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