How to connect signal on derived class of GstBin?

[fastcoding]

For example,

local Gst=lgi.Gst
local bin=Gst.parse_bin_from_description('rtspsrc location="rtsp://...."  name=mysrc ! rtph264depay ', true)
local rtspsrc=bin:get_by_name('mysrc')
function rtspsrc:on_on_sdp(sdp,ud)
	print('got sdp:',sdp)
end

when connecting to 'on-sdp' signal on a GstRtspSrc object, the above codes would fail with error message:

Gst.Bin: no `on_on_sdp'

It seems signals are loaded from GI typelib rather than through gobject runtime query. Maybe the way I'm using signals for such case is wrong?

[pavouk]

Most probably lgi does not have loaded GI typelib for GstRtspSrc object, so it treats it as its parent, GstBin. Try loading GstRtspSrc first using something like local GstRtsp = lgi.GstRtsp.

[fastcoding]

Thanks for reply. But unfortunately GstRtspSrc namespace doesn't exist. I just checked the meson.build file in GstRtspSrc (in gst-plugins-good), it doesn't include a g-ir-scanner command in the build like other Gst components do. I don't know whether it's intentional or not. That means the only way is to manually create typelib or cannot use lgi for this case.

I tried directly calling GObject.signal_connect_closure_by_id. It can connect signal successfully, but fail when closure gets invoked.

Error raised while calling 'lgi.cbk (function: 0x0100e72ea0): GObject': attempt to index 
a nil value

Is it possible to create a user defined GICallableInfo to make the closure callback marshalling work?

[pavouk]

Interesting, this page https://packages.debian.org/sid/i386/gir1.2-gst-plugins-base-1.0/filelist
indicates that GstRtsp should exist. Note that typelib is GstRtsp, not GstRtspSrc.
I don't remember how to connect signal manually, sorry.

[fastcoding]

yes, GstRtsp and GstRtspSrc are different components. The latter can be directly used in pipeline and what I'd like to use lgi to control. I've mentioned using GObject.signal_connect_closure_by_id to manually connect to 'on-sdp' signal, the code was basically copied from lgi/overides/GObject-Object.lua, like this:

local signal_lookup = GObject.signal_lookup
local Closure=GObject.Closure
local signal_connect_closure_by_id = GObject.signal_connect_closure_by_id
-- Connects signal to specified object instance.
local function connect_signal(obj, name, closure, detail, after)
   assert(obj._gtype)
   local id=signal_lookup(name, obj._gtype)
   assert(id)
   print('id=',id)
   return signal_connect_closure_by_id(
      obj,id ,
      detail and quark_from_string(detail) or 0,
      closure, after or false)
end
local clo=Closure( function(sdp,ud)
	print('rtsp got sdp:',sdp)
	return 1
end)
assert(clo)
connect_signal(rtspsrc,'on-sdp',clo)

The issue for this work-around is it will throw error when receiving callback. I guess it's due to lack of the GICallbackInfo interface, lgi doesn't know how to unpack the parameters. Thanks anyway, I will look for other ways around this.

[fastcoding]

The above problem was eventually solved by me. Here is one line patch:

--- a/lgi/override/GObject-Closure.lua
+++ b/lgi/override/GObject-Closure.lua
@@ -282,7 +282,7 @@ function Closure:_new(target, callback_info)
         -- Create marshaller based only on Value types.
         function marshaller(closure, retval, params)
            local args = {}
-           for i, val in ipairs(params) do args[i] = val.value end
+           for i, val in ipairs(params) do args[i] = val end
            local ret = target(unpack(args, 1, #params))
            if retval then retval.value = ret end
         end

In my case, whenever receiving 'on-sdp' callback, one of the params (userdata) passed to the marshaller is nil,

    args[i]=nil.value

and thus results in lgi.callable throw error.