Control on Tree Row collapsing

["Ashwin Ramachandran" <rashwin28 gmail com>]

Hi All,
I need to have  a mechanism in which I can control the collapsing of tree rows in my application.
To elaborate a bit more for the given parent, child relationship:

+ ABC
  -- DEF
  -- MNP <-- Row selected.

When the row MNP is selected, and its parent ABC is collapsed, then I want to popout a dialog box, mentioning that
the row MNP is going to be un-selected and whether the user wants it or not. If the user is fine with it, then the collapsing
goes through or if the user does not want it, then I need to cancel the collapse signal.
[This behaviour is required for my application needs, where some extra information is required for a given tree row to be taken as inputs
from the user].

The TreeSelection is connected to 'changed' signal and the TreeView to row-activated and collapsed signals, 
When the row MNP is selected and ABC is collapsed, I got the following signal events,
a. first changed signal ['unselection ']of row MNP [In fact, I even have a set_filter_function set for my Treeselection, which is not called in such a case. Am not sure why]
b. collapse of ABC

I tried a blocking dialog box at  signal a.[which muppet had suggested for a similar query in this list]
and tried returning based on the dialog's return to stop the signal b. But somehow it does not work.
The collapse[signal b.] always goes through irrespective of the blocking dialog box's return.

Am I missing something here? Any help in this regard will be highly appreciated.

Thanks
Ashwin