Re: Object Widgets

["Mark Hadfield" <m.hadfield(at)niwa.cri.nz>]

A few more items of feedback on your widget classes, Struan:

1. In your widget cleanup routine it is a good idea to check that the object
is valid before trying to destroy it, i.e.

pro SLFoWid_Cleanup, myID

  widget_control, myID, get_uvalue=myObjRef
  if obj_valid(myObjRef) then begin
    myObjRef -> GetProperty, no_block=no_block
    if no_block eq 1 then obj_destroy, myObjRef $
      else myObjRef -> cleanup
  endif

end   ; pro SLFoWid_Cleanup

This is advisable because it is possible for the object to have been
destroyed (by OBJ_DESTROY, or HEAP_GC) behind XMANAGER's back.

2. Similarly it is a good idea in the object cleanup routine to check if the
widget hierarchy is still valid and, if it is, destroy it, i.e.

pro SLFoWid::Cleanup

  print, 'SLFoWid::Cleanup'
  print, '  widget ID: ', self.myWidID
  print, '  object ID: ', self

  if widget_info(self.myWidID, /VALID_ID) then widget_control, self.myWidID,
/DESTROY

end   ; pro SLFoWid::Cleanup

3. There is a problem in heap cleanup for blocking widgets. If I create a
new widget with

o = obj_new('SLFow_minimal')

then hit the quit button, the heap is cleaned up. But if I do the same with

o = obj_new('SLFow_minimal', /BLOCK)

then the 'SLFow_minimal' object is left on the heap. The problem, as you
have noted in comments in your code, is that for a blocking widget the
procedure that handles the 'quit' event is called from the XMANAGER event
loop, which is called from inside the Init method, and you can't destroy an
object from inside it's own Init method. One solution is for
SLFow_minimal::Init to check whether it has been called with the BLOCK
keyword set. If it has, then it needs to leave out the call to Xmanage, and
leave this up to the user. This is an unfortunate complication. I guess the
other solution is for 'SLFow_minimal' to prevent the BLOCK keyword from
being passed to 'SLFoWid' so that the application always runs non-blocking.
(Perhaps you'd already thought of all this.)

Anyway, thanks very much for publishing this stuff. It provides a framework
for writing much cleaner widget applications.
---
Mark Hadfield
m.hadfield@niwa.cri.nz  http://katipo.niwa.cri.nz/~hadfield/
National Institute for Water and Atmospheric Research
PO Box 14-901, Wellington, New Zealand