bitcoin 代码中大量使用 boost::signal, boost::signal 实现了信号与槽的事件通知机制，或者说是一种消息的发布与订阅机制， signal 类型是一个可调用类型，slot 就是callback 对象，或者说事件的订阅者，signal 实例是一个可调用对象，调用signal 对象，就相当于发布了相应的事件, signal 的connect， disconnect 方法分别相当于对事件的订阅，取消。

#include <boost/signals2.hpp>
 #include 
 void print_args(float x, float y)
 {
 std::cout << "The arguments are " << x << " and " << y << std::endl;
 }
 void print_sum(float x, float y)
 {
 std::cout << "The sum is " << x + y << std::endl;
 }
 void print_product(float x, float y)
 {
 std::cout << "The product is " << x * y << std::endl;
 }
 void print_difference(float x, float y)
 {
 std::cout << "The difference is " << x - y << std::endl;
 }
 void print_quotient(float x, float y)
 {
 std::cout << "The quotient is " << x / y << std::endl;
 }
 int main() {
 boost::signals2::signal<void(float, float)> sig;
 sig.connect(print_args);
 sig.connect(print_sum);
 sig.connect(print_product);
 sig.connect(print_difference);
 sig.connect(print_quotient);
 sig(5., 3.);
 return 0;

上面这个例子, 有五个函数订阅sig 事件，sig(5. , 3.) 的调用触发事件，参数5，3，相当于事件携带的消息paylaod, 传给了五个事件订阅者。

bitcoin 中定义了类型CMainSignals 来统一管理各个功能模块的事件通知，CMainSignal 是一个资源管理类型， 主要工作代理给由unique_ptr 管理内存的成员 m_internals , 它的类型是MainSignalsInstance，内部定义十个boost signal 变量， 分别表达十种要通知的事件。

class CMainSignals {
 private:
 std::unique_ptr m_internals;
 friend void ::RegisterValidationInterface(CValidationInterface*);
 friend void ::UnregisterValidationInterface(CValidationInterface*);
 friend void ::UnregisterAllValidationInterfaces();
 friend void ::CallFunctionInValidationInterfaceQueue(std::function func); void MempoolEntryRemoved(CTransactionRef tx, MemPoolRemovalReason reason);public: /** Register a CScheduler to give callbacks which should run in the background (may only be called once) */
 void RegisterBackgroundSignalScheduler(CScheduler& scheduler);
 /** Unregister a CScheduler to give callbacks which should run in the background - these callbacks will now be dropped! */
 void UnregisterBackgroundSignalScheduler();
 /** Call any remaining callbacks on the calling thread */
 void FlushBackgroundCallbacks();
 size_t CallbacksPending();
 /** Register with mempool to call TransactionRemovedFromMempool callbacks */
 void RegisterWithMempoolSignals(CTxMemPool& pool);
 /** Unregister with mempool */
 void UnregisterWithMempoolSignals(CTxMemPool& pool);
 void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
 void TransactionAddedToMempool(const CTransactionRef &);
 void BlockConnected(const std::shared_ptr &, const CBlockIndex *pindex, const std::shared_ptr> &);
 void BlockDisconnected(const std::shared_ptr &);
 void SetBestChain(const CBlockLocator &);
 void Inventory(const uint256 &);
 void Broadcast(int64_t nBestBlockTime, CConnman* connman);
 void BlockChecked(const CBlock&, const CValidationState&);
 void NewPoWValidBlock(const CBlockIndex *, const std::shared_ptr&);
 };
 struct MainSignalsInstance {
 boost::signals2::signal UpdatedBlockTip;
 boost::signals2::signal TransactionAddedToMempool;
 boost::signals2::signal &, const CBlockIndex *pindex, const std::vector&)> BlockConnected;
 boost::signals2::signal &)> BlockDisconnected;
 boost::signals2::signal TransactionRemovedFromMempool;
 boost::signals2::signal SetBestChain;
 boost::signals2::signal Inventory;
 boost::signals2::signal Broadcast;
 boost::signals2::signal BlockChecked;
 boost::signals2::signal&)> NewPoWValidBlock;
 // We are not allowed to assume the scheduler only runs in one thread,
 // but must ensure all callbacks happen in-order, so we end up creating
 // our own queue here :(
 SingleThreadedSchedulerClient m_schedulerClient;
 explicit MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}
 };

类型CValidationInterface 主要是统一表示某些对MainSignalsInstance 中定义的十个事件感兴趣的对象， 即这些事件的订阅者， 所有对这些事件感兴趣代码继承CValidationInterface类型，提供自己版本的这些虚成员函数的实现，覆盖baseCValidationInterface 中对应的空方法， 表达对相应的事件感兴趣， 不感兴趣的事件的回调方法继续是那些继承自base class 的空方法。

class CValidationInterface {
 protected:
 virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {}
 virtual void TransactionAddedToMempool(const CTransactionRef &ptxn) {}
 virtual void TransactionRemovedFromMempool(const CTransactionRef &ptx) {}
 virtual void BlockConnected(const std::shared_ptr &block, const CBlockIndex *pindex, const std::vector &txnConflicted) {}
 virtual void BlockDisconnected(const std::shared_ptr &block) {}
 virtual void SetBestChain(const CBlockLocator &locator) {}
 virtual void Inventory(const uint256 &hash) {}
 virtual void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) {}
 virtual void BlockChecked(const CBlock&, const CValidationState&) {}
 virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr& block) {};
 friend void ::RegisterValidationInterface(CValidationInterface*);
 friend void ::UnregisterValidationInterface(CValidationInterface*);
 friend void ::UnregisterAllValidationInterfaces();
 };

CValidationInterface 有四个子类， CWallet , CZMQNotificationInterface, submitblock_StateCatcher, PeerLogicValidation 分别对应四个对MainSignalsInstance 中的事件感兴趣的订阅者。

 class CWallet final : public CCryptoKeyStore, public CValidationInterface
 {
 ...............
 void TransactionAddedToMempool(const CTransactionRef& tx) override;
 void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex *pindex, const std::vector& vtxConflicted) override;
 void BlockDisconnected(const std::shared_ptr& pblock) override;
 void TransactionRemovedFromMempool(const CTransactionRef &ptx) override;
 void ResendWalletTransactions(int64_t nBestBlockTime, CConnman* connman) override;
 void SetBestChain(const CBlockLocator& loc) override;
 void Inventory(const uint256 &hash) override
 {
 {
 LOCK(cs_wallet);
 std::map<uint256, int>::iterator mi = mapRequestCount.find(hash);
 if (mi != mapRequestCount.end())
 (*mi).second++;
 }
 }
 ...............
 };
 class CZMQNotificationInterface final : public CValidationInterface
 {
 ................
 // CValidationInterface
 void TransactionAddedToMempool(const CTransactionRef& tx) override;
 void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex* pindexConnected, const std::vector& vtxConflicted) override;
 void BlockDisconnected(const std::shared_ptr& pblock) override;
 void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;
 .................
 };
 class submitblock_StateCatcher : public CValidationInterface
 {
 public:
 uint256 hash;
 bool found;
 CValidationState state;
 explicit submitblock_StateCatcher(const uint256 &hashIn) : hash(hashIn), found(false), state() {}
 protected:
 void BlockChecked(const CBlock& block, const CValidationState& stateIn) override {
 if (block.GetHash() != hash)
 return;
 found = true;
 state = stateIn;
 }
 };
 class PeerLogicValidation : public CValidationInterface, public NetEventsInterface
 {
 private:
 CConnman* const connman;
 public:
 explicit PeerLogicValidation(CConnman* connman, CScheduler &scheduler);
 void BlockConnected(const std::shared_ptr& pblock, const CBlockIndex* pindexConnected, const std::vector& vtxConflicted) override;
 void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;
 void BlockChecked(const CBlock& block, const CValidationState& state) override;
 void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr& pblock) override;
 ........
 };

这四个订阅者通过调用RegisterValidationInterface， UnregisterValidationInterface 订阅，取消事件通知，函数接受参数是指向订阅者的指针。

void RegisterValidationInterface(CValidationInterface* pwalletIn) {
 g_signals.m_internals->UpdatedBlockTip.connect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
 g_signals.m_internals->TransactionAddedToMempool.connect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
 g_signals.m_internals->BlockConnected.connect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
 g_signals.m_internals->BlockDisconnected.connect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
 g_signals.m_internals->TransactionRemovedFromMempool.connect(boost::bind(&CValidationInterface::TransactionRemovedFromMempool, pwalletIn, _1));
 g_signals.m_internals->SetBestChain.connect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
 g_signals.m_internals->Inventory.connect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
 g_signals.m_internals->Broadcast.connect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
 g_signals.m_internals->BlockChecked.connect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
 g_signals.m_internals->NewPoWValidBlock.connect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }
 void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
 g_signals.m_internals->BlockChecked.disconnect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
 g_signals.m_internals->Broadcast.disconnect(boost::bind(&CValidationInterface::ResendWalletTransactions, pwalletIn, _1, _2));
 g_signals.m_internals->Inventory.disconnect(boost::bind(&CValidationInterface::Inventory, pwalletIn, _1));
 g_signals.m_internals->SetBestChain.disconnect(boost::bind(&CValidationInterface::SetBestChain, pwalletIn, _1));
 g_signals.m_internals->TransactionAddedToMempool.disconnect(boost::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, _1));
 g_signals.m_internals->BlockConnected.disconnect(boost::bind(&CValidationInterface::BlockConnected, pwalletIn, _1, _2, _3));
 g_signals.m_internals->BlockDisconnected.disconnect(boost::bind(&CValidationInterface::BlockDisconnected, pwalletIn, _1));
 g_signals.m_internals->TransactionRemovedFromMempool.disconnect(boost::bind(&CValidationInterface::TransactionRemovedFromMempool, pwalletIn, _1));
 g_signals.m_internals->UpdatedBlockTip.disconnect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
 g_signals.m_internals->NewPoWValidBlock.disconnect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }

在程序启动，初始化阶段，启动调度器线程

bool AppInitMain()
 {
 .................
 CScheduler::Function serviceLoop = boost::bind(&CScheduler::serviceQueue, &scheduler);
 threadGroup.create_thread(boost::bind(&TraceThread, "scheduler", serviceLoop));
 .................
 }

调用RegisterBackgroundSignalScheduler(), 初始化全局MainSignalsInstance 实例; 调用RegisterWithMempoolSignals(), 订阅全局内存池对象的NotifyEntryRemoved 事件通知, MainSignalsInstance 只对由于超时，大小限制， blockchian 重组，替换等原因发生的离开内存池事件感兴趣， 收到后MainSignalsInstance 再作为事件发布者， 转发给其他订阅者， 如CWallet。

bool AppInitMain()
 {
 ......
 GetMainSignals().RegisterBackgroundSignalScheduler(scheduler);
 GetMainSignals().RegisterWithMempoolSignals(mempool);
 ........
 }

初始化全局的连接管理对象connman 后， 初始化全局的peerLogic 对象， 然后调用RegisterValidationInterface(), peerLogic 成为MainSignalsInstance 对象的订阅者, 如果用户编译了zeromq支持模块，调用RegisterValidationInterface(), pzmqNotificationInterface 订阅MainSignalsInstance 。

bool AppInitMain()
 {
 .............
 assert(!g_connman);
 g_connman = std::unique_ptr(new CConnman(GetRand(std::numeric_limits<uint64_t>::max()), GetRand(std::numeric_limits<uint64_t>::max())));
 CConnman& connman = *g_connman;

peerLogic.reset(new PeerLogicValidation(&connman, scheduler));
 RegisterValidationInterface(peerLogic.get());
 .............
 #if ENABLE_ZMQ
 pzmqNotificationInterface = CZMQNotificationInterface::Create();
 if (pzmqNotificationInterface) {
 RegisterValidationInterface(pzmqNotificationInterface);
 }
 #endif
 ...............
 }

如果钱包功能开启， 启动后打开钱包过程，会把钱包注册成为MainSignalsInstance的订阅者。

bool AppInitMain()
 {
 ........
 #ifdef ENABLE_WALLET
 if (!OpenWallets())
 return false;
 #else
 LogPrintf("No wallet support compiled in!\n");
 #endif
 ........
 }
 bool OpenWallets()
 {
 if (gArgs.GetBoolArg("-disablewallet", DEFAULT_DISABLE_WALLET)) {
 LogPrintf("Wallet disabled!\n");
 return true;
 }
 for (const std::string& walletFile : gArgs.GetArgs("-wallet")) {
 CWallet * const pwallet = CWallet::CreateWalletFromFile(walletFile);
 if (!pwallet) {
 return false;
 }
 vpwallets.push_back(pwallet);
 }
 return true;
 }

CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
 {
 ......
 CWallet *walletInstance = new CWallet(std::move(dbw));
 RegisterValidationInterface(walletInstance);
 ......
 }

在submitblock rpc调用中， 用户提交hex编码的原始block， 解析后， 调用ProcessNewBlock()检查处理，使用类型submitblock_StateCatcher 的对象sc 作为MainSignalsInstance 的订阅者，
对提交过去的block 的验证结果，作为事件通知返回给rpc 调用。

UniValue submitblock(const JSONRPCRequest& request)
 {
 ...........
 submitblock_StateCatcher sc(block.GetHash());
 RegisterValidationInterface(&sc);
 bool fAccepted = ProcessNewBlock(Params(), blockptr, true, nullptr);
 UnregisterValidationInterface(&sc);
 ...........
 }

CMainSignals 类型上面定义了一堆触发事件的方法, 别的代码模块调用这些方法， 触发相应的事件，把事件通知发给相关的订阅者。

void CMainSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {
 m_internals->m_schedulerClient.AddToProcessQueue([pindexNew, pindexFork, fInitialDownload, this] {
 m_internals->UpdatedBlockTip(pindexNew, pindexFork, fInitialDownload);
 });
 }
 void CMainSignals::TransactionAddedToMempool(const CTransactionRef &ptx) {
 m_internals->m_schedulerClient.AddToProcessQueue([ptx, this] {
 m_internals->TransactionAddedToMempool(ptx);
 });
 }
 void CMainSignals::BlockConnected(const std::shared_ptr &pblock, const CBlockIndex *pindex, const std::shared_ptr>& pvtxConflicted) {
 m_internals->m_schedulerClient.AddToProcessQueue([pblock, pindex, pvtxConflicted, this] {
 m_internals->BlockConnected(pblock, pindex, *pvtxConflicted);
 });
 }
 void CMainSignals::BlockDisconnected(const std::shared_ptr &pblock) {
 m_internals->m_schedulerClient.AddToProcessQueue([pblock, this] {
 m_internals->BlockDisconnected(pblock);
 });
 }
 void CMainSignals::SetBestChain(const CBlockLocator &locator) {
 m_internals->m_schedulerClient.AddToProcessQueue([locator, this] {
 m_internals->SetBestChain(locator);
 });
 }
 void CMainSignals::Inventory(const uint256 &hash) {
 m_internals->m_schedulerClient.AddToProcessQueue([hash, this] {
 m_internals->Inventory(hash);
 });
 }
 void CMainSignals::Broadcast(int64_t nBestBlockTime, CConnman* connman) {
 m_internals->Broadcast(nBestBlockTime, connman);
 }
 void CMainSignals::BlockChecked(const CBlock& block, const CValidationState& state) {
 m_internals->BlockChecked(block, state);
 }
 void CMainSignals::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr &block) {
 m_internals->NewPoWValidBlock(pindex, block);
 }

CChainState 的ActivateBestChain方法里， 发布BlockConnected， UpdatedBlockTip 事件:

bool CChainState::ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr pblock) {
 ..............................
 for (const PerBlockConnectTrace& trace : connectTrace.GetBlocksConnected()) {
 assert(trace.pblock && trace.pindex);
 GetMainSignals().BlockConnected(trace.pblock, trace.pindex, trace.conflictedTxs);
 }
 ..............................
 GetMainSignals().UpdatedBlockTip(pindexNewTip, pindexFork, fInitialDownload);
 ...............................
 }

CChainState 的AcceptBlock 方法里，发布NewPoWValidBlock 事件:

bool CChainState::AcceptBlock(const std::shared_ptr& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
 {
 .......................
 if (!IsInitialBlockDownload() && chainActive.Tip() == pindex->pprev)
 GetMainSignals().NewPoWValidBlock(pindex, pblock);
 ......................
 }

CChainState 的DisconnectTip 方法里，发布 BlockDisconnected 事件:

bool CChainState::DisconnectTip(CValidationState& state, const CChainParams& chainparams, DisconnectedBlockTransactions *disconnectpool)
 {
 ...................
 GetMainSignals().BlockDisconnected(pblock);
 ..............
 }

CChainState 的ConnectTip 方法里，发布 BlockChecked 事件:

bool CChainState::ConnectTip(CValidationState& state, const CChainParams& chainparams, CBlockIndex* pindexNew, const std::shared_ptr& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions &disconnectpool)
 {
 ................
 CCoinsViewCache view(pcoinsTip.get());
 bool rv = ConnectBlock(blockConnecting, state, pindexNew, view, chainparams);
 GetMainSignals().BlockChecked(blockConnecting, state);
 ................
 }

PeerLogicValidation 的 SendMessage 方法里， 发布Broadcast事件, 通知钱包重新发送未确认的交易:

bool PeerLogicValidation::SendMessages(CNode* pto, std::atomic& interruptMsgProc)
 {
 .............
 if (!fReindex && !fImporting && !IsInitialBlockDownload())
 {
 GetMainSignals().Broadcast(nTimeBestReceived, connman);
 }
 .............
 }

从网络上收到INV消息后，通知给钱包，更新内部状态。

bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStream& vRecv, int64_t nTimeReceived, const CChainParams& chainparams, CConnman* connman, const std::atomic& interruptMsgProc)
 {
 .................
 for (CInv &inv : vInv)
 {
 ............
 GetMainSignals().Inventory(inv.hash);
 }
 ................
 }

validation.cpp 里面的 ProcessNewBlock在内部调用 CheckBlock 后，检查失败后，发布 BlockCheck s事件， 通告给相关订阅者。

bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr pblock, bool fForceProcessing, bool *fNewBlock)
 {
 ............
 bool ret = CheckBlock(*pblock, state, chainparams.GetConsensus());
 if (!ret) {
 GetMainSignals().BlockChecked(*pblock, state);
 return error("%s: AcceptBlock FAILED (%s)", __func__, state.GetDebugMessage());
 }
 ...........
 }

FlushStateToDisk, 发布SetBestChain 事件， 通知钱包

bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &state, FlushStateMode mode, int nManualPruneHeight) {
 ...............
 if (fDoFullFlush || ((mode == FLUSH_STATE_ALWAYS || mode == FLUSH_STATE_PERIODIC) && nNow > nLastSetChain + (int64_t)DATABASE_WRITE_INTERVAL * 1000000)) {
 // Update best block in wallet (so we can detect restored wallets).
 GetMainSignals().SetBestChain(chainActive.GetLocator());
 nLastSetChain = nNow;
 }
 ...............
 }

validation.cpp 里面的AcceptToMemoryPoolWorker, 在结束前， 发布TransactionAddedToMempool 事件。

static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool& pool, CValidationState& state, const CTransactionRef& ptx,
 bool* pfMissingInputs, int64_t nAcceptTime, std::list* plTxnReplaced,
 bool bypass_limits, const CAmount& nAbsurdFee, std::vector& coins_to_uncache)
 {
 ................
 GetMainSignals().TransactionAddedToMempool(ptx);
 }