Re: Re: Re: Getting a Cairo context

[Emmanuele Bassi <ebassi gmail com>]

Hi;

On 25 March 2015 at 13:45, Sergei Naumov <vomus rambler ru> wrote:

> >     1. create the widget instance
> >     2. realize it
> >     3. create a similar Cairo surface from the GdkWindow of the widget
> >     4. store Cairo surface on the widget instance; you can use a
> > subclass or g_object_set_data(), with a preference for the former
> >     5. connect a callback to/override the class closure of the ::draw
> > signal
> >         a. in the draw signal handler, you take the surface and use it a
> > source to draw on the cairo_t given to you; you can use a boolean flag
> > to know whether or not the contents of the surface are okay to use
> >     6. fire off a thread, and pass the pointer to widget to the thread
> > function
> >     7. inside the thread function, draw on the surface
> >     8. at the end of the thread, schedule a redraw in the main thread
> >         a. use gdk_threads_add_idle() with a callback and pass the widget
> > pointer as the data
> >         b. from within the callback, set the boolean flag that tells the
> > ::draw callback to use the surface as a source, and call
> > gtk_widget_queue_draw()
> >
> > It's safe to draw on Cairo surfaces from different threads, assuming
> > only a thread at a time does it; it's not safe to use GTK API from
> > different threads, though. That's why you can create a surface from
> > the main thread, and draw on it in a separate thread — as long as you
> > ensure that all the operations involving GTK happen on the main
> > thread.
>
>
> Ok. I am going to try it all out but I have a few questions:
>
> 1) do you know any example code that does what you are describing?

Not off hand, no.

> 2) there is something I do not understand in (5). Do you mean something like
> this:
>
> plotwindow_surface =
> gdk_window_create_similar_surface(gtk_widget_get_window(gcu.plotwindow),
>
> CAIRO_CONTENT_COLOR,_DISPLAY_X,_DISPLAY_Y/2);
>
> g_object_set_data(G_OBJECT(gcu.plotwindow),"plotwindow",(gpointer)plotwindow_surface);
>  g_signal_connect( G_OBJECT( gcu.plotwindow ), "draw", G_CALLBACK(
> ev_plotwindow_draw ), NULL );
>
> in the ev_plotwindow_draw() I extract data (i.e. the surface) from
> gcu.plotwindow and use cairo drawing functions to draw in it.

More or less, yes, that's how you could structure your code.

Another option is to subclass the GtkDrawingArea widget and add
members to the instance structures.

> This is going
> to be my setup draw (axes, tickmarks, etc). Then because I use
> g_timeout_add_seconds() to fire up the thread, I'd have to change it to
> something like that

Why are you using a timeout to fire of a thread? Timeouts do not use a
separate thread: they are part of the main loop. Are you calling
g_thread_new() from a timeout callback?

I sense some underlying confusion, here.

Ciao,
 Emmanuele.

-- 
https://www.bassi.io
[ ] ebassi [ gmail com]