Automated UI testing question

I have an application whose user interface is a single canvas called .ui with some items as buttons, e.g. btn_1
I need a test routine that simulates mouse clicks on those buttons.

Here is my approach:

The following bindings are defined for item btn_1:

.ui bind btn_1 <ButtonPress-1> "key_press b1"
.ui bind btn_1 <ButtonRelease-1> "key_release b1"

This is the click routine:


# get top right edge of .ui on screen
set uix [winfo rootx .ui]
set uiy [winfo rooty .ui]

# compute center ot item btn_1 in screen coords
lassign [.ui bbox btn_1] bx0 by0 bx1 by1
set mx [expr int($uix + $bx0 + ($bx1-$bx0)/2.0)]
set my [expr int($uiy + $by0 + ($by1-$by0)/2.0)]

# simulate mouse click
focus .ui
event generate .ui <ButtonPress-1> -x $mx -y $my -when tail
event generate .ui <ButtonRelease-1> -x $mx -y $my -when tail


When I run the code nothing happens; key_press/key_release are not called.
What am I doing wrong?

Torsten
.