widget,widget/material: add Float and Slider

Signed-off-by: Gordon Klaus <gordon.klaus@gmail.com>

widget/float.go

@@ -0,0 +1,88 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package widget
+
+import (
+	"image"
+
+	"gioui.org/gesture"
+	"gioui.org/io/pointer"
+	"gioui.org/layout"
+	"gioui.org/op"
+)
+
+// Float is for selecting a value in a range.
+type Float struct {
+	Value float32
+
+	drag    gesture.Drag
+	pos     float32 // position normalized to [0, 1]
+	length  float32
+	changed bool
+}
+
+// Layout processes events.
+func (f *Float) Layout(gtx layout.Context, pointerMargin int, min, max float32) layout.Dimensions {
+	size := gtx.Constraints.Min
+	f.length = float32(size.X)
+
+	var de *pointer.Event
+	for _, e := range f.drag.Events(gtx.Metric, gtx, gesture.Horizontal) {
+		if e.Type == pointer.Press || e.Type == pointer.Drag {
+			de = &e
+		}
+	}
+
+	value := f.Value
+	if de != nil {
+		f.pos = de.Position.X / f.length
+		value = min + (max-min)*f.pos
+	} else if min != max {
+		f.pos = value/(max-min) - min
+	}
+	// Unconditionally call setValue in case min, max, or value changed.
+	f.setValue(value, min, max)
+
+	if f.pos < 0 {
+		f.pos = 0
+	} else if f.pos > 1 {
+		f.pos = 1
+	}
+
+	defer op.Push(gtx.Ops).Pop()
+	rect := image.Rectangle{Max: size}
+	rect.Min.X -= pointerMargin
+	rect.Max.X += pointerMargin
+	pointer.Rect(rect).Add(gtx.Ops)
+	f.drag.Add(gtx.Ops)
+
+	return layout.Dimensions{Size: size}
+}
+
+func (f *Float) setValue(value, min, max float32) {
+	if min > max {
+		min, max = max, min
+	}
+	if value < min {
+		value = min
+	} else if value > max {
+		value = max
+	}
+	if f.Value != value {
+		f.Value = value
+		f.changed = true
+	}
+}
+
+// Pos reports the selected position.
+func (f *Float) Pos() float32 {
+	return f.pos * f.length
+}
+
+// Changed reports whether the value has changed since
+// the last call to Changed.
+func (f *Float) Changed() bool {
+	changed := f.changed
+	f.changed = false
+	return changed
+}