PointerHandler: add grabPermissions, enforce in setExclusiveGrab

As soon as we enable the concept that PointerHandlers can use passive
grabs to lurk, monitor all movements, and then steal the passive grab,
they can fight over the grab. For example if there are two items with
PinchHandlers, and two or more touches occur within bounds for both,
then each update event can cause the other PinchHandler to steal the
grabs and become active.

So we replace stealing with negotiation: the handler which wants to
take over the grab checks its own flags to see whether that's allowed,
and the handler which is about to lose its grab also has the right
to approve or deny the takeover (just as QQuickItem has had
keepMouseGrab and keepTouchGrab for a long time.)  Additionally,
if one handler wants to cancel another handler's grab without
taking over (simply set the grabber to null), it must be approved.

A single-point handler can simply call setExclusiveGrab, with the
expectation that permission may be granted or denied.  A multi-point
handler only wants to grab all points if grabbing all of them will
be allowed, otherwise grab none; so it calls canGrab on each point
to check beforehand.  Thus, when two handlers are competing for the
same grabs, one or both can be prevented from stealing from each other,
or from Handlers in general, or from Items, or some combination.

Change-Id: I5c733b2b8995ce686da0be42244394eeee82a268
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

0 files changed, 0 insertions, 0 deletions