Generating and handling multiple requests
The previous example illustrated a single request and response. Often, your application may be required to send multiple requests to the server to accomplish its task. These must be sequenced and controlled by the second library response.
Here is an example which generates two requests to get the discrete inputs and the busy input (The busy input is a single discrete input sometimes used to determine an overall remote machine state; ie, running, loading, calibrating, etc. You could build an application to check the busy input before making any further requests or sending any data).
Assume the user is connected and has a dialog named InputOutput opened. The user taps the Get Input button. Here is the function that handles the tap:
<![CDATA[
void CInputOutput::OnGetIn()
{
// TODO: Add your control notification handler code here
m_getInput.SetCheck(BST_UNCHECKED);
m_haveInputs = FALSE;
m_haveBusy = FALSE;
if(GetInput())
m_status = _T("Requested Input");
else
m_status=_T("Unable to send.");
UpdateData(FALSE);
}
]]>
Notice that this function generates one server request GetInput() and exits when the library's first response is received. It also sets two flags to indicate that two responses are required to complete the task.
The library response comes through the main dialog's DataReceivedMessage just as in the Com Port example. Except now the InputOutput Dialog is the active window.
The Message Map for InputOutput Modal Dialog includes the same mapping for the DataReceivedMessage:
<![CDATA[
BEGIN_MESSAGE_MAP(CInputOutput, CDialog)
//{{AFX_MSG_MAP(CInputOutput)
ON_BN_CLICKED(IDC_GET_IN, OnGetIn)
ON_BN_CLICKED(IDC_SEND_OUTPUTS, OnSendOutputs)
//}}AFX_MSG_MAP
ON_REGISTERED_MESSAGE(SD_DATARECEIVEDMESSAGE, DataReceivedMessage)
END_MESSAGE_MAP()
]]>
So the message is routed to the DataReceivedMessage for this dialog.
Here is the function that handles the library's second response:
<![CDATA[
long CInputOutput::DataReceivedMessage(WPARAM wParam, LPARAM lParam)
{
CDataReceived *pData = (CDataReceived *)lParam;
UINT16 size=0; // size of data received
size = *pData->length;
switch(pData->pWideData[0])
{
case 'B': //response to get busy input
{
m_busyValue = (unsigned char)pData->pWideData[1];
m_haveBusy = TRUE;
if(m_haveInputs == TRUE)
SetInputDisplay(m_inputValue,m_busyValue);
else
GetInput();
m_status = _T("Busy Response received.");
UpdateData(FALSE);
break;
}
case 'F': //response to get inputs
{
m_inputValue = (unsigned char)pData->pWideData[1];
m_haveInputs = TRUE;
if(m_haveBusy == TRUE) SetInputDisplay(m_inputValue,m_busyValue);
else
GetBusyInput();
m_status = _T("Input Response received.");
UpdateData(FALSE);
break;
}
case 'N': //response to set outputs
{
m_status = _T("Response received.");
UpdateData(FALSE);
break;
}
default:
m_status = _T("Wrong response");
UpdateData(FALSE);
break;
}
return 1;
}
]]>
The InputOutput dialog handles three responses, so there are three cases: responses to GetInput(), GetBusyInput(), and SetOutputs().
The response ("F") is examined, received data asssigned to a variable and the approriate flag is set. The busy input flag is clear, so a GetBusyInput() call is made to complete the task. With the library's first response to GetBusyInput(), the function exits.
The library generates its second response to the GetBusyInput() call and the message is routed to the DataReceivedMessage function for handling. The response ("B") is examined, data assigned to a variable and the appropriate flag cleared. Since all data has been received, the user display is updated and the function exits.
The user has tapped on the screen. The application and library have sequenced a set of requests to get the data and update the display.
This shows how the two requests are sequenced through the library's second response, insuring single thread processing through the wireless link.
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Library Messages
Discussion has focused on the DataReceivedMessage generated by the library. This is one of three messages the library generates. Here are the full set of library messages:
