[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<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。