Attribute observer callbacks are called with heartbeat until disabled - after first called

[eliao]

This seems odd to me, particularly because they are labeled "__on_change".

eg: if I do a
vehicle.add_attribute_observer('mode', mode_callback)

mode_callback is called every heartbeat, regardless of mode change.

[mrpollo]

https://github.com/diydrones/dronekit-python/blob/master/droneapi/module/api.py#L440

        elif typ == "HEARTBEAT":
            self.__on_change('mode', 'armed')

not sure why we did this, @geeksville do you remember this?

[geeksville]

Using heartbeat to detect mode changes is a good thing. But yep, it would be much better to also keep a cache of mode and armed inside of the vehicle object and only publish mode and armed if the corresponding boolean changes state.

[mrpollo]

perfect, I think this is what @eliao is calling odd here, I'll submit a PR with some caching, if the mode is still the same we won't publish __on_change('mode') same with armed

[mhct]

@mrpollo Perhaps the cache should be done for all other properties as well, since there is no check for evaluating if they have changed or not.

[hamishwillee]

@mrpollo @mhct Yes. The bug shows up on all properties not just mode.

[hamishwillee]

@mrpollo FYI, it appears that the callback is called more often than the heartbeat - when testing #184 I was getting "lots" of records a second and the heartbeat only comes in once a second. I suspect this means that it is updated whenever a relevant message containing the attribute comes from the vehicle. That would certainly make more sense than updating on heartbeat. Can you confirm? (Note, I assumed this to be correct so that is what #184 states)

In #184 I explain how you might cache the value and provide notification only when the value changes. I still think this caching should be done by our API.

When do you think we might get some resource on this?

[tcr3dr]

@hamishwillee I'll take over this issue. I think classifying it as a feature (attribute caching) makes it worth a minor release, so it's worth documenting for the 1.3.x branch. I will focus effort on #184 first.