[android-developers] Multi-touch pointer ids on Sony Xperia Play & spurious ACTION_DOWN events

A user reported an ArrayIndexOutOfBoundsException today in our multi-
touch handler. We store x/y/touchState on a per pointer id basis in an
array. Dirty? Yes, but it worked on all devices so far. Usually
pointer ids are handed out like this:

first finger goes down -> pointer Id 0
second finger goes down -> pointer id 1
second finger lifted
second finger goes down -> pointer id 1
second finger lifted
second finger goes down -> pointer id 1

This works on all phones and some discussions in this group also point
out this exact behaviour. Sadly, MotionEvent is underdocumented so it
seems this was an assumption after all, not a rule all manufacturers
would follow. On the Sony Xperia Play the following behaviour is
observed.

first finger goes down -> pointer Id 0
second finger goes down -> pointer id 1
second finger lifted
second finger goes down -> pointer id 2
second finger lifted
second finger goes down -> pointer id 3

Is this a bug in the Xperia touch driver or is this actually wanted
behaviour? Did we all (and there are a couple of people i know of
having this issue) created faulty apps based on a wrong assumption?

On a related note: The MotionEvent documentation has a new addition
which reads like this:

"The view implementation should be prepared to handle ACTION_CANCEL
and should tolerate anomalous situations such as receiving a new
ACTION_DOWN without first having received an ACTION_UP for the prior
gesture."

The handling of ACTION_CANCEL is a no-brainer of course. What is
making me a bit nervous is the fact that spurious ACTION_DOWN events
can now happen. It is not clear whether this will only happen if a
parent View consumed the accompanying ACTION_UP event or if this is
behaviour to be expected even in single View applications.

Any pointers would be appreciated, no pun intended.

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