1// Copyright 2013 The Chromium Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style license that can be 3// found in the LICENSE file. 4 5#ifndef SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_ 6#define SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_ 7 8#include "base/synchronization/lock.h" 9#include "sync/base/sync_export.h" 10 11namespace syncer { 12 13class CancelationObserver; 14 15// This class is used to allow one thread to request that another abort and 16// return early. 17// 18// The signalling thread owns this class and my call Signal() at any time. 19// After that call, this class' IsSignalled() will always return true. The 20// intended use case is that the task intending to support early exit will 21// periodically check the value of IsSignalled() to see if it should return 22// early. 23// 24// The receiving task may also choose to register an CancelationObserver whose 25// OnSignalReceived() method will be executed on the signaller's thread when 26// Signal() is called. This may be used for sending an early Signal() to a 27// WaitableEvent. The registration of the handler is necessarily racy. If 28// Signal() is executes before TryRegisterHandler(), TryRegisterHandler() will 29// not perform any registration and return false. That function's caller must 30// handle this case. 31// 32// This class supports only one handler, though it could easily support multiple 33// observers if we found a use case for such a feature. 34class SYNC_EXPORT_PRIVATE CancelationSignal { 35 public: 36 CancelationSignal(); 37 ~CancelationSignal(); 38 39 // Tries to register a handler to be invoked when Signal() is called. 40 // 41 // If Signal() has already been called, returns false without registering 42 // the handler. Returns true when the registration is successful. 43 // 44 // If the registration was successful, the handler must be unregistered with 45 // UnregisterHandler before this CancelationSignal is destroyed. 46 bool TryRegisterHandler(CancelationObserver* handler); 47 48 // Unregisters the abort handler. 49 void UnregisterHandler(CancelationObserver* handler); 50 51 // Returns true if Signal() has been called. 52 bool IsSignalled(); 53 54 // Sets the stop_requested_ flag and calls the OnSignalReceived() method of 55 // the registered handler, if there is one registered at the time. 56 // SignalReceived() will be called with the |signal_lock_| held. 57 void Signal(); 58 59 private: 60 // Protects all members of this class. 61 base::Lock signal_lock_; 62 63 // True if Signal() has been invoked. 64 bool signalled_; 65 66 // The registered abort handler. May be NULL. 67 CancelationObserver* handler_; 68}; 69 70} // namespace syncer 71 72#endif // SYNC_INTERNAL_API_PUBLIC_BASE_CANCELATION_SIGNAL_H_ 73