chromium/base/synchronization/waitable_event_watcher.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
#define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_

#include "base/base_export.h"
#include "base/memory/raw_ptr.h"
#include "base/task/sequenced_task_runner.h"
#include "build/build_config.h"

#if BUILDFLAG(IS_WIN)
#include "base/win/object_watcher.h"
#include "base/win/scoped_handle.h"
#elif BUILDFLAG(IS_APPLE)
#include <dispatch/dispatch.h>

#include <memory>

#include "base/memory/weak_ptr.h"
#include "base/synchronization/waitable_event.h"
#else
#include "base/sequence_checker.h"
#include "base/synchronization/waitable_event.h"
#endif

#if !BUILDFLAG(IS_WIN)
#include "base/functional/callback.h"
#endif

namespace base {

class Flag;
class AsyncWaiter;
class WaitableEvent;

// This class provides a way to wait on a WaitableEvent asynchronously.
//
// Each instance of this object can be waiting on a single WaitableEvent. When
// the waitable event is signaled, a callback is invoked on the sequence that
// called StartWatching(). This callback can be deleted by deleting the waiter.
//
// Typical usage:
//
//   class MyClass {
//    public:
//     void DoStuffWhenSignaled(WaitableEvent *waitable_event) {
//       watcher_.StartWatching(waitable_event,
//           base::BindOnce(&MyClass::OnWaitableEventSignaled, this);
//     }
//    private:
//     void OnWaitableEventSignaled(WaitableEvent* waitable_event) {
//       // OK, time to do stuff!
//     }
//     base::WaitableEventWatcher watcher_;
//   };
//
// In the above example, MyClass wants to "do stuff" when waitable_event
// becomes signaled. WaitableEventWatcher makes this task easy. When MyClass
// goes out of scope, the watcher_ will be destroyed, and there is no need to
// worry about OnWaitableEventSignaled being called on a deleted MyClass
// pointer.
//
// BEWARE: With automatically reset WaitableEvents, a signal may be lost if it
// occurs just before a WaitableEventWatcher is deleted. There is currently no
// safe way to stop watching an automatic reset WaitableEvent without possibly
// missing a signal.
//
// NOTE: you /are/ allowed to delete the WaitableEvent while still waiting on
// it with a Watcher. But pay attention: if the event was signaled and deleted
// right after, the callback may be called with deleted WaitableEvent pointer.

class BASE_EXPORT WaitableEventWatcher
#if BUILDFLAG(IS_WIN)
    : public win::ObjectWatcher::Delegate
#endif
{ … };

}  // namespace base

#endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_