Better indicate when there is no GPS signal

[westnordost]

fixes #3944

@mnalis Would you be available to test this? I have no idea how you were able to simulate a situation where the GPS signal was cut off.

[westnordost]

I don't know what this gif is supposed to show, I don't see the GPS icon blinking. So anyway, @mnalis or @matkoniecz are you available to test this?

[mnalis]

I've managed to compile it and make a quick test; it seems that:

• blinking navigation arrow icon is working
• hiding the blue dot on the map (and blue transparent circle sector showing direction user is looking at) is not working (if I understand it correctly what PR was supposed to do)

Here is the example video just after signal loss:

small_SVID_20220425_194115_1.mp4

(Steps: I had SC in foreground with working GPS signal just outside the building, then I turned screen off and entered the building thus losing GPS fix; after turning screen back on the SC which was still in foreground looked like video above).

[westnordost]

Thank you for testing it! I will try to find the bug. Maybe I will also build a tinfoil box in which I could test it (if that is enough to block the signal)

[westnordost]

Hm, it looks like there is not any callback that is called when location is lost. The only reason why the GPS button blinks in that case is that on initialization (when the app comes back into view), that button's state is first set to "searching" and only when the first location is reported back from the Android system, the state is set to "has location".

The same cane be done for the location dot of course. But note that this fix wouldn't help in the case when you lose location while the app is in foreground.

[westnordost]

And it leads to some ugly blinking, i.e. the blue dot vanishes when the app comes back into view for ~1 second and pops back in after that. Every time you tab out and in. Given it would not really be a wholesome fix anyway, I rather not do that.

So, I'll merge it as is.

All works as expected when one disables location.