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// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef COMPONENTS_SYNC_ENGINE_CANCELATION_SIGNAL_H_
#define COMPONENTS_SYNC_ENGINE_CANCELATION_SIGNAL_H_
#include "base/memory/raw_ptr.h"
#include "base/synchronization/lock.h"
namespace syncer {
// This class is used to allow one thread to request that another abort and
// return early.
//
// The signalling thread owns this class and my call Signal() at any time.
// After that call, this class' IsSignalled() will always return true. The
// intended use case is that the task intending to support early exit will
// periodically check the value of IsSignalled() to see if it should return
// early.
//
// The receiving task may also choose to register an observer whose
// OnCancelationSignalReceived() method will be executed on the signaller's
// thread when Signal() is called. This may be used for sending an early
// Signal() to a WaitableEvent. The registration of the handler is necessarily
// racy. If Signal() is executes before TryRegisterHandler(),
// TryRegisterHandler() will not perform any registration and return false. That
// function's caller must handle this case.
//
// This class supports only one handler, though it could easily support multiple
// observers if we found a use case for such a feature.
class CancelationSignal {
public:
class Observer {
public:
Observer() = default;
virtual ~Observer() = default;
// This may be called from a foreign thread while the CancelationSignal's
// lock is held. The callee should avoid performing slow or blocking
// operations.
virtual void OnCancelationSignalReceived() = 0;
};
CancelationSignal();
~CancelationSignal();
// Tries to register a handler to be invoked when Signal() is called.
//
// If Signal() has already been called, returns false without registering
// the handler. Returns true when the registration is successful.
//
// If the registration was successful, the handler must be unregistered with
// UnregisterHandler before this CancelationSignal is destroyed.
bool TryRegisterHandler(Observer* handler);
// Unregisters the abort handler.
void UnregisterHandler(Observer* handler);
// Returns true if Signal() has been called.
bool IsSignalled();
// Sets the stop_requested_ flag and calls the OnCancelationSignalReceived()
// method of the registered handler, if there is one registered at the time.
// SignalReceived() will be called with the `signal_lock_` held.
void Signal();
private:
// Protects all members of this class.
base::Lock signal_lock_;
// True if Signal() has been invoked.
bool signalled_ = false;
// The registered abort handler. May be null.
raw_ptr<Observer> handler_ = nullptr;
};
} // namespace syncer
#endif // COMPONENTS_SYNC_ENGINE_CANCELATION_SIGNAL_H_
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