Throttle map rendering when device unplugged

[ericrwolfe]

Right now, rendering 60fps animations between location updates is taxing on the device + battery. When the device is unplugged, we should not animate between location updates.

cc @bsudekum @1ec5

[1ec5]

This requires #352, because the course tracking animation built into MGLMapView can’t be disabled.

[ericrwolfe]

This also depends on a fix for the general battery monitoring crash: #371

[ericrwolfe]

@1ec5 Is this something we could achieve in a first pass without a complete overhaul of course tracking, for example with a category on MGLMapView?

[1ec5]

Unfortunately not without changes to the map SDK proper or a risky hack. The CADisplayLink frame rate is determined by a constant. We could swizzle -[MGLMapView didUpdateLocationIncrementallyAnimated:], but I’m not a fan of that approach.

[ericrwolfe]

Yeah, overriding -[MGLMapView didUpdateLocationIncrementallyAnimated:] is along the lines of what I was thinking. Could we override with a category or in the NavigationMapView subclass without needing to swizzle and break instances outside of the navigation UI?

[1ec5]

I agree that swizzling would be a bad idea for the reason that it would break non-NavigationMapView instances of MGLMapView. A category method override would be nondeterministic, right? Overriding in NavigationMapView would be safer, although -_setCenterCoordinate:edgePadding:zoomLevel:direction:duration:animationTimingFunction:completionHandler:, -edgePaddingForFollowing, and -directionByFollowingWithCourse are also internal, so they would need to be forward-declared as well.

[frederoni]

This is being implemented as part of #402 but it's blocked upstream by some camera refinements.

[1ec5]

By the way, if we need a tightly-scoped fix for a patch release, we can override -_setCenterCoordinate:edgePadding:zoomLevel:direction:duration:animationTimingFunction:completionHandler: in a separate PR while #402 bakes.

[frederoni]

Unfortunately, _setCenterCoordinate:edgePadding:zoomLevel:direction:duration:animationTimingFunction:completionHandler: doesn't take pitch into account so when the device is plugged in and we're using animations, setCenter or pitch will cancel the previous transition.

[ericrwolfe]

I think we should continue to consider throttling when the device is unplugged, though from tests anything that involves a significant change in course (e.g. turns) looks pretty bad without animations.

Could we disable animations if the device is unplugged and the change in course/heading is small (i.e. <10-15°), otherwise animate between location updates?

[ericrwolfe]

Reopening as a follow on to #402.