[ios] Cannot determine in completion block whether camera transition was cancelled

[julianrex]

If one of

• _setVisibleCoordinates:count:edgePadding:direction:duration:animationTimingFunction:completionHandler:
• _setCenterCoordinate:edgePadding:zoomLevel:direction:duration:animationTimingFunction:completionHandler:
• setCamera:withDuration:animationTimingFunction:edgePadding:completionHandler:
• _flyToCamera:edgePadding:withDuration:peakAltitude:completionHandler:

is interrupted, for example by a pan gesture, then its completion block is correctly called. However, it's currently impossible to tell that this transition was cancelled. If you check cameraChangeReasonBitmask during the completion block, you'll see that MGLCameraChangeReasonTransitionCancelled is not set.

This is because the completion block is called from a dispatch_async, and by the time the completion block is called the bit mask has been changed.

Ideally, the completion block should take a parameter from which the user could determine whether it had been cancelled (a BOOL, or perhaps some kind of transition state object). This will change the API, so I'm marking as a SEMVER-MAJOR change - however, we can improve the situation by doing some cameraChangeReasonBitmask dancing (though this should only be viewed as a short-term measure).

iOS SDK: ios-v5.3.0-beta.1 (and earlier)
Discovered during testing of #15122.

[julianrex]

Closing as stale.