[Chrome源码阅读]IPC通信初探

RenderWidgetHost继承于IPC::Channel::Listener，所以它必然需要实现Listener::OnMessageReceived纯虚函数。

Chrome对这个函数进行了宏的封装，宏的写法有点类似于MFC/wxWidget的事件响应表，不同点在于Chrome里的宏是对函数的实现封装，而MFC/wxWidget里却是对事件表的初始化。

IPC::Channel::Sender类的实现

class Channel : public Message ::Sender {
  // Security tests need access to the pipe handle.
  friend class ChannelTest ;

  public:
  // Implemented by consumers of a Channel to receive messages.
  class Listener {
   public:
    virtual ~ Listener () {}

    // Called when a message is received.
    virtual void OnMessageReceived ( const Message & message ) = 0;

    // Called when the channel is connected and we have received the internal
    // Hello message from the peer.
    virtual void OnChannelConnected ( int32 peer_pid ) {}

    // Called when an error is detected that causes the channel to close.
    // This method is not called when a channel is closed normally.
    virtual void OnChannelError () {}
  };

RenderWidgetHost类需要处理RenderProcess(进程)发过来的一部分Message，更准确的说是RenderProcessHost接收到Render Process的消息，然后一层一层传递过来的。

RenderWidgetHost里的宏，用来实现OnMessageReceived函数。以ViewHostMsg开头的message class是从Render Process发向Render Process Host(browser)进程的。相反，ViewMsg开头的类是从Render Process Host(browser) 发向Render Process的。它们都定义在文件\src\chrome\common\render_messages_internal.h中。

IPC_DEFINE_MESSAGE_MAP (RenderWidgetHost )
  IPC_MESSAGE_HANDLER( ViewHostMsg_RenderViewReady , OnMsgRenderViewReady )
  IPC_MESSAGE_HANDLER( ViewHostMsg_RenderViewGone , OnMsgRenderViewGone )
  IPC_MESSAGE_HANDLER( ViewHostMsg_Close , OnMsgClose )
  IPC_MESSAGE_HANDLER( ViewHostMsg_RequestMove , OnMsgRequestMove )
  IPC_MESSAGE_HANDLER( ViewHostMsg_PaintRect , OnMsgPaintRect )
  IPC_MESSAGE_HANDLER( ViewHostMsg_ScrollRect , OnMsgScrollRect )
  IPC_MESSAGE_HANDLER( ViewHostMsg_HandleInputEvent_ACK , OnMsgInputEventAck )
  IPC_MESSAGE_HANDLER( ViewHostMsg_Focus , OnMsgFocus )
  IPC_MESSAGE_HANDLER( ViewHostMsg_Blur , OnMsgBlur )
  IPC_MESSAGE_HANDLER( ViewHostMsg_SetCursor , OnMsgSetCursor )
  IPC_MESSAGE_HANDLER( ViewHostMsg_ImeUpdateStatus , OnMsgImeUpdateStatus )
  IPC_MESSAGE_UNHANDLED_ERROR()
IPC_END_MESSAGE_MAP ()

我们现在尝试着来拆解这些宏。这些宏定义在文件 \src\chrome\common\ipc_message_macros.h中。

首先是IPC_DEFINE_MESSAGE_MAP宏，传递的参数是一个类的名字：

#define IPC_DEFINE_MESSAGE_MAP(class_name) \
void class_name::OnMessageReceived(const IPC::Message& msg) \
  IPC_BEGIN_MESSAGE_MAP(class_name, msg)

如果传递的class_name是 RenderWidgetHost，那么这个宏展开就是:

void RenderWidgetHost::OnMessageReceived(const IPC::Message& msg)
 IPC_BEGIN_MESSAGE_MAP(RenderWidgetHost, msg)

很明显的就是它是函数OnMessageReceived的定义，接下来展开IPC_BEGIN_MESSAGE_MAP宏：

#define IPC_BEGIN_MESSAGE_MAP(class_name, msg) \
  { \
    typedef class_name _IpcMessageHandlerClass; \
    const IPC::Message& ipc_message__ = msg; \
    bool msg_is_ok__ = true; \
    switch (ipc_message__.type()) { \

继续展开来就是：

{
    typedef RenderWidgetHost _IpcMessageHandlerClass;
    const IPC::Message& ipc_message__ = msg;
    bool msg_is_ok__ = true;
    switch (ipc_message__.type()) {

看到这里，差不多明了函数要干的事情了，就是switch...case的组合，来判断当前的Message是什么类型，然后执行响应的函数体。

宏IPC_MESSAGE_HANDLER(ViewHostMsg_RenderViewReady, OnMsgRenderViewReady)：

#define IPC_MESSAGE_FORWARD(msg_class, obj, member_func) \
  case msg_class::ID: \
    msg_is_ok__ = msg_class::Dispatch(&ipc_message__, obj, &member_func); \
    break;

#define IPC_MESSAGE_HANDLER(msg_class, member_func) \
  IPC_MESSAGE_FORWARD(msg_class, this, _IpcMessageHandlerClass::member_func)

看到case...关键字了。展开来就是：

   case ViewHostMsg_RenderViewReady::ID:
      msg_is_ok__ = ViewHostMsg_RenderViewReady::Dispatch(&ipc_message__, this, _IpcMessageHandlerClass::OnMsgRenderViewReady);
      break;

所有的Msg_class类都继承于IPC::Message，各种message class也是用宏来定义的。

#define IPC_MESSAGE_ROUTED0(msg_class) \
  class msg_class : public IPC::Message { \
   public: \
    enum { ID = msg_class##__ID }; \
    msg_class(int32 routing_id) \
        : IPC::Message(routing_id, ID, PRIORITY_NORMAL) {} \
  };

ID就是类的名字与"__ID"字符串的链接。

IPC::Message这个类 根据消息响应函数参数的不同，提供了4个重载的Dispatch模板函数：

 template
  static bool Dispatch(const Message* msg, T* obj, void (T::*func)()) {
    (obj->*func)();
    return true;
  }

  template
  static bool Dispatch(const Message* msg, T* obj, void (T::*func)() const) {
    (obj->*func)();
    return true;
  }

  template
  static bool Dispatch(const Message* msg, T* obj,
                       void (T::*func)(const Message&)) {
    (obj->*func)(*msg);
    return true;
  }

  template
  static bool Dispatch(const Message* msg, T* obj,
                       void (T::*func)(const Message&) const) {
    (obj->*func)(*msg);
    return true;
  }

仅仅是简单的转调用传入进来的类成员函数。

所以总体来看，这一套宏定义了一个IPC::Channel::Listener继承类的OnMessageReceived函数体，根据message的不同类型，转调用响应的成员函数体。