widget/material: add Float.Invert

Setting Float.Invert=true not only inverts the order of values (which was already easily done by swapping min and max), it also draws the widget inverted so that the track is darkened on the opposite side from usual.

This patch also fixes a bug wherein a vertical slider was drawn inverted by default.

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

widget/float.go

@@ -13,8 +13,9 @@ import (
 
 // Float is for selecting a value in a range.
 type Float struct {
-	Value float32
-	Axis  layout.Axis
+	Value  float32
+	Axis   layout.Axis
+	Invert bool
 
 	drag    gesture.Drag
 	pos     float32 // position normalized to [0, 1]
@@ -43,7 +44,10 @@ func (f *Float) Layout(gtx layout.Context, pointerMargin int, min, max float32)
 	if de != nil {
 		xy := de.Position.X
 		if f.Axis == layout.Vertical {
-			xy = de.Position.Y
+			xy = f.length - de.Position.Y
+		}
+		if f.Invert {
+			xy = f.length - xy
 		}
 		f.pos = xy / f.length
 		value = min + (max-min)*f.pos