ui/layout: add Inset example

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

ui/layout/layout_test.go

@@ -0,0 +1,55 @@
+package layout_test
+
+import (
+	"fmt"
+	"image"
+	"time"
+
+	"gioui.org/ui"
+	"gioui.org/ui/layout"
+)
+
+type config struct{}
+
+var cfg = new(config)
+
+var cs = layout.Constraints{
+	Width: layout.Constraint{
+		Max: 100,
+	},
+	Height: layout.Constraint{
+		Max: 100,
+	},
+}
+
+func ExampleInset() {
+	ops := new(ui.Ops)
+
+	// Inset all edges by 10.
+	inset := layout.UniformInset(ui.Dp(10))
+	cs = inset.Begin(cfg, ops, cs)
+	// Lay out a 50x50 sized widget.
+	dims := layoutWidget(50, 50, cs)
+	dims = inset.End(dims)
+
+	fmt.Println(dims.Size)
+
+	// Output: (70,70)
+}
+
+func layoutWidget(width, height int, cs layout.Constraints) layout.Dimens {
+	return layout.Dimens{
+		Size: image.Point{
+			X: width,
+			Y: height,
+		},
+	}
+}
+
+func (config) Now() time.Time {
+	return time.Now()
+}
+
+func (config) Px(v ui.Value) int {
+	return int(v.V + .5)
+}