Wangle源码分析：Pipeline、Handler、Context

基本概念

      Wangle中的Pipeline和Netty中的Pipeline是很相似的，既可以将它看为一种职责链模式的实现也可以看作是Handler的容器。Pipeline中的handler都是串行化执行的，前一个handler完成自己的工作之后把事件传递给下一个handler，理论上Pipeline中的所有handler都是在同一个IO线程中执行的，但是为了防止某些handler（比如序列化、编解码handler等）耗时过长，Netty中允许为某些handler指定其它线程（eventloop）异步执行，类似的功能在Wangle中也有体现，只是在实现方式上有些区别。和Netty中一个较大的区别是，Wangle中并没有专门的Channel定义，Wangle中的Pipeline兼有了Channel的角色和功能。下面分别就Pipeline、Handler和Context的顺序进行源码分析。

Pipeline

     PipelineBase作为Pipeline的基类，提供了一些最为通用、核心的api实现，比如对handler的操作：addBack及其变体、addFront及其变体、remove及其变体等，下面看一下addBack的一个实现版本：

template <class H>
PipelineBase& PipelineBase::addBack(std::shared_ptr<H> handler) {
  typedef typename ContextType<H>::type Context;// 声明Conetxt类型,ContextImpl<Handler>、InboundContextImpl<Handler>、OutboundContextImpl<Handler>其中之一
  // 使用Context包装Handler后，将其添加到pipeline中，Context中还持有pipeline的引用
  return addHelper(
           std::make_shared<Context>(shared_from_this(), std::move(handler)),
           false);// false标识添加到尾部
}

      首先，会根据要添加的handler类型定义一个Context（Context可以看成是Handler的外套,后面还会单独介绍）类型，然后根据这个Context类型创建一个Context：参数为Pipeline指针和handler，最终addHelper会将Context添加到容器管理起来：

template <class Context>
PipelineBase& PipelineBase::addHelper(std::shared_ptr<Context>&& ctx,bool front) {
  // 先加入总的Context （std::vector<std::shared_ptr<PipelineContext>>）
  // 该vector种使用的是智能指针，可以保持对Context的引用
  ctxs_.insert(front ? ctxs_.begin() : ctxs_.end(), ctx);
  // 然后根据方向（BOTH、IN、OUT分别加入相应的vector中）
  // std::vector<PipelineContext*> 这里放的是Context的指针，因为引用在上面的容器中已经保持
  if (Context::dir == HandlerDir::BOTH || Context::dir == HandlerDir::IN) {
    inCtxs_.insert(front ? inCtxs_.begin() : inCtxs_.end(), ctx.get());
  }
  if (Context::dir == HandlerDir::BOTH || Context::dir == HandlerDir::OUT) {
    outCtxs_.insert(front ? outCtxs_.begin() : outCtxs_.end(), ctx.get());
  }
  return *this;
}

      Context内部包含了Pipeline、Handler，和Handler一样，Context也有方向：BOTH、IN、OUT，首先，无论Context 什么方向，都会在ctxs_容器上添加这个Context，然后会根据Context方向的不同，分别在inCtxs_和outCtxs_上添加该Context。接下来看一下这三个容器的定义：

  std::vector<std::shared_ptr<PipelineContext>> ctxs_;  // 所有的PipelineContext
  std::vector<PipelineContext*> inCtxs_;  // inbound 类型的PipelineContext
  std::vector<PipelineContext*> outCtxs_; // outbound 类型的PipelineContext

     由于handler的其他操作（addFront、remove等）都是对这三个容器的增删操作，原理一样，此处不再赘述。

     PipelineBase中还提供了设置PipelineManager的接口，从字面理解，PipelineManager就是管理Pipeline的接口，其定义如下：

class PipelineManager {
 public:
  virtual ~PipelineManager() = default;
  virtual void deletePipeline(PipelineBase* pipeline) = 0;
  virtual void refreshTimeout() {};
};

      其中，deletePipeline会在显示调用一个pipeline的close方法时被调用，一般用来完成该Pipeline相关的资源释放，而refreshTimeout主要在Pipeline发生读写事件时被回调，主要用来刷新Pipeline的空闲时间。因此，如果你需要监听Pipeline的delete和refresh事件，那么可以自己实现一个PipelineManager并设置到Pipeline上。

      在Wangle中没有定义专门的Channel结构，其实Wangle中的Pipeline兼有Channel的功能，比如要判断一个Channel是否还处于连接状态，在Netty中代码如下：

channel.isConnected();

     那么Wangle中的Pipeline并没有此类方法可供使用，怎么办呢？其实，Wangle的Pipeline提供了一个更强大的方法：getTransport，该方法可以获得一个底层的AsyncTransport，而该AsyncTransport拥有所有的底层连接信息，比如（仅列出主要接口）：

class AsyncTransport : public DelayedDestruction, public AsyncSocketBase {
public:
  typedef std::unique_ptr<AsyncTransport, Destructor> UniquePtr;
  virtual void close() = 0;
  virtual void closeNow() = 0;
  virtual void closeWithReset() {
    closeNow();
  }

  virtual void shutdownWrite() = 0;
  virtual void shutdownWriteNow() = 0;
  virtual bool good() const = 0;
  virtual bool readable() const = 0;
  virtual bool isPending() const {
    return readable();
  }
  virtual bool connecting() const = 0;
  virtual bool error() const = 0;
  virtual void attachEventBase(EventBase* eventBase) = 0;
  virtual void detachEventBase() = 0;
  virtual bool isDetachable() const = 0;
  virtual void setSendTimeout(uint32_t milliseconds) = 0;
  virtual uint32_t getSendTimeout() const = 0;
  virtual void getLocalAddress(SocketAddress* address) const = 0;
  virtual void getAddress(SocketAddress* address) const {
    getLocalAddress(address);
  }
  virtual void getPeerAddress(SocketAddress* address) const = 0;
  virtual ssl::X509UniquePtr getPeerCert() const { return nullptr; }
};

       至此，PipelineBase中的主要功能分析完毕。

       Pipeline是PipelineBase的子类，其具体定义如下：

template <class R, class W = folly::Unit>
class Pipeline : public PipelineBase {
 public:
  using Ptr = std::shared_ptr<Pipeline>;

  static Ptr create() {
    return std::shared_ptr<Pipeline>(new Pipeline());
  }

  ~Pipeline();

  // 模板方法
  template <class T = R>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value>::type
  read(R msg);//front_->read(std::forward<R>(msg)); --> this->handler_->read(this, std::forward<Rin>(msg));

  template <class T = R>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value>::type
  readEOF();//front_->readEOF();

  template <class T = R>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value>::type
  readException(folly::exception_wrapper e);//front_->readException(std::move(e));

  template <class T = R>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value>::type
  transportActive();// front_->transportActive();

  template <class T = R>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value>::type
  transportInactive();//front_->transportInactive();

  template <class T = W>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value,
                          folly::Future<folly::Unit>>::type
  write(W msg);//back_->write(std::forward<W>(msg));

  template <class T = W>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value,
                          folly::Future<folly::Unit>>::type
  writeException(folly::exception_wrapper e);//back_->writeException(std::move(e));

  template <class T = W>
  typename std::enable_if<!std::is_same<T, folly::Unit>::value,
                          folly::Future<folly::Unit>>::type
  close();//back_->close()

  void finalize() override;

 protected:
  Pipeline();
  explicit Pipeline(bool isStatic);

 private:
  bool isStatic_{false};

  InboundLink<R>* front_{nullptr};// inbound类型Context（read）
  OutboundLink<W>* back_{nullptr};// outbound类型Context (write)
};

      可以看到，Pipeline主要定义和实现了一些和Handler对应的常用方法：read、readEOF、readException、transportActive、transportInactive、write、writeException、close。同时，Pipeline还定义了两个私有成员：front_和back_，从类型可以看出这是两个不同的方向，首先看一下InboundLink定义：

template <class In>
class InboundLink {
public:
  virtual ~InboundLink() = default;
  virtual void read(In msg) = 0;
  virtual void readEOF() = 0;
  virtual void readException(folly::exception_wrapper e) = 0;
  virtual void transportActive() = 0;
  virtual void transportInactive() = 0;
};

       可以看出，InboundLink只是把Pipeline主要方法中的IN方向单独抽象出来，都是一个IN事件（输入事件），那么可想而知OutboundLink的定义：

template <class Out>
class OutboundLink {
public:
  virtual ~OutboundLink() = default;
  virtual folly::Future<folly::Unit> write(Out msg) = 0;
  virtual folly::Future<folly::Unit> writeException(
    folly::exception_wrapper e) = 0;
  virtual folly::Future<folly::Unit> close() = 0;
};

      的确，OutboundLink定义的都是OUT事件类型的操作。

      前文在讲PipelineBase时，addBack之类的操作都是只针对那三个容器进行的，没有地方对front_链表和back_链表进行操作啊？其实，front_链表和back_链表的设置是在Pipeline的finalize中完成的：

template <class R, class W>
void Pipeline<R, W>::finalize() {
  front_ = nullptr;
  if (!inCtxs_.empty()) {
    front_ = dynamic_cast<InboundLink<R>*>(inCtxs_.front());
    for (size_t i = 0; i < inCtxs_.size() - 1; i++) {
      inCtxs_[i]->setNextIn(inCtxs_[i + 1]);
    }
    inCtxs_.back()->setNextIn(nullptr);
  }

  back_ = nullptr;
  if (!outCtxs_.empty()) {
    back_ = dynamic_cast<OutboundLink<W>*>(outCtxs_.back());
    for (size_t i = outCtxs_.size() - 1; i > 0; i--) {
      outCtxs_[i]->setNextOut(outCtxs_[i - 1]);
    }
    outCtxs_.front()->setNextOut(nullptr);
  }

  if (!front_) {
    detail::logWarningIfNotUnit<R>(
      "No inbound handler in Pipeline, inbound operations will throw "
      "std::invalid_argument");
  }
  if (!back_) {
    detail::logWarningIfNotUnit<W>(
      "No outbound handler in Pipeline, outbound operations will throw "
      "std::invalid_argument");
  }

  for (auto it = ctxs_.rbegin(); it != ctxs_.rend(); it++) {
    (*it)->attachPipeline();
  }
}

     代码很简单，以IN方向为例，遍历inCtxs_容器，对容器中的每一个Context调用其setNextIn方法将Context组成一个单向链表front_。同理，outCtxs_最终会变为back_单向链表。最后，还会遍历Context的总容器ctxs_，为每一个Context调用attachPipeline方法，该方法主要工作就是把Context绑定到对应的Handler上（最终是Context和Handler都互相持有对方的引用），还会回调Handler的attachPipeline方法。

    此处还有一个细节，Pipeline是一个模板类，具有两个模板参数template <class R, class W = folly::Unit>，分别代表Pipeline的 read（IN事件）的数据类型和write（out事件）数据类型，这些类型的设置要和Pipeline中的handler类型向匹配（后文还会详细讲解）。

     下面就以Pipeline中的write方法来看一下事件的流动过程：

template <class R, class W>
template <class T>
typename std::enable_if < !std::is_same<T, folly::Unit>::value,
         folly::Future<folly::Unit >>::type
Pipeline<R, W>::write(W msg) {
  if (!back_) {
    throw std::invalid_argument("write(): no outbound handler in Pipeline");
  }
  return back_->write(std::forward<W>(msg));
}

      Pipeline的write方法只是简单的调用back_的wirte方法，也就是OUT类型的事件会从Pipeline的最后一个Context依次向前传递(只传递给OUT类型的handler)。

Handler

      Handler在继承层次上类似于Pipeline，首先有一个基类HandlerBase，其定义如下：

template <class Context>
class HandlerBase {
public:
  virtual ~HandlerBase() = default;

  virtual void attachPipeline(Context* /*ctx*/) {}
  virtual void detachPipeline(Context* /*ctx*/) {}

  // 获取绑定的Context
  Context* getContext() {
    if (attachCount_ != 1) {
      return nullptr;
    }
    CHECK(ctx_);
    return ctx_;
  }

private:
  friend PipelineContext;    // 设置PipelineContext为友元类，便于PipelineContext操作自己
  uint64_t attachCount_{0};  // 绑定计数，同一个handler可以被同时绑定到不同的pipeline中
  Context* ctx_{nullptr};    // 该Handler绑定的Context
};

      HandlerBase内部组合了一个绑定的Context指针，并提供了getContext接口用于获取这个Handler绑定的Context。

      Handler作为HandlerBase的子类，它具有四个模板参数： Rin、Rout、Win、Wout，其中Rin作为Handler和Context中read方法中消息的数据类型，Rout是作为Context中fireRead方法的参数类型。同理，Win是作为Handler和Context中wirte方法的消息参数类型，而Wout是作为Context中fireWrite的消息参数类型。可以这么理解：Xout是作为以fire开头的事件方法的参数类型。

template <class Rin, class Rout = Rin, class Win = Rout, class Wout = Rin>
class Handler : public HandlerBase<HandlerContext<Rout, Wout>> {
public:
  static const HandlerDir dir = HandlerDir::BOTH; // 方向为双向

  typedef Rin rin;
  typedef Rout rout;
  typedef Win win;
  typedef Wout wout;
  typedef HandlerContext<Rout, Wout> Context;  // 声明该HandlerContext类型
  virtual ~Handler() = default;

  // inbound类型事件
  virtual void read(Context* ctx, Rin msg) = 0;
  virtual void readEOF(Context* ctx) {
    ctx->fireReadEOF();
  }
  virtual void readException(Context* ctx, folly::exception_wrapper e) {
    ctx->fireReadException(std::move(e));
  }
  virtual void transportActive(Context* ctx) {
    ctx->fireTransportActive();
  }
  virtual void transportInactive(Context* ctx) {
    ctx->fireTransportInactive();
  }

  // outbound类型事件
  virtual folly::Future<folly::Unit> write(Context* ctx, Win msg) = 0;
  virtual folly::Future<folly::Unit> writeException(Context* ctx,
      folly::exception_wrapper e) {
    return ctx->fireWriteException(std::move(e));
  }
  virtual folly::Future<folly::Unit> close(Context* ctx) {
    return ctx->fireClose();
  }
};

      类似于Pipeline，Handler也相应的定义了inbound类型和outbound类型事件，分别对应方法：read、readEOF、readException、transportActive、transportInactive、write、writeException、close（这些方法和Pipeline中一一对应）。其中，除了read和write两个方法是纯虚接口之外，其他的方法都提供了默认实现：就是将事件进行透传（调用Context里fireXxx方法）。

       同理，根据事件类型的不同，还可以进一步细分Handler类型，比如InboundHandler类型为：

// inbound类型的Handler （默认情况下读入和读出的类型是一致）
template <class Rin, class Rout = Rin>
class InboundHandler : public HandlerBase<InboundHandlerContext<Rout>> {
public:
  static const HandlerDir dir = HandlerDir::IN;  // 方向为输入

  typedef Rin rin;
  typedef Rout rout;
  typedef folly::Unit win;
  typedef folly::Unit wout;
  typedef InboundHandlerContext<Rout> Context; // 声明inbound类型的InboundHandlerContext
  virtual ~InboundHandler() = default;

  // 纯虚函数。由子类实现
  virtual void read(Context* ctx, Rin msg) = 0;
  // 下面的默认实现都是事件的透传
  virtual void readEOF(Context* ctx) {
    ctx->fireReadEOF();
  }
  virtual void readException(Context* ctx, folly::exception_wrapper e) {
    ctx->fireReadException(std::move(e));// std::move
  }
  virtual void transportActive(Context* ctx) {
    ctx->fireTransportActive();
  }
  virtual void transportInactive(Context* ctx) {
    ctx->fireTransportInactive();
  }
};

       相应的，OutboundHandler类型定义为：

// outbound类型的Handler (默认写入类型和写出类型一致，如果不一致就会产生很多的转换)
template <class Win, class Wout = Win>
class OutboundHandler : public HandlerBase<OutboundHandlerContext<Wout>> {
public:
  static const HandlerDir dir = HandlerDir::OUT; // 方向为输出

  typedef folly::Unit rin;
  typedef folly::Unit rout;
  typedef Win win;
  typedef Wout wout;
  typedef OutboundHandlerContext<Wout> Context;
  virtual ~OutboundHandler() = default;

  // 纯虚函数。由子类实现
  virtual folly::Future<folly::Unit> write(Context* ctx, Win msg) = 0;
  // 下面的默认实现都是事件的透传
  virtual folly::Future<folly::Unit> writeException(
    Context* ctx, folly::exception_wrapper e) {
    return ctx->fireWriteException(std::move(e));
  }
  virtual folly::Future<folly::Unit> close(Context* ctx) {
    return ctx->fireClose();
  }
};

      前文所说，Handler所有的事件方法中只有read和write是纯虚接口，这样用户每次实现自己的Handler时都需要override这两个方法（即使只是完成简单的事件透传），因此，为了方便用户编写自己的Handler，Wangle提供了HandlerAdapter，HandlerAdapter其实很简单，就是以事件透传的方式重写（override）了read个write两个方法。代码如下：

// Handler适配器
template <class R, class W = R>
class HandlerAdapter : public Handler<R, R, W, W> {
public:
  typedef typename Handler<R, R, W, W>::Context Context;

  // 将read事件直接进行透传
  void read(Context* ctx, R msg) override {
    ctx->fireRead(std::forward<R>(msg));
  }

  // 将write事件直接进行透传
  folly::Future<folly::Unit> write(Context* ctx, W msg) override {
    return ctx->fireWrite(std::forward<W>(msg));
  }
};

Context

      如前文所述，Pipeline中直接管理的并不是Handler，而是Context，为了便于理解，此处再把Pipeline中的addBack源码列出来：

template <class H>
PipelineBase& PipelineBase::addBack(std::shared_ptr<H> handler) {
  typedef typename ContextType<H>::type Context;// 声明Conetxt类型,ContextImpl<Handler>、InboundContextImpl<Handler>、OutboundContextImpl<Handler>其中之一
  // 使用Context包装Handler后，将其添加到pipeline中，Context中还持有pipeline的引用
  return addHelper(
           std::make_shared<Context>(shared_from_this(), std::move(handler)),
           false);// false标识添加到尾部
}

      其中，ContextType的定义如下，它会根据Handler的类型（具体来说是方向）决定Context的类型，如果Handler是双向的，那么Context类型为ContextImpl<Handler>，如果Handler的方向为IN，那么Context类型为InboundContextImpl<Handler>，如果Handler的方向为OUT，那么Context类型为OutboundContextImpl<Handler>。

template <class Handler>
struct ContextType {
  // template< bool B, class T, class F >
  // type T if B == true, F if B == false
  typedef typename std::conditional <
  Handler::dir == HandlerDir::BOTH,        //如果是双向
          ContextImpl<Handler>,            //类型就是ContextImpl<Handler>
          typename std::conditional<       //如果不是双向，那么还需要细分
          Handler::dir == HandlerDir::IN,  //如果是IN类型
          InboundContextImpl<Handler>,     //那么类型就是InboundContextImpl<Handler>
          OutboundContextImpl<Handler>     //否则就是OutboundContextImpl<Handler>
          >::type >::type
          type;                            // Context类型
};

     其实，InboundContextImpl和OutboundContextImpl都是ContextImpl的子类，ContextImpl的继承关系为：

template <class H>
class ContextImpl
  : public HandlerContext<typename H::rout, typename H::wout>,
    public InboundLink<typename H::rin>,
    public OutboundLink<typename H::win>,
    public ContextImplBase<H, HandlerContext<typename H::rout, typename H::wout>> 

      可以看到，ContextImpl一个继承自四个父类：HandlerContext、InboundLink、OutboundLink和ContextImplBase，其中HandlerContext中主要定义了以fire开头的事件传递方法；InboundLink和OutboundLink分别定义了Handler中Inbound和Outbound类型的方法接口，还记得Pipeline中用于管理IN方向和OUT方向的两个链表：front_和back_，它们就分别是InboundLink和OutboundLink类型；ContextImplBase主要提供了Pipeline中Context在组装链表时的接口，比如：setNextIn、setNextOut，以及用于将Context绑定到handler上的attachPipeline方法。

      首先来看HandlerContext基类：

// HandlerContext定义(集inbound和outbound类型于一身)
// 以fire开始的方法都是Context中的事件方法
template <class In, class Out>
class HandlerContext {
public:
  virtual ~HandlerContext() = default;

  // inbound类型事件接口
  virtual void fireRead(In msg) = 0;
  virtual void fireReadEOF() = 0;
  virtual void fireReadException(folly::exception_wrapper e) = 0;
  virtual void fireTransportActive() = 0;
  virtual void fireTransportInactive() = 0;

  // outbound类型事件接口
  virtual folly::Future<folly::Unit> fireWrite(Out msg) = 0;
  virtual folly::Future<folly::Unit> fireWriteException(
    folly::exception_wrapper e) = 0;
  virtual folly::Future<folly::Unit> fireClose() = 0;


  virtual PipelineBase* getPipeline() = 0;
  virtual std::shared_ptr<PipelineBase> getPipelineShared() = 0;
  std::shared_ptr<folly::AsyncTransport> getTransport() {
    return getPipeline()->getTransport();
  }

  virtual void setWriteFlags(folly::WriteFlags flags) = 0;
  virtual folly::WriteFlags getWriteFlags() = 0;

  virtual void setReadBufferSettings(
    uint64_t minAvailable,
    uint64_t allocationSize) = 0;
  virtual std::pair<uint64_t, uint64_t> getReadBufferSettings() = 0;
};

    HandlerContext主要定义了以fire开头的事件传播方法：fireRead、fireReadEOF、fireReadException、fireTransportActive、fireTransportInactive、fireWrite、fireWriteException、fireClose，以及getPipeline用于获取Context绑定的Pipeline、getPipelineShared以智能指针的形式获取Pipeline、getTransport用于获取Pipeline对应的Transport。

     根据事件流向的不同，Context也可以细分定义，InboundHandlerContext定义为：

// inbound 类型的InboundHandlerContext
template <class In>
class InboundHandlerContext {
public:
  virtual ~InboundHandlerContext() = default;

  virtual void fireRead(In msg) = 0;
  virtual void fireReadEOF() = 0;
  virtual void fireReadException(folly::exception_wrapper e) = 0;
  virtual void fireTransportActive() = 0;
  virtual void fireTransportInactive() = 0;

  virtual PipelineBase* getPipeline() = 0;
  virtual std::shared_ptr<PipelineBase> getPipelineShared() = 0;
  std::shared_ptr<folly::AsyncTransport> getTransport() {
    return getPipeline()->getTransport();
  }
};

     同理，OutboundHandlerContext定义为：

// outbound 类型的OutboundHandlerContext
template <class Out>
class OutboundHandlerContext {
public:
  virtual ~OutboundHandlerContext() = default;

  virtual folly::Future<folly::Unit> fireWrite(Out msg) = 0;
  virtual folly::Future<folly::Unit> fireWriteException(
    folly::exception_wrapper e) = 0;
  virtual folly::Future<folly::Unit> fireClose() = 0;

  virtual PipelineBase* getPipeline() = 0;
  virtual std::shared_ptr<PipelineBase> getPipelineShared() = 0;
  std::shared_ptr<folly::AsyncTransport> getTransport() {
    return getPipeline()->getTransport();
  }
};

      如前文所述，PipelineContext主要定义了如何在Pipeline中组织Context链表的操作接口，比如setNextIn用于设置下一个IN类型的Context，setNextOut用来设置下一个OUT类型Context，具体定义如下：

class PipelineContext {
public:
  virtual ~PipelineContext() = default;

  // 依附到一个pipeline中
  virtual void attachPipeline() = 0;
  // 从pipeline中分离
  virtual void detachPipeline() = 0;

  // 将一个HandlerContext绑定到handler上
  template <class H, class HandlerContext>
  void attachContext(H* handler, HandlerContext* ctx) {
    // 只有第一次绑定的时候才会设置
    if (++handler->attachCount_ == 1) {
      handler->ctx_ = ctx;
    } else {
      // 为何在此设置的时候就为nullptr
      handler->ctx_ = nullptr;
    }
  }

  // 设置下一个inbound类型的Context
  virtual void setNextIn(PipelineContext* ctx) = 0;
  // 设置下一个outbound类型的Context
  virtual void setNextOut(PipelineContext* ctx) = 0;

  // 获取方向(Context方向依赖于Handler方向)
  virtual HandlerDir getDirection() = 0;
};

      ContextImplBase主要实现了PipelineContext接口方法，同时它的两个成员：nextIn_和nextOut_就是链表的指针，用来串联起整个Context。

template <class H, class Context>
class ContextImplBase : public PipelineContext {
public:
  ~ContextImplBase() = default;

  // 获取Context绑定的Handler
  H* getHandler() {
    return handler_.get();
  }

  // Context初始化，参数为Context所属的Pipeline weak_ptr，Context要绑定的Handler  shared_ptr
  void initialize(std::weak_ptr<PipelineBase> pipeline,std::shared_ptr<H> handler) {
    pipelineWeak_ = pipeline;
    pipelineRaw_ = pipeline.lock().get();//裸指针
    handler_ = std::move(handler);
  }

  // PipelineContext overrides
  void attachPipeline() override {
    // 如果该Context还没有被绑定
    if (!attached_) {
      this->attachContext(handler_.get(), impl_);// 将该Context绑定到handler上
      handler_->attachPipeline(impl_); // 调用Handler的attachPipeline，有具体的Handler实现
      attached_ = true;//标记Context已经attached到一个pipeline中
    }
  }

  // 从pipeline中分离
  void detachPipeline() override {
    handler_->detachPipeline(impl_);// 调用Handler的detachPipeline，有具体的Handler实现
    // 依附标志位为false
    attached_ = false;
  }

  void setNextIn(PipelineContext* ctx) override {
    if (!ctx) {
      nextIn_ = nullptr;
      return;
    }

    // 转成InboundLink，因为Context是InboundLink子类
    auto nextIn = dynamic_cast<InboundLink<typename H::rout>*>(ctx);
    if (nextIn) {
      nextIn_ = nextIn;
    } else {
      throw std::invalid_argument(folly::sformat(
                                    "inbound type mismatch after {}", folly::demangle(typeid(H))));
    }
  }

  void setNextOut(PipelineContext* ctx) override {
    if (!ctx) {
      nextOut_ = nullptr;
      return;
    }
    auto nextOut = dynamic_cast<OutboundLink<typename H::wout>*>(ctx);
    if (nextOut) {
      nextOut_ = nextOut;
    } else {
      throw std::invalid_argument(folly::sformat(
                                    "outbound type mismatch after {}", folly::demangle(typeid(H))));
    }
  }

  // 获取Context的方向
  HandlerDir getDirection() override {
    return H::dir;
  }

protected:
  Context* impl_;                                    // 具体的Context实现
  std::weak_ptr<PipelineBase> pipelineWeak_;         //
  PipelineBase* pipelineRaw_;                        // 该Context绑定的pipeline
  std::shared_ptr<H> handler_;                       // 该Context包含的Handler
  InboundLink<typename H::rout>* nextIn_{nullptr};   // 下一个inbound类型的Context地址
  OutboundLink<typename H::wout>* nextOut_{nullptr}; // 下一个outbound类型的Context地址

private:
  bool attached_{false}; // 这个Context是否已经被绑定
};

      ContextImpl就是最终的Context实现，也就是要被添加到Pipeline中（比如使用addBack）的容器（ctxs_，inCtxs_，outCtxs_）的最终Context，在最后的finalize方法中还会进一步将容器中的Context组装成front_和back_单向链表。

      ContextImpl的主要功能就是实现了各种事件传递方法（以fire开头的方法），以fireRead为例，这是一个IN类型的事件，由于Context中持有的Pipeline是一个weak类型的指针，因此先尝试lock，保证在事件传播阶段这个Pipeline不会销毁，然后会去调用下一个IN类型的Context的read方法。read方法是InboundLink中定义的接口（注意这里的read不是Handler中的也不是Pipeline中的），ContextImpl的也实现了这个read方法，它的功能很简单，首先还是先lock住这个Pipeline，然后直接调用Context内部包含的Handler的read方法。

template <class H>
class ContextImpl
  : public HandlerContext<typename H::rout, typename H::wout>,
    public InboundLink<typename H::rin>,
    public OutboundLink<typename H::win>,
    public ContextImplBase<H, HandlerContext<typename H::rout, typename H::wout>> {
public:
  typedef typename H::rin Rin;
  typedef typename H::rout Rout;
  typedef typename H::win Win;
  typedef typename H::wout Wout;
  static const HandlerDir dir = HandlerDir::BOTH;

  explicit ContextImpl(std::weak_ptr<PipelineBase> pipeline,std::shared_ptr<H> handler) {
    this->impl_ = this;//实现就是自己
    this->initialize(pipeline, std::move(handler));//初始化
  }

  // For StaticPipeline
  ContextImpl() {
    this->impl_ = this;
  }

  ~ContextImpl() = default;

  // HandlerContext overrides
  // Inbound类型的事件：read事件
  void fireRead(Rout msg) override {
    auto guard = this->pipelineWeak_.lock();// 锁住，确保一旦锁住成功，在操作期间，pipeline不会被销毁
    // 如果还没有到最后
    if (this->nextIn_) {
      //  将事件继续向下传播（传给下一个Inbound类型的Context）
      //  注意：这里调用的是下一个Contex的read而不是fireRead
      //  即调用下一个Context里面的Handler方法
      this->nextIn_->read(std::forward<Rout>(msg));
    } else {
      LOG(WARNING) << "read reached end of pipeline";
    }
  }

  void fireReadEOF() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->readEOF();
    } else {
      LOG(WARNING) << "readEOF reached end of pipeline";
    }
  }

  void fireReadException(folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->readException(std::move(e));
    } else {
      LOG(WARNING) << "readException reached end of pipeline";
    }
  }

  void fireTransportActive() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->transportActive();
    }
  }

  void fireTransportInactive() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->transportInactive();
    }
  }

  //Outbound类型的事件传播
  folly::Future<folly::Unit> fireWrite(Wout msg) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->write(std::forward<Wout>(msg));
    } else {
      LOG(WARNING) << "write reached end of pipeline";
      // 如果到了最后，返回一个future
      return folly::makeFuture();
    }
  }

  folly::Future<folly::Unit> fireWriteException(
    folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->writeException(std::move(e));
    } else {
      LOG(WARNING) << "close reached end of pipeline";
      return folly::makeFuture();
    }
  }

  folly::Future<folly::Unit> fireClose() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->close();
    } else {
      LOG(WARNING) << "close reached end of pipeline";
      return folly::makeFuture();
    }
  }
 
  // 获取Context绑定的pipeline指针
  PipelineBase* getPipeline() override {
    return this->pipelineRaw_;
  }

  // 获取Context绑定的pipeline引用
  std::shared_ptr<PipelineBase> getPipelineShared() override {
    return this->pipelineWeak_.lock();
  }

  // 设置和获取wirte标志位
  void setWriteFlags(folly::WriteFlags flags) override {
    this->pipelineRaw_->setWriteFlags(flags);
  }

  folly::WriteFlags getWriteFlags() override {
    return this->pipelineRaw_->getWriteFlags();
  }

  // 设置read缓冲区参数 minAvailable、allocationSize
  void setReadBufferSettings(
    uint64_t minAvailable,
    uint64_t allocationSize) override {
    this->pipelineRaw_->setReadBufferSettings(minAvailable, allocationSize);
  }

  std::pair<uint64_t, uint64_t> getReadBufferSettings() override {
    return this->pipelineRaw_->getReadBufferSettings();
  }

  // InboundLink overrides
  void read(Rin msg) override {
    // 保证pipeline不会被删除
    auto guard = this->pipelineWeak_.lock();
    // 调用该Context绑定的Handler的read方法，至于事件是都需要继续传播，完全受read中的实现
    this->handler_->read(this, std::forward<Rin>(msg));
  }

  void readEOF() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->readEOF(this);
  }

  void readException(folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->readException(this, std::move(e));
  }

  void transportActive() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->transportActive(this);
  }

  void transportInactive() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->transportInactive(this);
  }

  // OutboundLink overrides
  folly::Future<folly::Unit> write(Win msg) override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->write(this, std::forward<Win>(msg));
  }

  folly::Future<folly::Unit> writeException(
    folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->writeException(this, std::move(e));
  }

  folly::Future<folly::Unit> close() override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->close(this);
  }
};

      同样，Context也可以根据传输方向进行细分，首先是InboundContextImpl：

template <class H>
class InboundContextImpl
  : public InboundHandlerContext<typename H::rout>,
    public InboundLink<typename H::rin>,
    public ContextImplBase<H, InboundHandlerContext<typename H::rout>> {
public:
  typedef typename H::rin Rin;
  typedef typename H::rout Rout;
  typedef typename H::win Win;
  typedef typename H::wout Wout;
  static const HandlerDir dir = HandlerDir::IN;

  explicit InboundContextImpl(
    std::weak_ptr<PipelineBase> pipeline,
    std::shared_ptr<H> handler) {
    this->impl_ = this;
    this->initialize(pipeline, std::move(handler));
  }

  // For StaticPipeline
  InboundContextImpl() {
    this->impl_ = this;
  }

  ~InboundContextImpl() = default;

  // InboundHandlerContext overrides
  void fireRead(Rout msg) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->read(std::forward<Rout>(msg));
    } else {
      LOG(WARNING) << "read reached end of pipeline";
    }
  }

  void fireReadEOF() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->readEOF();
    } else {
      LOG(WARNING) << "readEOF reached end of pipeline";
    }
  }

  void fireReadException(folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->readException(std::move(e));
    } else {
      LOG(WARNING) << "readException reached end of pipeline";
    }
  }

  void fireTransportActive() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->transportActive();
    }
  }

  void fireTransportInactive() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextIn_) {
      this->nextIn_->transportInactive();
    }
  }

  PipelineBase* getPipeline() override {
    return this->pipelineRaw_;
  }

  std::shared_ptr<PipelineBase> getPipelineShared() override {
    return this->pipelineWeak_.lock();
  }

  // InboundLink overrides
  void read(Rin msg) override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->read(this, std::forward<Rin>(msg));
  }

  void readEOF() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->readEOF(this);
  }

  void readException(folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->readException(this, std::move(e));
  }

  void transportActive() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->transportActive(this);
  }

  void transportInactive() override {
    auto guard = this->pipelineWeak_.lock();
    this->handler_->transportInactive(this);
  }
};

       其次是OutboundContextImpl：

template <class H>
class OutboundContextImpl
  : public OutboundHandlerContext<typename H::wout>,
    public OutboundLink<typename H::win>,
    public ContextImplBase<H, OutboundHandlerContext<typename H::wout>> {
public:
  typedef typename H::rin Rin;
  typedef typename H::rout Rout;
  typedef typename H::win Win;
  typedef typename H::wout Wout;
  static const HandlerDir dir = HandlerDir::OUT;

  explicit OutboundContextImpl(
    std::weak_ptr<PipelineBase> pipeline,
    std::shared_ptr<H> handler) {
    this->impl_ = this;
    this->initialize(pipeline, std::move(handler));
  }

  // For StaticPipeline
  OutboundContextImpl() {
    this->impl_ = this;
  }

  ~OutboundContextImpl() = default;

  // OutboundHandlerContext overrides
  folly::Future<folly::Unit> fireWrite(Wout msg) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->write(std::forward<Wout>(msg));
    } else {
      LOG(WARNING) << "write reached end of pipeline";
      return folly::makeFuture();
    }
  }

  folly::Future<folly::Unit> fireWriteException(
    folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->writeException(std::move(e));
    } else {
      LOG(WARNING) << "close reached end of pipeline";
      return folly::makeFuture();
    }
  }

  folly::Future<folly::Unit> fireClose() override {
    auto guard = this->pipelineWeak_.lock();
    if (this->nextOut_) {
      return this->nextOut_->close();
    } else {
      LOG(WARNING) << "close reached end of pipeline";
      return folly::makeFuture();
    }
  }

  PipelineBase* getPipeline() override {
    return this->pipelineRaw_;
  }

  std::shared_ptr<PipelineBase> getPipelineShared() override {
    return this->pipelineWeak_.lock();
  }

  // OutboundLink overrides
  folly::Future<folly::Unit> write(Win msg) override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->write(this, std::forward<Win>(msg));
  }

  folly::Future<folly::Unit> writeException(
    folly::exception_wrapper e) override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->writeException(this, std::move(e));
  }

  folly::Future<folly::Unit> close() override {
    auto guard = this->pipelineWeak_.lock();
    return this->handler_->close(this);
  }
};

        按照惯例，还是来一张图总结一下吧：

              

本系列文章

Wangle源码分析：Service

Wangle源码分析：ServerBootstrap

Wangle源码分析：编解码Handler 

Wangle源码分析：EventBaseHandler、AsyncSocketHandler 

Wangle源码分析：Pipeline、Handler、Context

Wangle源码分析：ClientBootstrap

来源：oschina

链接：https://my.oschina.net/u/2896894/blog/881236