The future of MirInputEvent

[Daniel van Vugt]

Rather than querying types and aborting you can simplify things for 
clients by just returning NULL instead:

if (MirTouchEvent* touch = mir_event_as_touch_event(event))
{
     ...
}

Segfaulting in dereferencing NULL is just as reliable as aborting, but 
this way requires less effort to use the client API.


On 26/06/15 16:01, Alexandros Frantzis wrote:
> On Thu, Jun 25, 2015 at 10:45:10AM -0700, Robert Carr wrote:
>> Hi!
>
>> I think the value of MirInputEvent is pretty limited.
>
> Having a base MirInputEvent and deriving specialized input event types
> seems like a natural model for the domain. However, it's problematic
> since the interface shared by the types is small, so the benefits of
> inheritance are mostly lost, because most code needs to know the exact
> type it's working with.
>
> That being said, input events are usually treated specially as a
> group at the higher levels, so it's useful to have a way to check if an
> event is an input event. This can be achieved without a base
> MirInputEvent type by checking the type manually:
>
>    if (type == mir_event_keyboard || type == mir_event_motion || ...)
>
> However, I think it's worth having a convenience function:
>
>    if (mir_event_is_input_event(event))
>
>> I think it's possible to defend as correct, for example something may
>> wish to access the time of events without knowing the event
>> type...even then though this seems pretty marginal.
>
> Writing a wrapper to provide this functionality is trivial, although not
> elegant from a user's point of view, since users need to explicitly care
> about the types.
>
>> On the other hand there is no way to implement the conversion we
>> want without casting inm C so I think we'd probably be better to remove it
>> in the end...this would also enable us to keep MirEvent as a discriminated
>> union...where as it stands thats become awkward with MirInputEvent becoming
>> this like...fake type.
>
> Perhaps we could reach a middle ground by trading compile time
> type safety for convenience by introducing functions for the common
> properties of input events, without having an explicit MirInputEvent
> type. The accessor functions would abort if passed an non-input event.
>
> bool mir_event_is_input_event(MirEvent const*)
> int64_t mir_input_event_timestamp(MirEvent const*)
> MirInputDeviceId mir_input_event_device_id(MirEvent const*)
>
> Thanks,
> Alexandros
>