GTK3 Crystal Bindings

This is a fork of hugopl/gtk4.cr, but with GTK3. Please see that other repo for details.

There's also the more popular and more battle-tested jhass/crystal-gobject for GTK3. Both bindings are not compatible, but migrating is not hard. I made this very alternative because when I used the other one, there were rare different rare crashes which I couldn't resolve.

If you need AT-SPI, you can use https://github.com/phil294/atspi.cr.

GC-related problems

Crystal's garbage collector [GC] normally runs in somewhat unpredictable intervals. This can lead to crashes, sometimes. The following precautions seem to resolve them (entirely?). Maybe they only occur with -Dpreview_mt only anyway.

You'll see a lot of GLib-GObject-CRITICAL **: etc. error messages in the console all the time. I guess they're not avoidable for now.

1. Call GC manually before and after running Gtk code. Example: Instead of

   GLib.idle_add do { gtk code... }

   do

   GC.collect
   GLib.idle_add do { gtk code...}
   GC.collect

   Those Glib error messages mentioned above will still show up, but now exactly when you instruct it to, so it doesn't interfere with the Gtk thread.
2. Avoid begin/rescue inside destroy handlers. Example:

   window.destroy_signal.connect do
     begin
       raise "asdf"
     rescue e
       STDERR.puts e
   end

   this will somehow lead to crashes
3. When you have many invocations of idle_add in a short timeframe (e.g. 1000 within 1 second), you should wait for each of those to complete before querying the next to avoid crashes. Example: Instead of

   def my_gtk_act_function(&block : -> nil)
       GC.collect
       GLib.idle_add do
           block.call
           false
       end
       GC.collect
   end
   1000.times { my_gtk_act_function { ... } }

   do

   def my_gtk_act_function(&block : -> nil)
       channel = Channel(Nil).new
       GC.collect
       GLib.idle_add do
           block.call
           channel.send(nil)
           false
       end
       channel.receive
       GC.collect
   end
   1000.times { my_gtk_act_function { ... } }

   Where the second method also offers the capability of error handling or returning values as well, at the expense of speed.