Previously we used a bunch of heuristics for this. We checked if velocity
was directed towards the nearest snap point and its value was larger than
a threshold. If it is, we completed the swipe, otherwise we cancelled it.
This was good enough at the time, because this code was originally written
for back/forward swipe. Since then, the swipe tracker was extended to
handle arbitrary snap points and not just 0 and 1, or -1 and 0, depending
on text direction. After that it was iterated on, but never significantly
redone.
This worked well enough, but had two problems:
1. In some cases, notably overview, it may be wanted to be able to swipe
through multiple pages at once. This wasn't really possible because we
always picked the adjacent snap point.
2. Since we can't do that well, we want to restrict swipes to one page at a
time. It was done in a rather hacky way by clamping the position into
[-1, 1] range from the place where we started the swipe. This works
if we start the swipe from idle position, but if an animation was
already going, the range would be clamped to arbitrary values, and very
likely containing only one snap point, which we already swiped past at
this point. In this case, finishing the swipe would cancel it regardless
of velocity. This means that if one tries to quickly move through
carousel pages via swiping, half of the swipes will be inexplicably
cancelled.
We'll use the deceleration formula from
https://medium.com/@esskeetit/how-uiscrollview-works-e418adc47060#10ce
to calculate then projection point, then pick the nearest snap point and
calculate the duration as we did before. It works well enough for short
distances, but has two problems:
1. It caps the maximum distance at a pretty low value - about 5 pages in my
testing.
2. With how we pick the nearest snap point, it's too easy to accidentally
cancel the swipe,
To combat the first problem, we can modify the curve: only use linear
function at small distances, and smoothly transition it to a parabola
further.
For the second problem we can add two special cases: first, if the swipe
ended up between the initial snap point and the next one, we always prefer
the latter. Second, a good old velocity threshold for cancelling.
We'll also use a slightly smaller deceleration value for touchpad: 0.997
instead of 0.998.
Now that we can pick any snap point, the [-1, 1] clamping doesn't make
sense anymore, so instead let's replace it with a more flexible
mechanism: if we're near a snap point, pick its adjacent snap points.
Otherwise, take the two nearest snap points, and take their adjacent
snap points. This way we have 3 snap points to choose from when
starting a swipe from an idle position, and 4 if we start during an
ongoing transition.
This way, if we've just swiped from snap point n to n+1, the transition
will pick snap points n-1, n, n+1, n+2 and if we swipe again, we will
likely land on n+2. During that transition, if we swipe again, it will
likely have already passed the snap point n+1, so this time the available
snap points will be n, n+1, n+2, n+3, so we can swipe again and it will
still complete, and so on.
This will make it easy to allow multi-page swipes as well, by just
removing the clamping.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1647>
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