Thanks friends. I tried using a callback hook and MUIM_Group_InitChange/ExitChange and I've at least achieved what I was trying to do.

Here's a working mini-app in case this helps anyone in the future:

#include <stdio.h>
#include <exec/types.h>
#include <exec/libraries.h>
#include <utility/hooks.h>
#include <libraries/mui.h>

#include <proto/exec.h>
#include <proto/intuition.h>
#include <proto/muimaster.h>
#include <proto/utility.h>
#include <proto/alib.h>
#include <clib/alib_protos.h>

static struct Hook addItemHook;

static ULONG AddItem(struct Hook *h, Object *o, APTR *msg)
{

Object *sometext;

sometext = TextObject,
MUIA_Text_Contents, "Sometimes you eat the bear.",
End;

if (DoMethod(o, MUIM_Group_InitChange))
{
DoMethod(o, MUIM_Group_AddTail, sometext);
DoMethod(o, MUIM_Group_ExitChange);
}

return 1;
}

int main(void)
{
addItemHook.h_Entry = HookEntry;
addItemHook.h_SubEntry = (HOOKFUNC)AddItem;

Object *app, *wnd, *button, *group;

app = ApplicationObject,
SubWindow, wnd = WindowObject,
MUIA_Window_Title, "Todo Sample App",
WindowContents, group = VGroup,
Child, button = SimpleButton("_Add"),
End,
End,
End;

if (app != NULL)
{
ULONG sigs=0;

DoMethod(wnd, MUIM_Notify, MUIA_Window_CloseRequest, TRUE,
(IPTR)app, 2,
MUIM_Application_ReturnID, MUIV_Application_ReturnID_Quit);

DoMethod(button, MUIM_Notify, MUIA_Pressed, FALSE,
(IPTR)group,2, MUIM_CallHook, (IPTR)&addItemHook);

set(wnd, MUIA_Window_Open, TRUE);

if (XGET(wnd, MUIA_Window_Open))
{
while((LONG)DoMethod(app, MUIM_Application_NewInput, (IPTR)&sigs)
!= MUIV_Application_ReturnID_Quit)
{
if (sigs)
{
sigs = Wait(sigs | SIGBREAKF_CTRL_C);
if (sigs & SIGBREAKF_CTRL_C)
break;
}
}
}
else
{
puts("window not opened!");
}
MUI_DisposeObject(app);
}

return 0;
}

I've done this:

Child, HGroup,
Child, tick1 = MUI_MakeObject(MUIO_Checkmark, NULL),
Child, MUI_MakeObject(MUIO_Label,"640x480", 0),
End,

and then:

DoMethod(tick1, MUIM_Notify, MUIA_Selected, TRUE, (IPTR)custom, 3, MUIM_Set, MUIA_Resolution, TRUE);
DoMethod(tick1, MUIM_Notify, MUIA_Selected, FALSE,(IPTR)custom, 3, MUIM_Set, MUIA_Resolution, FALSE);

I don't know about the group class and what the addtail is supposed to do on your code.

My code can be found here:https://trac.aros.org/trac/browser/AROS/trunk/AROS/rom/usb/guiapps/ps3eye/ps3eye.c

Only hack in that code is when I change the window size. There's a need to close and reopen the window after the new size is selected.

SetAttrs(_win(obj), MUIA_Window_Open, FALSE, MUIA_Window_Open, TRUE, TAG_DONE);

Maybe try to close and reopen the window if it doesn't update the window.

Just a wild guess,

but usually you have to set the Group to "Exchange mode" to add/remove items in runtime.

see here:

http://www.sasg.com/mui/autodocs/MUI_Group.html#MUIM_Group_InitChange

Maybe that the reason it does not work.

You could try the Notify events with an other method, like window, MUIM_Window_ToBack, to test if the method calling is working.

I thought I understood how DoNotify works but I'm not so sure now. Lets say I have a VGroup ("group") inside a window to which I want to add new Test:

DoMethod(group, MUIM_Add_Tail, sometext);

This works fine. :)

I thought I could be notified of an event and call a method on a Zune/MUI object... like:

DoMethod(button, MUIM_Notify, MUIA_Pressed, FALSE,
(IPTR)group, 2, MUIM_Add_Tail, sometext);

but this doesn't work at all :(

Other examples I've seen simply drop a magic value into the event loop using MUIM_Application_ReturnID and use this in a switch to dispatch to a function call, or alternatively use a callback hook.

Both approaches are reasonable but I don't understand the limitations of what is permitted in MUIM_Notify.

Can anyone offer any insight here?

Cheers!