ui/gesture,ui/internal/fling: extract fling logic into its own package

We're going to re-use fling extrapolation and animation for Wayland touchpads.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/gesture/gesture.go

@@ -11,12 +11,11 @@ package gesture
 
 import (
 	"math"
-	"runtime"
-	"time"
 
 	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/input"
+	"gioui.org/ui/internal/fling"
 	"gioui.org/ui/pointer"
 )
 
@@ -46,8 +45,8 @@ type ClickType uint8
 type Scroll struct {
 	dragging  bool
 	axis      Axis
-	estimator estimator
-	flinger   flinger
+	estimator fling.Extrapolation
+	flinger   fling.Animation
 	pid       pointer.ID
 	grab      bool
 	last      int
@@ -57,15 +56,6 @@ type Scroll struct {
 
 type ScrollState uint8
 
-type flinger struct {
-	// Current offset in pixels.
-	x float32
-	// Initial time.
-	t0 time.Time
-	// Initial velocity in pixels pr second.
-	v0 float32
-}
-
 type Axis uint8
 
 const (
@@ -102,16 +92,7 @@ const (
 	StateFlinging
 )
 
-var (
-	touchSlop = ui.Dp(3)
-	// Pixels/second.
-	minFlingVelocity = ui.Dp(50)
-	maxFlingVelocity = ui.Dp(8000)
-)
-
-const (
-	thresholdVelocity = 1
-)
+var touchSlop = ui.Dp(3)
 
 // Add the handler to the operation list to receive click events.
 func (c *Click) Add(ops *ui.Ops) {
@@ -168,7 +149,7 @@ func (s *Scroll) Add(ops *ui.Ops) {
 
 // Stop any remaining fling movement.
 func (s *Scroll) Stop() {
-	s.flinger = flinger{}
+	s.flinger = fling.Animation{}
 }
 
 // Scroll detects the scrolling distance from the available events and
@@ -190,7 +171,7 @@ func (s *Scroll) Scroll(cfg ui.Config, q input.Queue, axis Axis) int {
 				break
 			}
 			s.Stop()
-			s.estimator = estimator{}
+			s.estimator = fling.Extrapolation{}
 			v := s.val(e.Position)
 			s.last = int(math.Round(float64(v)))
 			s.estimator.Sample(e.Time, v)
@@ -201,16 +182,8 @@ func (s *Scroll) Scroll(cfg ui.Config, q input.Queue, axis Axis) int {
 				break
 			}
 			fling := s.estimator.Estimate()
-			if slop, d := float32(cfg.Px(touchSlop)), fling.Distance; d >= slop || -slop >= d {
-				if min, v := float32(cfg.Px(minFlingVelocity)), fling.Velocity; v >= min || -min >= v {
-					max := float32(cfg.Px(maxFlingVelocity))
-					if v > max {
-						v = max
-					} else if v < -max {
-						v = -max
-					}
-					s.flinger.Init(cfg.Now(), v)
-				}
+			if slop, d := float32(cfg.Px(touchSlop)), fling.Distance; d < -slop || d > slop {
+				s.flinger.Start(cfg, fling.Velocity)
 			}
 			fallthrough
 		case pointer.Cancel:
@@ -270,56 +243,6 @@ func (s *Scroll) State() ScrollState {
 	}
 }
 
-func (f *flinger) Init(now time.Time, v0 float32) {
-	f.t0 = now
-	f.v0 = v0
-	f.x = 0
-}
-
-func (f *flinger) Active() bool {
-	return f.v0 != 0
-}
-
-// Tick computes and returns a fling distance since
-// the last time Tick was called.
-func (f *flinger) Tick(now time.Time) int {
-	if !f.Active() {
-		return 0
-	}
-	var k float32
-	if runtime.GOOS == "darwin" {
-		k = -2 // iOS
-	} else {
-		k = -4.2 // Android and default
-	}
-	t := now.Sub(f.t0)
-	// The acceleration x''(t) of a point mass with a drag
-	// force, f, proportional with velocity, x'(t), is
-	// governed by the equation
-	//
-	// x''(t) = kx'(t)
-	//
-	// Given the starting position x(0) = 0, the starting
-	// velocity x'(0) = v0, the position is then
-	// given by
-	//
-	// x(t) = v0*e^(k*t)/k - v0/k
-	//
-	ekt := float32(math.Exp(float64(k) * t.Seconds()))
-	x := f.v0*ekt/k - f.v0/k
-	dist := x - f.x
-	idist := int(math.Round(float64(dist)))
-	f.x += float32(idist)
-	// Solving for the velocity x'(t) gives us
-	//
-	// x'(t) = v0*e^(k*t)
-	v := f.v0 * ekt
-	if v < thresholdVelocity && v > -thresholdVelocity {
-		f.v0 = 0
-	}
-	return idist
-}
-
 func (a Axis) String() string {
 	switch a {
 	case Horizontal: