在C++程序中添加逻辑流程控制-2

 源：

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　　解决方案

　　为减少麻烦，可在程序主流程之外再创建一个子过程，这个子过程用于执行某些逻辑，在需要满足某些条件之后才能继续执行时，它可以先停下来，直到主流程告之它再次进行检查。对于上面的示例，可以写成如下的代码：

class Connection
{
SOCKET socket;
Connection(SOCKET s) : socket(s)
{
FLOW_START conn.flow_start();
}

void flow_start()
{
while (TRUE)
{
HTTP_REQUEST http_request;
try {
read(&http_request);
}
catch (EXCEPTION e)
{
break;
}

FILE file;
fp = fopen(http_request.uri);
if (fp == NULL)
{
write(fp);
fclose(file);
}
else
write(504);
}

socket.close();
delete this;
}

void read(HTTP_REQUEST* http_request)
{
read(&http_request.header);
read(&http_request.body, &http_request.header);
}

void read(HTTP_REQUEST_HEADER* header)
{ …}

void read(BYTE[]& body, HTTP_REQUEST_HEADER& header)
{ …}

string read_line()
{
while (TRUE)
{
FLOW_WAIT (m_buffer += )
char* str = strchr(m_buffer, 'n');
if (!str)
continue;
}

string s(m_buffer, 0, str - m_buffer);
memcpy(m_buffer, str);
buf_avail -= str - m_buffer;
return s;
}

BYTE[] read_bytes(int sz)
{
while (TRUE)
{
WAIT (m_buffer += );
if (m_buffer.length < sz)
continue;
}

BYTE[] data = m_buffer.extract(0, sz);
return data;
}

void write(FILE* fp)
{
int filesize = fp.size();

string header;
header << "200 OKrnContent-Length: " << filesize << ";rn"
<<"rn";
write(header.c_str(), header.size());

int szBulk;
for (int i = 0; i < filesize; i += szBulk)
{
szBulk = min(filesize - i, 8192);
data = fread(fp, szBulk);
write(data, szBulk);
}
}

void write(WORD error_status)
{
string header;
header << error_status << " Errorrn"
<<"rn";
write(header.c_str(), header.size());
}

void write(BYTE[] data, int len)
{
while (len > 0)
{
int ret = socket.write(data, len);
if (ret > 0)
{
data += ret;
len -= ret;
}
if (len)
{
WAIT (bWritable == TRUE);
}
}
}

void OnRead()
{
int avail = socket.avail();
m_buffer += socket.read(avail);
}

void OnWrite()
{
bWritable = TRUE;
}

void OnClose()
{
delete this;
}
};

main {
Socket listen_socket;
listen_socket.listen(http_port);
socket_add(listen_socket, NULL);

socket_loop(socket_callback);
}

void socket_callback(void* user_data, SOCKET s, int msg,
int lParam, void* pParam)
{
switch (msg)
{
case READ:
if (user_data == NULL)
{
SOCKET s2 = accept(s);
Connection conn = new Connection(socket);
socket_add(s2, conn);
break;
}
((Connection*)user_data)->OnRead();
break;

case WRITE:
((Connection*)user_data)->OnWrite();
break;

case EXCEPT:
((Connection*)user_data)->OnExcept();
break;
}
}

　　这涉及到两个新的原语：一个为FLOW_START，其创建了一个新的子过程；另一个为FLOW_WAIT，其告之系统何时将被调用以继续程序流程。例如，FLOW_WAIT(m_buffer += )意味着m_buffer的符+=被执行，FLOW_WAIT (bWritable = TRUE)意味着bWritable被设为TRUE。

　　当一个对象创建后，因为FLOW_START这条指令，一个子过程也会被创建，执行流程会沿着此过程执行下去，直至碰到FLOW_WAIT，然后，它会继续执行主流程；当它追加m_buffer或设置bWritable为TRUE时，它将继续执行子过程，直至碰到另一个FLOW_WAIT，此时再返回到主流程当中。