MFC - Posting data via PostMessage to the GUI - multithreading

Question

I Read a few threads on how passing data from Workerthread to Main window(Dialog), but I still don't understand and still need help.
My workerthread should process some calculations and display the result during each loop to an edit on the GUI. I know, I should use PostMessage but since the calculation I'm doing implies a control element, I don't know how to solve this problem ...
//CWorkerThreadMgr.h manages the second thread
HRESULT Start (HWND hWnd);// from where the workerthread will be started
HWND m_hWnd ; //Window handle to the UI ;
HANDLE m_hTread; //handle of the worker thread
static UINT WINAPI ThreadProc( LPVOID lptest );
static UINT WINAPI ThreadProc( LPVOID lptest )
{
CWorkerThreadMgr* pCalculateMgr = reinterpret_cast< CWorkerThreadMgr*(lptest);
//The following operation:rand() *m_Slider.GetPos() should
//should be calculated and the result displayed each time in the edit box in the gui
for( UINT uCount = 0; uCount < 40; uCount++ ){
pCalculateMgr->rand() *m_Slider.GetPos();//?don't allowed to touch the gui!!
PostMessage(pCalculateMgr-> m_hWnd, WM_SENDCALCULATED_VALUE,wparam(rand() *m_Slider.GetPos(),0);
}
}
LRESULT CStartCalculationDlg::OnSendCalculatedValue( WPARAM Result, LPARAM )
{
// The resut should be displayed in the edit box
m_Calculation.Format(_T("%d"),???);
SetDlgItemText(IDC_CALCULATION, m_Calculation);
return 1;
}
void CStartCalculationDlg::OnHScroll(UINT nSBCode, UINT nPos,CScrollBar* pScrollBar)
{
m_SliderValue.Format(_T("%d"),m_Slider.GetPos());
SetDlgItemText(IDC_SLIDER_VALUE,m_SliderValue);
}
// Implementation in the CStartCalculationDlg.h
CWorkerThreadMgr m_WorkerThreadMgr //instance of the WorkerThreadMgr
CSliderCtrl m_Slider //member variable of the slider control
CString m_SliderValue // member variable of the edit box, where the current value of the
//slider will be displayed
CString m_Calculation // member variable of the edit box where the calculated
//result from the workerthread will be displayed via PostMessage
afx_msg LRESULT OnSendCalculatedValue( WPARAM, LPARAM );
afx_msg void OnHScroll(UINT nSBCode, UINT nPos, CScrollBar* pScrollBar);
The next probem is that, when my slider control is moved and gets a new value, the thread procedure should know it and update the value of the slider. How can I manage to do it?

Answer 1

Instead of reading the slider position from the worker thread, read it in the UI thread and make it available to the worker thread.
I'm really no expert in this field, but I've done some worker threads as explained here.
What I would do in your case is create a static variable, like the "running" flag used in the article linked above, to hold the current position of the slider. Then, in the OnHScroll handler set it to the appropriate value.
As long as you only write to that variable from one thread, you should not have synchronization issues.

Answer 2

From worker thread:
m_data = foo();
PostMessage(hWndMain, UWM_SOME_CUSTOM_MESSAGE, 0, 0);
From UI thread:
LRESULT CMainFrame::OnSomeCustomMessage(WPARAM wParam, LPARAM lParam)
{
CMyData data = m_pWorker->GetData();
// Do stuff...
return 0;
}
GetData must be guarded by a critical section:
CMyData CMyWorker::GetData()
{
// This critical section is used in the worker thread too, whenever m_data is accessed.
m_lock.Lock();
CMyData data = m_data;
m_lock.Unlock();
return data;
}
See CCriticalSection on MSDN.