Re: How to maintain a reference to Cairo::Context or Cairo::Surface?

["Diego A. Fons" <diegofons apexar com>]

Milosz Derezynski wrote:
> It does not seem neccessary to create 2 separate Cairo Contexts:
>
> class MyArea : public Gtk::DrawingArea
> {
> public:
>     virtual ~MyArea();
> protected:
> //Override default signal handler:
>     virtual bool on_expose_event(
>
>     GdkEventExpose* event);
>     };
>     MyArea::MyArea()
>     {
>     }
>     MyArea::~MyArea()
>     {
>     }
>     bool MyArea::on_expose_event(GdkEventExpose* event)
>     {
>     // This is where we draw on the window
>         Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
>         cr->set_line_width(10.0);
>         cr->move_to(0, 0);
>         cr->line_to(100, 100);
>         cr->stroke();   
>         return true;
>     }
>
>
> This will work equally well. Is there any specific reason why you want 
> to keep the surface member in the class and use it the way you wrote 
> it in the preceding mail?
>
> M.
>
>
> 2008/12/3 BC Zhu <zhubicen gmail com <mailto:zhubicen gmail com>>
>
>     Now  i am using Gtkmm. I want to maintain a reference to
>     Cairo::Context or Cairo::Surface, so i can draw something on it
>     anywhere.
>     I am trying to do this , but failed.
>     With following code , i am trying to keep a reference to
>     Cairo:::Surface，but when on_expose_event was called secondly , it
>     will fail.
>     And i have tried to maintain a reference to Cairo::Context too,
>     but failed either.
>     Could some body give me an advice?
>     Thanks in advance!
>
>
>     class MyArea : public Gtk::DrawingArea
>     {
>     public:
>         virtual ~MyArea();
>     protected:
>     //Override default signal handler:
>         virtual bool on_expose_event(GdkEventExpose* event);
>     private:
>         Cairo::RefPtr<Cairo::Surface> surf_;
>     };
>     MyArea::MyArea()
>     {
>     }
>     MyArea::~MyArea()
>     {
>     }
>     bool MyArea::on_expose_event(GdkEventExpose* event)
>     {
>     // This is where we draw on the window
>         if(!surf_){
>             Glib::RefPtr<Gdk::Window> window = get_window();
>             Cairo::RefPtr<Cairo::Context> cr =
>     window->create_cairo_context();
>             surf_ = cr->get_target();
>         }
>         Cairo::RefPtr<Cairo::Context> cr = Cairo::Context::create(surf_);
>         cr->set_line_width(10.0);
>         cr->move_to(0, 0);
>         cr->line_to(100, 100);
>         cr->stroke();   
>         return true;
>     }
>
>
>     -- 
>     Best Regards
>     Bicen.Zhu
>
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>
>
>
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Hi,

I also have this problem, my reason to keep the context (or surface) is 
that i want to draw and i don't want to loose the previous drawings i 
made in the context.

The last code works fine but when you call
 Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();
in the on_expose_event(), you are creating a new context, an empty context.

My solution was drawing in a context i create and then copy this context 
over the one i get from the component, the problem is that this method 
is _very_ inefficient.

Regards,
Diego A. Fons.