From 692d6ab2219fa2e52061290f7ae5bffd18b6be26 Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Wed, 8 Jul 2020 22:52:58 +0200
Subject: [PATCH] widget/material: add Loader for indeterminate progress widget

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 widget/material/loader.go | 130 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)
 create mode 100644 widget/material/loader.go

diff --git a/widget/material/loader.go b/widget/material/loader.go
new file mode 100644
index 00000000..fc537fb3
--- /dev/null
+++ b/widget/material/loader.go
@@ -0,0 +1,130 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package material
+
+import (
+	"image"
+	"image/color"
+	"math"
+	"time"
+
+	"gioui.org/f32"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+)
+
+type LoaderStyle struct {
+	Color color.RGBA
+}
+
+func Loader(th *Theme) LoaderStyle {
+	return LoaderStyle{
+		Color: th.Color.Primary,
+	}
+}
+
+func (l LoaderStyle) Layout(gtx layout.Context) layout.Dimensions {
+	diam := gtx.Px(unit.Dp(24))
+	if minX := gtx.Constraints.Min.X; minX > diam {
+		diam = minX
+	}
+	if minY := gtx.Constraints.Min.Y; minY > diam {
+		diam = minY
+	}
+	sz := gtx.Constraints.Constrain(image.Pt(diam, diam))
+	radius := float64(sz.X) * .5
+	defer op.Push(gtx.Ops).Pop()
+	op.Offset(f32.Pt(float32(radius), float32(radius))).Add(gtx.Ops)
+
+	dt := (time.Duration(gtx.Now.UnixNano()) % (time.Second)).Seconds()
+	startAngle := dt * math.Pi * 2
+	endAngle := startAngle + math.Pi*1.5
+
+	clipLoader(gtx.Ops, startAngle, endAngle, radius)
+	paint.ColorOp{
+		Color: l.Color,
+	}.Add(gtx.Ops)
+	op.Offset(f32.Pt(-float32(radius), -float32(radius))).Add(gtx.Ops)
+	paint.PaintOp{
+		Rect: f32.Rectangle{Max: layout.FPt(sz)},
+	}.Add(gtx.Ops)
+	op.InvalidateOp{}.Add(gtx.Ops)
+	return layout.Dimensions{
+		Size: sz,
+	}
+}
+
+func clipLoader(ops *op.Ops, start, end, radius float64) {
+	const thickness = .2
+
+	outer := float32(radius)
+	inner := float32(radius) * (1. - thickness)
+
+	var p clip.Path
+	p.Begin(ops)
+
+	sine, cose := math.Sincos(start)
+
+	pen := f32.Pt(float32(cose), float32(sine)).Mul(outer)
+	p.Move(pen)
+	angle := start
+	// The clip path uses quadratic beziér curves to approximate
+	// a circle arc. Minimize the error by capping the length of
+	// each curve segment.
+	arcPrRadian := radius * math.Pi
+	const maxArcLen = 20.
+	anglePerSegment := maxArcLen / arcPrRadian
+	// Outer arc.
+	for angle < end {
+		angle += anglePerSegment
+		if angle > end {
+			angle = end
+		}
+		sins, coss := sine, cose
+		sine, cose = math.Sincos(angle)
+
+		// https://pomax.github.io/bezierinfo/#circles
+		div := 1. / (coss*sine - cose*sins)
+		ctrl := f32.Point{
+			X: float32((sine - sins) * div),
+			Y: -float32((cose - coss) * div),
+		}.Mul(outer)
+
+		endPt := f32.Pt(float32(cose), float32(sine)).Mul(outer)
+
+		p.Quad(ctrl.Sub(pen), endPt.Sub(pen))
+		pen = endPt
+	}
+
+	// Arc cap.
+	cap := f32.Pt(float32(cose), float32(sine)).Mul(inner)
+	p.Line(cap.Sub(pen))
+	pen = cap
+
+	// Inner arc.
+	for angle > start {
+		angle -= anglePerSegment
+		if angle < start {
+			angle = start
+		}
+		sins, coss := sine, cose
+		sine, cose = math.Sincos(angle)
+
+		div := 1. / (coss*sine - cose*sins)
+		ctrl := f32.Point{
+			X: float32((sine - sins) * div),
+			Y: -float32((cose - coss) * div),
+		}.Mul(inner)
+
+		endPt := f32.Pt(float32(cose), float32(sine)).Mul(inner)
+
+		p.Quad(ctrl.Sub(pen), endPt.Sub(pen))
+		pen = endPt
+	}
+
+	// Second arc cap automatically completed by End.
+	p.End().Add(ops)
+}