CODEWITHSUNDEEP
c++winapivisual-c++
Dawamaha
Dawamaha

Reputation: 287

C++ Win32: Calling WM_PAINT from another thread using SendMessage, drawing has no effect

I am currently developing a program (C++ Win32, Visual Studio 2012) that captures audio signals from a microphone, computes a Fourier Transformation (you know, to get the frequencies in the audio) in another thread, and finally plots the results on a screen.

Everything works fine, except for the plotting, so this is a GUI related topic.

In WndProc, I get a message when a buffer is full and ready to be processed, and I call

boost::thread t(&ComputationThread,data);

to start the computation thread.

At the end of ComputationThread I send a message to the GUI thread to plot the data, using a predefined fixed value and the data pointer as arguments:

SendMessage(hWnd, WM_PAINT, PARAM_FFT_COMPUTED, (LPARAM)data);

This works fine as well, I get the message and I can also write the computed data to a file, however, drawing on the form (e.g. using SetPixel) has no effect, while it has an effect when wParam is not PARAM_FFT_COMPUTED, for example when the form is resized.

InvalidateRecthas no effect either.

What am I doing wrong?

Thanks in advance!

Upvotes: 1

Views: 3691

Answers (1)

David Heffernan
David Heffernan

Reputation: 612993

You are not meant to send the WM_PAINT. It's a specially synthesised message. You never post it or send it. Instead you call InvalidateRect or one of the related functions to get the system to generate WM_PAINT.

And you are not meant to draw on a window outside of a WM_PAINT message handler. Or from a thread other than the GUI thread. Perhaps you aren't doing any of those things, but they are common mistakes so it doesn't hurt to mention it.

If InvalidateRect does not work then something fundamental is wrong with your code. You need to fix that rather than try to make painting work by some other mechanism.

So, the solution is to use InvalidateRect and fix whatever is broken in your WM_PAINT handler. Without seeing your WM_PAINT handler I would not like to speculate as to what is wrong.

Upvotes: 15

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