues
// running, the target may NEVER terminate. It is very easy to livelock (run
// forever) in such a case.
void Quit();
void QuitWhenIdle();
// Convenience methods to get a closure that safely calls Quit() or
// QuitWhenIdle() (has no effect if the RunLoop instance is gone).
// The resulting Closure is thread-safe (note however that invoking the
// QuitClosure() from another thread than this RunLoop's will result in an
// asynchronous rather than immediate Quit()).
// Example:
// RunLoop run_loop;
// PostTask(run_loop.QuitClosure());
// run_loop.Run();
base::Closure QuitClosure();
base::Closure QuitWhenIdleClosure();
// Returns true if there is an active RunLoop on this thread.
// Safe to call before RegisterDelegateForCurrentThread().
static bool IsRunningOnCurrentThread();
// Returns true if there is an active RunLoop on this thread and it's nested
// within another active RunLoop.
// Safe to call before RegisterDelegateForCurrentThread().
static bool IsNestedOnCurrentThread();
// A NestingObserver is notified when a nested RunLoop begins and ends.
class BASE_EXPORT NestingObserver {
// Notified before a nested loop starts running work on the current thread.
virtual void OnBeginNestedRunLoop() = 0;
// Notified after a nested loop is done running work on the current thread.
virtual void OnExitNestedRunLoop() {}
virtual ~NestingObserver() = default;
static void AddNestingObserverOnCurrentThread(NestingObserver* observer);
static void RemoveNestingObserverOnCurrentThread(NestingObserver* observer);
// Returns true if nesting is allowed on this thread.
static bool IsNestingAllowedOnCurrentThread();
// Disallow nesting. After this is called, running a nested RunLoop or calling
// Add/RemoveNestingObserverOnCurrentThread() on this thread will crash.
static void DisallowNestingOnCurrentThread();
// A RunLoop::Delegate is a generic interface that allows RunLoop to be
// separate from the underlying implementation of the message loop for this
// thread. It holds private state used by RunLoops on its associated thread.
// One and only one RunLoop::Delegate must be registered on a given thread
// via RunLoop::RegisterDelegateForCurrentThread() before RunLoop instances
// and RunLoop static methods can be used on it.
class BASE_EXPORT Delegate {
// A Callback which returns true if the Delegate should return from the
// topmost Run() when it becomes idle. The Delegate is responsible for
// probing this when it becomes idle.
using ShouldQuitWhenIdleCallback = RepeatingCallback<bool(void)>;
virtual ~Delegate();
// Used by RunLoop to inform its Delegate to Run/Quit. Implementations are
// expected to keep on running synchronously from the Run() call until the
// eventual matching Quit() call. Upon receiving a Quit() call it should
// return from the Run() call as soon as possible without executing
// remaining tasks/messages. Run() calls can nest in which case each Quit()
// call should result in the topmost active Run() call returning. The only
// other trigger for Run() to return is the
// |should_quit_when_idle_callback_| which the Delegate should probe before
// sleeping when it becomes idle. |application_tasks_allowed| is true if
// this is the first Run() call on the stack or it was made from a nested
// RunLoop of Type::kNestableTasksAllowed (otherwise this Run() level should
// only process system tasks).
virtual void Run(bool application_tasks_allowed) = 0;
virtual void Quit() = 0;
// Invoked right before a RunLoop enters a nested Run() call on this
// Delegate iff this RunLoop is of type kNestableTasksAllowed. The Delegate
// should ensure that the upcoming Run() call will result in processing
// application tasks queued ahead of it without further probing. e.g.
// message pumps on some platforms, like Mac, need an explicit request to
// process application tasks when nested, otherwise they'll only wait for
// system messages.
virtual void EnsureWorkScheduled() = 0;
// Returns the result of this Delegate's |should_quit_when_idle_callback_|.
// "protected" so it can be invoked only by the Delegate itself.
bool ShouldQuitWhenIdle();
// While the state is owned by the Delegate subclass, only RunLoop can use
// it.
friend class RunLoop;
// A vector-based stack is more memory efficient than the default
// deque-based stack as the active RunLoop stack isn't expected to ever
// have more than a few entries.
using RunLoopStack = base::stack<RunLoop*, std::vector<RunLoop*>>;
bool allow_nesting_ = true;
RunLoopStack active_run_loops_;
ObserverList<RunLoop::NestingObserver> nesting_observers_;
bool allow_running_for_testing_ = true;
// True once this Delegate is bound to a thread via
// RegisterDelegateForCurrentThread().
bool bound_ = false;
ShouldQuitWhenIdleCallback should_quit_when_idle_callback_;
// Thread-affine per its use of TLS.
// Registers |delegate| on the current thread. Must be called once and only
// once per thread before using RunLoop methods on it. |delegate| is from then
// on forever bound to that thread (including its destruction).
static void RegisterDelegateForCurrentThread(Delegate* delegate);
// Akin to RegisterDelegateForCurrentThread but overrides an existing Delegate
// (there must be one). Returning the overridden Delegate which the caller is
// now in charge of driving. |override_should_quit_when_idle_callback|
// specifies will replace the overridden Delegate's
// |should_quit_when_idle_callback_|, giving full control to |delegate|.
static Delegate* OverrideDelegateForCurrentThreadForTesting(
Delegate* delegate,
// Quits the active RunLoop (when idle) -- there must be one. These were
// introduced as prefered temporary replacements to the long deprecated
// MessageLoop::Quit(WhenIdle) methods. Callers should properly plumb a
// reference to the appropriate RunLoop instance (or its QuitClosure) instead
// of using these in order to link Run()/Quit() to a single RunLoop instance
// and increase readability.
static void QuitCurrentDeprecated();
static void QuitCurrentWhenIdleDeprecated();
// Run() will DCHECK if called while there's a ScopedDisallowRunningForTesting
// in scope on its thread. This is useful to add safety to some test
// constructs which allow multiple task runners to share the main thread in
// unit tests. While the main thread can be shared by multiple runners to
// deterministically fake multi threading, there can still only be a single
// RunLoop::Delegate per thread and RunLoop::Run() should only be invoked from
// it (or it would result in incorrectly driving TaskRunner A while in
// TaskRunner B's context).
class BASE_EXPORT ScopedDisallowRunningForTesting {
Delegate* current_delegate_;
const bool previous_run_allowance_;
#endif // DCHECK_IS_ON()
#if defined(OS_ANDROID)
// Android doesn't support the blocking RunLoop::Run, so it calls
// BeforeRun and AfterRun directly.
friend class base::MessagePumpForUI;
#if defined(OS_IOS)
// iOS doesn't support the blocking RunLoop::Run, so it calls
// BeforeRun directly.
friend class base::MessagePumpUIApplication;
// Return false to abort the Run.
bool BeforeRun();
void AfterRun();
// A copy of RunLoop::Delegate for the thread driven by tis RunLoop for quick
// access without using TLS (also allows access to state from another sequence
// during Run(), ref. |sequence_checker_| below).
Delegate* delegate_;
const Type type_;
bool run_called_ = false;
bool quit_called_ = false;
bool running_ = false;
// Used to record that QuitWhenIdle() was called on this RunLoop, meaning that
// the Delegate should quit Run() once it becomes idle (it's responsible for
// probing this state via ShouldQuitWhenIdle()). This state is stored here
// rather than pushed to Delegate to support nested RunLoops.
bool quit_when_idle_received_ = false;
// RunLoop is not thread-safe. Its state/methods, unless marked as such, may
// not be accessed from any other sequence than the thread it was constructed
// on. Exception: RunLoop can be safely accessed from one other sequence (or
// single parallel task) during Run() -- e.g. to Quit() without having to
// plumb ThreatTaskRunnerHandle::Get() throughout a test to repost QuitClosure
// to origin thread.
const scoped_refptr<SingleThreadTaskRunner> origin_task_runner_;
// WeakPtrFactory for QuitClosure safety.
base::WeakPtrFactory<RunLoop> weak_factory_;
} // namespace base
#endif // BASE_RUN_LOOP_H_