Re: deprecated gtk_cairo_create

[Emmanuele Bassi <ebassi gmail com>]

Hi;

On 30 December 2016 at 11:15, Sergei Kolomeeyets <kolomeeyets gmail com> wrote:
> Hi, everyone
> I'm struggling with migration from gtk2. Well gtk_cairo_create is deprecated
> in gtk3. And it looks  the changes are very serious (or I'm sach silly that
> even do not understand what should I do instead of it ). Is there anybody
> who use Drawing Area in GTK3 and can explain the right way of its cairo
> context creation and drawing on it to me. It would be so appreciated.

Connect to the GtkWidget::draw signal (or subclass GtkDrawingArea and
override the GtkWidgetClass.draw virtual function, which is
essentially the same) and use the Cairo context that's given to you.

Drawing *on widgets* outside of their rendering cycle is not possible
in GTK+ 3.x.

If you want to draw something off screen and then composite it inside
your widget, you'll need to create a Cairo surface yourself, using
something like:

  GdkWindow *window = gtk_widget_get_window (widget);
  cairo_surface_t *surface = gdk_window_create_similar_surface
(window, CAIRO_CONTENT_COLOR_ALPHA, width, height);

then create a Cairo context from it, and draw:

  cairo_t *cr = cairo_create (surface);
  // draw
  cairo_destroy (cr);

Now you can use the surface as a source on the Cairo context that
GtkWIdget::draw gives you:

  static gboolean
  on_draw (GtkWidget *widget, cairo_t *cr)
  {
    // render your surface on the widget's context
    cairo_set_source_surface (cr, surface, x, y);
    cairo_paint (cr);

    // do your own drawing
  }

> Documentation says the following:
> ***gtk_cairo_create
> gdk_cairo_create has been deprecated since version 3.22 and should not be
> used in newly-written code. Use gdk_window_begin_draw_frame() and
> gdk_drawing_context_get_cairo_context() instead.

This is only needed if you're literally implementing a toolkit (like
GTK) on top of GDK.


> gboolean draw_callback(GtkWidget *area, GdkEventExpose *event, GArray
> *ptLinePoints) {
> ...
> cairo_t *cr = gdk_drawing_context_get_cairo_context (area);
> ...
> }

This is *not* a GtkWidget::draw callback. You should read the API reference:

  https://developer.gnome.org/gtk3/stable/GtkWidget.html#GtkWidget-draw

The second argument of the callback is a Cairo context; use it.

> But linker (gcc package) writes  "undefined reference to
> `gdk_drawing_context_get_cairo_context"... :-(
> Everything indicated that I missed several lines of code before
> gdk_drawing_context_get_cairo_context. But what lines exactly? Googling
> gives nothing...

It means you're not using GTK+ 3.22, but likely an earlier version.

In any case, you should just forget about the drawing context, and use
the Cairo context that GTK itself gives you.

Ciao,
 Emmanuele.

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