VelocityHelper has been renamed as ScrollDamper

Introduce ScrollDamper

I wrote a utility class that helps developers to handle velocity in convenience. ScrollDamper is an abstract class which shouldn't be used directly but to extend. For now, I brought in four different scroll dampers, they are LinearScrollDamper, QuadScrollDamper, CubicScrollDamper and ExpoScrollDamper. The last one is similar to the original way, and it's the default ScrollDamper once SmoothScrollContainer is created in editor.

How to use

slide()

Fill in parameters it needs, it will return the next velocity and relative offset (not position).

attract()

Emulate force that attracts something to destination. Return the result of next velocity according to delta time.

rebound_strength

The higher the value, the faster attract() attracts.

_calculate_velocity_to_dest()

Return the exact velocity that makes something slide to destination.

How to extend

To show how scroll damper works, here we take LinearScrollDamper as example.
In convenience of calculation, we assume that x axis is time, but time comes from infinity to zero.

_calculate_velocity_by_time(time)

For linear velocity, we let velocity slows down as time count down to zero, which is:

if time <= 0.0: return 0.0
return time * _factor

_factor is a private variable of LinearScrollDamper, using friction to set its value indirectly is a simpler way. The greater the value, the faster the velocity decrease.
The differential of this formula must greater than 0 no matter what "time" it is, or it might cause some issue.

_calculate_time_by_velocity(velocity)

It's the reverse calculation of the previous formula.

return abs(velocity) / _factor

Notice that no matter what sign velocity is, we regard it as positive.

_calculate_offset_by_time(time)

This is the integral for the formula in _calculate_velocity_by_time(time). Constant is not needed.

time = max(time, 0.0)
return 1.0/2.0 * _factor* time*time

_calculate_time_by_offset(offset)

It's the reverse calculation of the previous formula.

return sqrt(abs(offset) * 2.0 / _factor)

Notice

Need more testing: A big part of the script has been rewritten, since it is involved deeply.
Incompatible changes: Friction and damping is removed which is not a compatible change.

Other changes

Useful methods

get_spare_size()

Scroll bar takes up some space of the container, which might cause some calculation mistakes. Using get_spare_size() to get spare space size.

get_child_size_diff()

Get the size differences between child node and container. Child node 's scale and scroll bar size are taken into consideration. It could be useful for future development like content scaling.

get_child_boundary_dist()

Get the left, right, top and bottom distance.

Redesigned interface

Since the exposed properties are getting more, it is a good time to reorder these properties.

[Edited]

Tween for scroll_to()

Tween for scroll_to function will fight against scrolling and dragging input when tween animation is not done. Now all the scroll_to tween will be killed if needed.