op/clip,widget: add clip.Border and widget.Border

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

widget/border.go

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package widget
+
+import (
+	"image/color"
+
+	"gioui.org/f32"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+)
+
+// Border lays out a widget and draws a border inside it.
+type Border struct {
+	Color        color.RGBA
+	CornerRadius unit.Value
+	Width        unit.Value
+}
+
+func (b Border) Layout(gtx layout.Context, w layout.Widget) layout.Dimensions {
+	dims := w(gtx)
+	sz := dims.Size
+	rr := float32(gtx.Px(b.CornerRadius))
+	st := op.Push(gtx.Ops)
+	width := gtx.Px(b.Width)
+	clip.Border{
+		Rect: f32.Rectangle{
+			Max: layout.FPt(sz),
+		},
+		NE: rr, NW: rr, SE: rr, SW: rr,
+		Width: float32(width),
+	}.Add(gtx.Ops)
+	dr := f32.Rectangle{
+		Max: layout.FPt(sz),
+	}
+	paint.ColorOp{Color: b.Color}.Add(gtx.Ops)
+	paint.PaintOp{Rect: dr}.Add(gtx.Ops)
+	st.Pop()
+	return dims
+}