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Multi Threads (Visual C++, MFC)

giora
giora asked
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Last Modified: 2013-12-03
I built an MDI application with Visual C++ 5 (automatically with "MFC appWizard - EXE").
The View Class draws itself to the child window through the CDC pDC variable in View::OnDraw, to the device context in the usual way.
I'd like to do the drawing in a separate thread, to create a new 'drawing' thread. (In my application the drawing is a long task). This is a very common thing to do.

I encountered severe problems doing that. Many critical errors were created.
note: I work with WinNT.

Please answer as clear as possible and give an example, because I had many problems with this issue.
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The problem is you need to protect access to objects which are used in multiple threads.  This includes MFC objects.  Otherwise you'll get synchronization problems which will produce unpredictable results.

Author

Commented:
Please give more details about protecting access, I don't understand.

Commented:
I would suggest that you allocate a memory DC and a memory Bitmap into that DC, which
has the same size like the painting area. Then, let your threads paint into this DC,
and - perhaps using a timer - send every 500 msec or so an Invalidate() command to the
main window, which blits the memory DC into the visible DC. This is also more common
because if the window gets overlapped, the repainting does not start from the beginning.
Murali Devarakonda4WeekStartup.guru

Commented:
I can give you code for a thread class that takes a handle to a static control and continuously draws the current time on it.

The thread is give handles to it's parent and the static control via the constructor.

You can extrapolate this code to your situation without much difficulty.

Murali Krishna Devarakonda
Murali Devarakonda4WeekStartup.guru

Commented:
I can give you code for a thread class that takes a handle to a static control and continuously draws the current time on it.

The thread is give handles to it's parent and the static control via the constructor.

You can extrapolate this code to your situation without much difficulty.

Murali Krishna Devarakonda
MFC Objects were not intended to be used by more then one thread. VC's reference states, that in case of need of sharing CWnd or CDC class, you should pass only the handle (m_hDC), and it should create the CDC object using CDC::FromHandle
Commented:
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