As a general rule, your event handlers may be called any number of
times. Event handlers should ensure that the proper pre-conditions
exist before taking any action. In formal terms, event handlers must
be [idempotent](http://en.wikipedia.org/wiki/Idempotence_
(computer_science)).
In this particular case, the construction of the view notices that
you have a handler for `ony`, so it is called when the view gets its
initial `y` value, then the scrollbar is instantiated, which causes
the views layout to be recomputed, resulting in a second `ony` event.
On 2007-01-25, at 00:44 EST, Norman Klein wrote:
Why does this code generate two "ony" events ??
<canvas>
<view name="A" width="100" height="100">
<view name="B" width="100" height="100">
<view name="C" width="100%">
<handler name="ony">
Debug.write("on y : " + A.y + A.B.y + A.B.C.y);
</handler>
</view>
<scrollbar/>
</view>
</view>
</canvas>
Produces the following output:
on y : 000
on y : 000
I added the third view, because I initially thought that the
instantiation of a view resulted in an "ony" event being sent, but it
doesn't. Adding the axis="x" attribute to the scrollbar turns them
off.
So why does the scrollbar object send this event and why does it
send it
twice?
