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161 lines
5.3 KiB
161 lines
5.3 KiB
// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
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#define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
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#include "base/base_export.h"
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#include "base/macros.h"
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#include "base/sequenced_task_runner.h"
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#include "build/build_config.h"
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#if defined(OS_WIN)
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#include "base/win/object_watcher.h"
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#include "base/win/scoped_handle.h"
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#elif defined(OS_MACOSX)
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#include <dispatch/dispatch.h>
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#include "base/mac/scoped_dispatch_object.h"
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#include "base/memory/weak_ptr.h"
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#include "base/synchronization/waitable_event.h"
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#else
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#include "base/sequence_checker.h"
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#include "base/synchronization/waitable_event.h"
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#endif
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#if !defined(OS_WIN)
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#include "base/callback.h"
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#endif
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namespace base {
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class Flag;
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class AsyncWaiter;
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class WaitableEvent;
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// This class provides a way to wait on a WaitableEvent asynchronously.
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//
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// Each instance of this object can be waiting on a single WaitableEvent. When
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// the waitable event is signaled, a callback is invoked on the sequence that
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// called StartWatching(). This callback can be deleted by deleting the waiter.
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//
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// Typical usage:
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//
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// class MyClass {
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// public:
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// void DoStuffWhenSignaled(WaitableEvent *waitable_event) {
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// watcher_.StartWatching(waitable_event,
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// base::BindOnce(&MyClass::OnWaitableEventSignaled, this);
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// }
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// private:
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// void OnWaitableEventSignaled(WaitableEvent* waitable_event) {
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// // OK, time to do stuff!
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// }
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// base::WaitableEventWatcher watcher_;
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// };
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//
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// In the above example, MyClass wants to "do stuff" when waitable_event
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// becomes signaled. WaitableEventWatcher makes this task easy. When MyClass
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// goes out of scope, the watcher_ will be destroyed, and there is no need to
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// worry about OnWaitableEventSignaled being called on a deleted MyClass
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// pointer.
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//
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// BEWARE: With automatically reset WaitableEvents, a signal may be lost if it
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// occurs just before a WaitableEventWatcher is deleted. There is currently no
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// safe way to stop watching an automatic reset WaitableEvent without possibly
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// missing a signal.
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//
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// NOTE: you /are/ allowed to delete the WaitableEvent while still waiting on
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// it with a Watcher. But pay attention: if the event was signaled and deleted
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// right after, the callback may be called with deleted WaitableEvent pointer.
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class BASE_EXPORT WaitableEventWatcher
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#if defined(OS_WIN)
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: public win::ObjectWatcher::Delegate
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#endif
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{
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public:
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using EventCallback = OnceCallback<void(WaitableEvent*)>;
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WaitableEventWatcher();
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#if defined(OS_WIN)
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~WaitableEventWatcher() override;
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#else
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~WaitableEventWatcher();
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#endif
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// When |event| is signaled, |callback| is called on the sequence that called
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// StartWatching().
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// |task_runner| is used for asynchronous executions of calling |callback|.
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bool StartWatching(WaitableEvent* event,
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EventCallback callback,
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scoped_refptr<SequencedTaskRunner> task_runner);
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// Cancel the current watch. Must be called from the same sequence which
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// started the watch.
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//
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// Does nothing if no event is being watched, nor if the watch has completed.
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// The callback will *not* be called for the current watch after this
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// function returns. Since the callback runs on the same sequence as this
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// function, it cannot be called during this function either.
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void StopWatching();
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private:
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#if defined(OS_WIN)
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void OnObjectSignaled(HANDLE h) override;
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// Duplicated handle of the event passed to StartWatching().
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win::ScopedHandle duplicated_event_handle_;
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// A watcher for |duplicated_event_handle_|. The handle MUST outlive
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// |watcher_|.
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win::ObjectWatcher watcher_;
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EventCallback callback_;
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WaitableEvent* event_ = nullptr;
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#elif defined(OS_MACOSX)
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// Invokes the callback and resets the source. Must be called on the task
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// runner on which StartWatching() was called.
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void InvokeCallback();
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// Closure bound to the event being watched. This will be is_null() if
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// nothing is being watched.
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OnceClosure callback_;
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// A reference to the receive right that is kept alive while a watcher
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// is waiting. Null if no event is being watched.
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scoped_refptr<WaitableEvent::ReceiveRight> receive_right_;
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// A TYPE_MACH_RECV dispatch source on |receive_right_|. When a receive event
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// is delivered, the message queue will be peeked and the bound |callback_|
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// may be run. This will be null if nothing is currently being watched.
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ScopedDispatchObject<dispatch_source_t> source_;
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// Used to vend a weak pointer for calling InvokeCallback() from the
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// |source_| event handler.
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WeakPtrFactory<WaitableEventWatcher> weak_ptr_factory_;
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#else
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// Instantiated in StartWatching(). Set before the callback runs. Reset in
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// StopWatching() or StartWatching().
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scoped_refptr<Flag> cancel_flag_;
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// Enqueued in the wait list of the watched WaitableEvent.
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AsyncWaiter* waiter_ = nullptr;
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// Kernel of the watched WaitableEvent.
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scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
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// Ensures that StartWatching() and StopWatching() are called on the same
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// sequence.
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SequenceChecker sequence_checker_;
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#endif
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DISALLOW_COPY_AND_ASSIGN(WaitableEventWatcher);
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};
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} // namespace base
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#endif // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
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