decelerated panning on iOS

[incanus]

Involves tying a dummy UIScrollView delegate to the CADisplayLink render pass.

[incanus]

I did this before over in https://github.com/mapbox/mapbox-me/blob/infinity-scroll/Infinity%20Scroll/ISMapView.m

[incanus]

We should actually not complicate this with a scroll view and instead use the -[UIPanGestureRecognizer velocityInView:] API for this.

[incanus]

This could benefit from both alternative timing curves and cancelable animations during successive pans, but those doesn't exist yet in util::animation. Will revisit when they do.

[kkaefer]

@incanus Can you please describe what you're going to need exactly? Do they just have to be cancellable, or should we somehow add several animations into one?

[incanus]

I don't think (yet) that animations will need to be combined, but here's a cancellation scenario:

• User pans and releases
• Momentum causes view to coast to a stop
• Before the stop, user pans in the opposite direction
• Currently, the animated coast needs to stop before the other direction takes effect.

For timing curves, ease out makes the most sense for inertial panning and zooming.