RenderWidgetHost繼承于IPC::Channel::Listener，所以它必然需要實現(xiàn)Listener::OnMessageReceived純虛函數(shù)。
Chrome對這個函數(shù)進行了宏的封裝，宏的寫法有點類似于MFC/wxWidget的事件響應(yīng)表，不同點在于Chrome里的宏是對函數(shù)的實現(xiàn)封裝，而MFC/wxWidget里卻是對事件表的初始化。
IPC::Channel::Sender類的實現(xiàn)
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類需要處理RenderProcess(進程)發(fā)過來的一部分Message，更準確的說是RenderProcessHost接收到Render Process的消息，然后一層一層傳遞過來的。
RenderWidgetHost里的宏，用來實現(xiàn)OnMessageReceived函數(shù)。以ViewHostMsg開頭的message class是從Render Process發(fā)向Render Process Host(browser)進程的。相反，ViewMsg開頭的類是從Render Process Host(browser) 發(fā)向Render Process的。它們都定義在文件\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 ()我們現(xiàn)在嘗試著來拆解這些宏。這些宏定義在文件\src\chrome\common\ipc_message_macros.h中。
首先是IPC_DEFINE_MESSAGE_MAP宏，傳遞的參數(shù)是一個類的名字：
#define IPC_DEFINE_MESSAGE_MAP(class_name) \void class_name::OnMessageReceived(const IPC::Message& msg) \ IPC_BEGIN_MESSAGE_MAP(class_name, msg)
如果傳遞的class_name是RenderWidgetHost，那么這個宏展開就是:
void RenderWidgetHost::OnMessageReceived(const IPC::Message& msg)IPC_BEGIN_MESSAGE_MAP(RenderWidgetHost, msg)很明顯的就是它是函數(shù)OnMessageReceived的定義，接下來展開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()) { \繼續(xù)展開來就是：{typedef RenderWidgetHost _IpcMessageHandlerClass;const IPC::Message& ipc_message__ = msg;bool msg_is_ok__ = true;switch (ipc_message__.type()) {看到這里，差不多明了函數(shù)要干的事情了，就是switch...case的組合，來判斷當(dāng)前的Message是什么類型，然后執(zhí)行響應(yīng)的函數(shù)體。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...關(guān)鍵字了。展開來就是：
case ViewHostMsg_RenderViewReady::ID:msg_is_ok__ = ViewHostMsg_RenderViewReady::Dispatch(&ipc_message__, this, _IpcMessageHandlerClass::OnMsgRenderViewReady);break;
所有的Msg_class類都繼承于IPC::Message，各種message class也是用宏來定義的。
#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就是類的名字與"__ID"字符串的鏈接。
IPC::Message這個類根據(jù)消息響應(yīng)函數(shù)參數(shù)的不同，提供了4個重載的Dispatch模板函數(shù)：
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;}僅僅是簡單的轉(zhuǎn)調(diào)用傳入進來的類成員函數(shù)。
所以總體來看，這一套宏定義了一個IPC::Channel::Listener繼承類的OnMessageReceived函數(shù)體，根據(jù)message的不同類型，轉(zhuǎn)調(diào)用響應(yīng)的成員函數(shù)體。
