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https://git.sr.ht/~eliasnaur/gio
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Elias Naur
3784ece6dd
Package ui is now only about units except for the Config.Now method. Remove Now and rename Config to Converter. Add layout.Config to replace the old ui.Config. Signed-off-by: Elias Naur <mail@eliasnaur.com>
99 lines
1.8 KiB
Go
99 lines
1.8 KiB
Go
// SPDX-License-Identifier: Unlicense OR MIT
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package fling
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import (
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"math"
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"runtime"
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"time"
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"gioui.org/unit"
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)
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type Animation struct {
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// Current offset in pixels.
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x float32
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// Initial time.
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t0 time.Time
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// Initial velocity in pixels pr second.
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v0 float32
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}
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var (
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// Pixels/second.
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minFlingVelocity = unit.Dp(50)
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maxFlingVelocity = unit.Dp(8000)
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)
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const (
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thresholdVelocity = 1
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)
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// Start a fling given a starting velocity. Returns whether a
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// fling was started.
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func (f *Animation) Start(c unit.Converter, now time.Time, velocity float32) bool {
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min := float32(c.Px(minFlingVelocity))
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v := velocity
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if -min <= v && v <= min {
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return false
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}
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max := float32(c.Px(maxFlingVelocity))
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if v > max {
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v = max
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} else if v < -max {
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v = -max
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}
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f.init(now, v)
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return true
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}
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func (f *Animation) init(now time.Time, v0 float32) {
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f.t0 = now
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f.v0 = v0
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f.x = 0
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}
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func (f *Animation) Active() bool {
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return f.v0 != 0
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}
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// Tick computes and returns a fling distance since
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// the last time Tick was called.
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func (f *Animation) Tick(now time.Time) int {
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if !f.Active() {
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return 0
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}
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var k float32
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if runtime.GOOS == "darwin" {
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k = -2 // iOS
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} else {
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k = -4.2 // Android and default
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}
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t := now.Sub(f.t0)
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// The acceleration x''(t) of a point mass with a drag
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// force, f, proportional with velocity, x'(t), is
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// governed by the equation
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//
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// x''(t) = kx'(t)
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//
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// Given the starting position x(0) = 0, the starting
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// velocity x'(0) = v0, the position is then
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// given by
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//
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// x(t) = v0*e^(k*t)/k - v0/k
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//
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ekt := float32(math.Exp(float64(k) * t.Seconds()))
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x := f.v0*ekt/k - f.v0/k
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dist := x - f.x
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idist := int(math.Round(float64(dist)))
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f.x += float32(idist)
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// Solving for the velocity x'(t) gives us
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//
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// x'(t) = v0*e^(k*t)
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v := f.v0 * ekt
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if -thresholdVelocity < v && v < thresholdVelocity {
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f.v0 = 0
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}
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return idist
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}
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